A

AAO Abbreviation of ANGLO-AUSTRALIAN OBSERVATORY

AAT Abbreviation of ANGLO-AUSTRALIAN TELESCOPE

AAVSO Abbreviation of AMERICAN ASSOCIATION OF VARIABLE STAR OBSERVERS

Abbot, Charles Greeley (1872-1961) American astronomer who specialized in solar radiation and its effects on the Earth's climate. He was director of the Smithsonian Astrophysical Observatory from 1907. Abbot made a very accurate determination of the solar constant, compiled the first accurate map of the Sun's infrared spectrum and studied the heating effect of the solar corona. He helped to design Mount Wilson Solar Observatory's 63-ft (19-m) vertical solar telescope.

Abell, George Ogden (1927-83) American astronomer who studied galaxies and clusters of galaxies. He is best known for his catalogue of 2712 'rich' clusters of galaxies (1958), drawn largely from his work on the PALOMAR OBSERVATORY SKY SURVEY. The Abell clusters, some of which are 3 billion l.y. distant, are important because they define the Universe's large-scale structure. Abell successfully calculated the size and mass of many of these clusters, finding that at least 90% of the mass necessary to keep them from flying apart must be invisible.

aberration (1) (aberration of starlight) Apparent displacement of the observed position of a star from its true position in the sky, caused by a combination of the Earth's motion through space and the finite velocity of the light arriving from the star. The effect was discovered by James BRADLEY in 1728 while he was attempting to measure the PARALLAX of nearby stars. His observations revealed that the apparent position of all objects shifted back and forth annually by up to 20" in a way that was not connected to the expected parallax effect.

The Earth's movement in space comprises two parts: its orbital motion around the Sun at an average speed of 29.8 km/s (18.5 mi/s), which causes annual aberration, and its daily rotation, which is responsible for the smaller of the two components, diurnal aberration. The former causes a star's apparent position to describe an ELLIPSE over the course of a year. For any star on the ECLIPTIC, this ellipse is flattened into a straight line, whereas a star at the pole of the ecliptic describes a circle. The angular displacement of the star, a, is calculated from the formula tan a = v/c, where v is the Earth's orbital velocity and c is the speed of light.

Diurnal aberration is dependent on the observer's position on the surface of the Earth. Its effect is maximized at the equator, where it produces a displacement of a stellar position of 0".32 to the east, but drops to zero for an observer at the poles.

Bradley's observations demonstrated both the motion of the Earth in space and the finite speed of light; they have influenced arguments in cosmology to the present day.

aberration Aberration can cause displacement in the position of a star relative to its true position as viewed in a telescope. Bending of the light path away from the optical axis produces coma, drawing the star image into a ‘tail’ (3). Offset of the star’s position (2) can reduce the effectiveness of the telescope for astrometry.

aberration (2) Defect in an image produced by a LENS or MIRROR. Six primary forms of aberration affect the quality of image produced by an optical system. One of these, CHROMATIC ABERRATION, is due to the different amount of refraction experienced by different wavelengths of light when passing through the boundary between two transparent materials; the other five are independent of colour and arise from the limitations of the geometry of the optical surfaces. They are sometimes referred to as Seidel aberrations after Ludwig von Seidel (1821-96), the mathematician who investigated them in detail.

The five Seidel aberrations are SPHERICAL ABERRATION, COMA, ASTIGMATISM, curvature and distortion. All but spherical aberration are caused when light passes through the optics at an angle to the optical axis. Optical designers strive to reduce or eliminate aberrations and combine lenses of different glass types, thickness and shape to produce a 'corrected lens'. Examples are the composite OBJECTIVES in astronomical refractors and composite EYEPIECES.

Curvature produces images that are not flat. When projected on to a flat surface, such as a photographic film, the image may be in focus in the centre or at the edges, but not at both at the same time. Astronomers using CCD cameras on telescopes can use a field flattener to produce a well-focused image across the whole field of view. Often this is combined with a focal reducer to provide a wider field of view.

Distortion occurs where the shape of the resulting image is changed. Common types of distortion are pincushion and barrel distortion, which describe the effects seen when an image of a rectangle is formed. Some binocular manufacturers deliberately introduce a small amount of pin-cushion distortion as they claim it produces a more natural experience when the binoculars are panned across a scene. Measuring the distortion in a telescope is extremely important for ASTROMETRY as it affects the precise position measurements being undertaken. Astromet-ric telescopes once calibrated are maintained in as stable a condition as possible to avoid changing the distortion.

Abetti, Giorgio (1882-1982) Italian solar physicist, director of ARCETRI ASTROPHYSICAL OBSERVATORY (1921-52). As a young postgraduate he worked at Mount Wilson Observatory, where pioneering solar astronomer George Ellery HALE became his mentor. Abetti designed and constructed the Arcetri solar tower, at the time the best solar telescope in Europe, and used it to investigate the structure of the chromosphere and the motion of sunspot penumbras (the Evershed-Abetti effect).

ablation Process by which the surface layers of an object entering the atmosphere (for example a spacecraft or a METEOROID) are removed through the rapid intense heating caused by frictional contact with the air. The heat shields of space vehicles have outer layers that ablate, preventing overheating of the spacecraft's interior.

absolute magnitude (M) Visual magnitude that a star would have at a standard distance of 10 PARSECS. If m = apparent magnitude and r = distance in parsecs:

M = m + 5 — 5 log r For a minor planet this term is used to describe the brightness it would have at a distance of 1 AU from the Sun, 1 AU from the Earth and at zero PHASE ANGLE (the Sun-Asteroid-Earth angle, which is a physical impossibility).

absolute temperature Temperature measured using the absolute temperature scale; the units (obsolete) are °A. This scale is effectively the same as the modern thermodynamic temperature scale, wherein temperature is defined via the properties of the Carnot cycle. The zero point of the scale is ABSOLUTE ZERO, and the freezing and boiling points of water are 273.15°A and 373.15°A, respectively. 1°A is equivalent to 1 K and kelvins are now the accepted SI unit. See also CELSIUS SCALE

absolute zero Lowest theoretically attainable temperature; it is equivalent to —273.15°C or 0 K. Absolute zero is the temperature at which the motion of atoms and molecules is the minimum possible, although that motion never ceases completely because of the operation of the Heisenberg uncertainty principle. (This principle states that an object does not have a measurable position and momentum at the same time, because the act of measuring disturbs the system.) Absolute zero can never be achieved in practice, but temperatures down to 0.001 K or less can be reached in the laboratory. The COSMIC MICROWAVE BACKGROUND means that 2.7 K is the minimum temperature found naturally in the Universe.

absorption As a beam of light, or other ELECTROMAGNETIC RADIATION, travels through any material medium, the intensity of the beam in the direction of travel gradually diminishes. This is partly due to scattering by particles of the medium and partly due to absorption within the medium. Energy that is absorbed in this way may subsequently be re-radiated at the same or longer wavelength and may cause a rise in temperature of the medium.

The absorption process may be general or selective in the way that it affects different wavelengths. Examples can be seen in the colours of various substances. Lamp black, or amorphous carbon, absorbs all wavelengths equally and reflects very little, whereas paints and pigments absorb all but the few wavelengths that give them their characteristic colours.

Spectral analysis of starlight reveals the selective absorption processe

s that tell us so much about the chemical and physical conditions involved. The core of a star is a hot, incandescent, high-pressure gas, which produces a CONTINUOUS SPECTRUM. The atoms of stellar material are excited by this high-temperature environment and are so close together that their electrons move easily from atom to atom, emitting energy and then being re-excited and so on. This gives rise to energy changes of all possible levels releasing all possible colours in the continuous SPECTRUM.

The cooler, low-pressure material that comprises the atmospheres of both star and Earth, and the interstellar medium that lies between them, can be excited by constituents of this continuous radiation from the star core, thus absorbing some of the radiation. Such selective absorption produces the dark ABSORPTION LINES that are so typical of stellar spectra. These lines are not totally black; they are merely fainter than the continuum because only a fraction of the absorbed energy is re-radiated in the original direction. See also FRAUNHOFER LINES; MOR-GAN-KEENAN CLASSIFICATION

absorption line Break or depression in an otherwise CONTINUOUS SPECTRUM. An ABSORPTION line is

caused by the absorption of photons within a specific (usually narrow) band of wavelengths by some species of atom, ion or molecule, any of which has its own characteristic set of absorptions. Absorption lines are produced when electrons associated with the various species absorb incoming radiation and jump to higher energy levels. The analysis of absorption lines allows the determination of stellar parameters such as temperatures, densities, surface gravities, velocities and chemical compositions (see SPECTRAL CLASSIFICATION).

absorption line The visible light spectrum of the cool giant star Arcturus ( Bootis) is shown here. The dark vertical lines in the spectrum are caused by atoms in the star’s atmosphere absorbing radiation. Because each element absorbs radiation at characteristic wavelengths, the spectrum of a star can be used to determine which elements are present.

absorption nebula See DARK NEBULA

Abu'l-Wafa' al BUzjanT, Muhammad (940-997/8) Persian astronomer and mathematician. His Kitab al-kamil ('Complete Book' [on Astronomy]) and his astronomical tables were used by many later astronomers, and he was the first to prove that the sine theorem is valid for spherical surfaces (for example, the celestial sphere). Abu'l-Wafa' discovered irregularities in the Moon's motion which were explained only by advanced theories of celestial mechanics developed centuries later.

Acamar The star 0 Eridani, visual mag. 2.88, distance 161 l.y. Through small telescopes it is seen to be double. The components are of mags. 3.2 and 4.3, with spectral types A5 IV and A1 Va. The name comes from the Arabic akhir al-nahr, meaning 'river's end', for in ancient times it marked the southernmost end of Eridanus, before the constellation was extended farther south to ACHERNAR.

acapulcoite-lodranite Association of two groups of ACHONDRITE meteorites. They show a range of properties that grade into each other, with similar oxygen isotopic compositions. Acapulcoite-lodranites are thought to be partial melts of chondritic precursors. They have been described as primitive achondrites, suggesting that they are a bridge between CHONDRITES and achondrites.

acceleration of free fall Acceleration experienced by a body falling freely in a gravitational field. A body in free fall follows a path determined only by the combination of its velocity and gravitational acceleration. This path may be a straight line, circle, ellipse, parabola or hyperbola. A freely falling body experiences no sensation of weight, hence the 'weightlessness' of astronauts, since the spacecraft is continuously free falling towards the Earth while its transverse motion ensures that it gets no closer. The free fall acceleration is 9.807 m/s2 at the Earth's surface. It varies as the inverse square of the distance from the Earth's centre.

accretion Process by which bodies gain mass; the term is applied both to the growth of solid objects by collisions that result in sticking and to the capture of gas by the gravity of a massive body. Both types of accretion are involved in the formation of a planetary system from a disk-shaped nebula surrounding a PROTOSTAR. When newly formed, such a disk consists mostly of gas, with a small fraction (c.1%) of solid material in small dust particles, with original sizes of the order of a micrometre. Grains settle through the gas towards the central plane of the disk; they drift inwards toward the protostar at rates that vary with their sizes and densities, resulting in collisions at low velocities. Particles may stick together by several mechanisms, depending on their compositions and local conditions, including surface forces (van der Waals bonding), electrical or magnetic effects, adsorbed layers of organic molecules forming a 'glue', and partial melting of ices. This sticking produces irregularly shaped fluffy aggregates, which can grow further by mechanical interlocking.

When bodies reach sizes of the order of a kilometre or larger, gravity becomes the dominant bonding mechanism. Such bodies are called PLANETESIMALS. Mutual perturbations cause their orbits to deviate from circularity, allowing them to cross, which results in further collisions. The impact velocity is always at least as large as the escape velocity from the larger body. If the energy density exceeds the mechanical strength of the bodies, they are shattered. However, a large fraction of the impact energy is converted into heat, and most fragments move at relatively low velocities, less than the impact velocity. These fragments will fall back to the common centre of gravity, resulting in a net gain of mass unless the impact velocity greatly exceeds the escape velocity. The fragments form a rubble pile held together by their mutual gravity. As this process is repeated, bodies grow ever larger. At sizes greater than a few hundred kilometres gravitational binding exceeds the strength of geological materials, and the mechanical properties of the planetesimals become unimportant.

Accretion is stochastic, that is, the number of collisions experienced by a body of a given size in an interval of time is a matter of chance. This process produces a distribution of bodies of various sizes. Often, the size distribution can be described by a power law, with an index s, defined so that the number of objects more massive than a specific mass, m, is proportional to m~s. If s is less than 1, most of the mass is in the larger objects; for larger values of s, the smaller bodies comprise most of the mass. Power law size distributions may be produced by either accretion or fragmentation, with the latter tending to have somewhat larger s values. However, accretion of planetesimals subject to gravitational forces can produce another type of distribution. If relative velocities are low compared with a body's escape velocity, its gravity can deflect the trajectories of other objects, causing impacts for encounters that would otherwise be near misses. This 'gravitational focusing' is more effective for more massive bodies, and it increases their rate of mass gain by accretion, allowing the largest bodies to grow still more rapidly. In numerical simulations of accretion, the first body to reach a size such that its escape velocity exceeds the mean relative velocity experiences 'runaway growth', quickly becoming much larger than the mean size. Its own perturbations then stir up velocities of the smaller bodies near its orbit, preventing them from growing in the same manner. At greater distances, its effects are weaker, and the process can repeat in another location. The result is a series of PROTOPLANETS in separated orbits; these can grow further by sweeping up the residual population of small planetesimals.

In the inner Solar System, the final stage of accretion probably involved collisions between protoplanets. Impacts of this magnitude would have involved enough energy to melt the planets; such an event is theorized to be responsible for the origin of the Moon. If a protoplan-et attained sufficient mass before dissipation of the SOLAR NEBULA, then its gravitation could overcome the pressure of the nebular gas, and it could accrete gas from the nebula. This process can begin at a critical mass that depends on a number of factors, including the density and temperature of the gas, and the protoplanet's distance from the Sun. The rate at which gas is accreted is limited by the escape of energy, which must be radiated away by the gas as it cools. The original protoplanet would then become the CORE of a planet that consists mostly of hydrogen. Plausible estimates imply that the critical mass is at least a few times Earth's mass, but pro-toplanets of this size could have accreted in the outer Solar System. Jupiter and Saturn probably formed by accretion of gas. See also COSMOGONY

accretion disk Disk of matter that surrounds an astronomical object and through which material is transferred to that object. In many circumstances, material does not transfer directly from one astronomical object to another. Instead, the material is pulled into equatorial orbit about the object before accreting. Such material transfer systems are known as ACCRETION disks. Accretion disks occur in protostellar clouds, close BINARY STAR systems and at the centre of galaxies.

Accretion disks are difficult to observe directly because of their small size or large distance from Earth. The disks that appear the largest (because they are nearest) are PROTOPLANETARY DISKS, around 100 AU in size, some of which have been imaged by the Hubble Space Telescope. Accretion disks in CLOSE BINARIES range in diameter from a few tenths to a few solar radii. Details about size, thickness and temperature of accretion disks can be provided by observing eclipses occurring between the disk and the secondary star from which the material is being pulled.

The energy output of the material being accreted depends on the mass and radius of the accreting object. The more massive and the smaller the accreting object, the higher the speed of material arriving, and the greater the amount of energy released on impact. Energy continues to be radiated as the material loses energy by slowing down within the disk. If the accreting star in a binary system is a WHITE DWARF, as in a CATACLYSMIC VARIABLE, the inner part of the disk will radiate in the ultraviolet, while the outer part radiates mostly in the visible. The MASS TRANSFER in such systems is often unstable, causing DWARF NOVAE outbursts.

In an X-RAY BINARY the accreting star is a NEUTRON STAR or stellar-mass BLACK HOLE and the inner disk radiates in X-rays. Unstable mass transfer across these disks produces soft X-RAY TRANSIENTS and sometimes relativis-tic JETS. The greatest amounts of energy are released when matter accretes on to a SUPERMASSIVE BLACK HOLE at the centre of a galaxy. This is the power source of an ACTIVE GALACTIC NUCLEUS, the central region of which radiates in ultraviolet and X-ray and can produce relativistic jets.

ACE Acronym for ADVANCED COMPOSITION EXPLORER

Achernar The star a Eridani, visual mag. 0.46, distance 144 l.y. Achernar is the ninth-brightest star in the sky and has a luminosity over a thousand times that of the Sun. Its spectral type is B3 V with additional features that suggest it is a SHELL STAR. The name, which comes from the Arabic meaning 'river's end' (the same origin as ACAMAR), was given to this star in Renaissance times when the constellation Eridanus was extended southwards.

Achilles First TROJAN ASTEROID to be recognized, by Max WOLF in 1906; number 588. It is c.147 km (c.91 mi) in size.

achondrite STONY METEORITE that formed from melted parts of its parent body. Achondrites usually have differentiated compositions. They generally do not contain CHONDRULES, and they have very low metal contents. There are many different groups of achondrites, some of which can be linked to form associations allied with specific parents. The separate associations have little, if any, genetic relationship to each other. See also ACAPULCOITE-LODRANITE; ANGRITE; AUBRITE; BRACHINITE; HOWARDITE-EUCRITE-DIOGENITE

ASSOCIATION; LUNAR METEORITE; MARTIAN METEORITE; UREILITE; WINONAITE

achromat (achromatic lens) Composite LENS designed to reduce CHROMATIC ABERRATION. The false colour introduced into an image by a lens can be reduced by combining the action of two or more lenses with different characteristics. In an achromat, two lenses of different materials are used together.

The most common example is the OBJECTIVE of a good quality but inexpensive astronomical REFRACTOR. This is usually made of a crown glass lens and a flint glass lens that have different refractive indices and introduce different levels of dispersion. By making one lens diverging and the other converging the optical designer can produce a converging composite lens that brings light of two different wavelengths to a focus at the same point. This reduces considerably the false colour that would be produced by a single lens, but it does not eliminate it altogether: bright objects observed against a dark background, such as the Moon at night, will have a coloured edge. There is also a reduction in overall contrast compared with a completely colour-corrected optical system such as an apochromat.

Acidalia Planitia Main dark area in the northern hemisphere of mars (47°.0N 22°.0W).

Acrux The star a Crucis (of which 'Acrux' is a contraction), visual mag. 0.77, distance 321 l.y. Small telescopes split it into two components of mags. 1.3 and 1.7. Their spectral types are B0.5 IV and B1 V, so both appear blue-white. Acrux is the southernmost first-magnitude star, declination — 63°.1.

actinometer (pyrheliometer) Instrument used for measuring at any instant the direct heating power of the Sun's radiation. Sir William herschel first noted, in 1800, that the heating effect of the Sun's rays was greatest beyond the red end of the spectrum. This infrared radiation was further investigated by his son Sir John herschel, who invented the actinometer around 1825.

active galactic nucleus (AGN) Central energy-producing region in some galaxies. AGNs are distinct in having substantial portions of their energy output coming from processes that are not associated with normal stars and their evolution. The observed guises of this energy release define the various types of active nuclei.

At the lowest power levels are liners (Low Ionization Nuclear Emission-line Regions), generally recognized only by the ratios of fairly weak emission lines; not all LINERS are genuine active nuclei. Activity characterized by h6, broad emission lines occurs in seyfert galaxies, most of which are spirals; Seyfert types 1 and 2 have different patterns of line width. Seyfert galaxies also show h6 X-ray emission and, often, far-infrared radiation. radio galaxies are most notable for their h6 radio emission, usually from a pair of lobes symmetrically placed about the galaxy, often accompanied by jets and radio emission from the nucleus itself. This activity may have little or no trace in the optical region, although some radio galaxies do have spectacular optical emission lines similar to those of both types of Seyfert galaxy. Higher-luminosity objects are quasistellar objects (QSOs), which are known as quasars (quasi-stellar radio sources) if they exhibit h6 radio emission. These objects are so bright that the surrounding galaxy can be lost for ordinary observations in the light of the nucleus. Members of another class, bl lacertae objects, show featureless spectra and rapid variability, suggesting that they are radio galaxies or quasars seen along the direction of a relativistic jet, the radiation of which is h6ly beamed along its motion. These categories share features of h6 X-ray emission, large velocities for the gas seen in emission lines, and a very small emitting region as seen from variability. Many show variation in the ultraviolet and X-ray bands on scales of hours to days, implying that the radiation is emitted in a region with light-crossing time no longer than these times.

The most popular model for energy production in all these kinds of active galactic nuclei involves material around a supermassive black hole (of millions to a few thousand million solar masses). The power is released during accretion, likely in an accretion disk, while jets may be a natural by-product of the disk geometry and magnetic fields.

active optics Technique for controlling the shape and alignment of the primary mirror of a large reflecting telescope. As a telescope tilts to track the path of a celestial object across the sky, its mirror is subject to changes in the forces acting upon it, as well as temperature variations and buffeting from the wind, which can cause it to flex, giving rise to spherical aberration or astigmatism of the image.

active optics The active optics actuators on the reverse of the primary mirror of the WIYN telescope at Kitt Peak National Observatory allow the mirror to be flexed continually to compensate for the effects of gravity as the telescope moves. This system means that far thinner mirrors can be used without risking distortion of the images and data obtained.

Active optics compensate for these effects, through the use of a number of computer-controlled motorized mirror supports, known as actuators, which continually monitor the shape of the mirror and adjust it into its correct form. These adjustments are typically only about 1/10,000 the thickness of a human hair but are enough to keep light from a star or galaxy precisely focused.

For many years it was considered impossible to build telescopes of the order of eight metres in diameter using a single mirror because it would have had to be so thick and heavy in order to maintain its correct shape as to make it impractical. The development of active optics technology has meant that relatively thin primary mirrors can now be built that are lighter and cheaper and are able to hold their precise shape, thereby optimizing image quality. The system also compensates for any imperfections in the surface of the mirror caused by minor manufacturing errors. See also adaptive optics; segmented mirror

active region Region of enhanced magnetic activity on the Sun often, but not always, associated with sunspots and extending from the solar photosphere to the corona. Where sunspots occur, they are connected by h6 magnetic fields that loop through the chromosphere into the low corona (coronal loops). Radio, ultraviolet and X-ray radiation from active regions is enhanced relative to neighbouring regions of the chromosphere and corona. Active regions may last from several hours to a few months. They are the sites of intense explosions, flares, which last from a few minutes to hours. NOAA (National Oceanic and Atmospheric Administration), which monitors solar activity, assigns numbers to active regions (for example, AR 9693) in order of their visibility or appearance. The occurrence and location of active regions varies in step with the approximately 11-year solar cycle. Loops of gas seen as filaments or prominences are often suspended in magnetic fields above active regions.

Adams, John Couch (1819-92) English mathematician and astronomer who played a part in the discovery of Neptune. In 1844, while at St John's College, Cambridge, he began to investigate the orbital irregularities of Uranus, which he concluded could be accounted for by gravitational perturbations by an undiscovered planet. He calculated an orbit for this planet, and identified a small region of sky where it might be found. He approached James challis, director of the Cambridge Observatory, and George Biddell airy, the Astronomer Royal. However, communications between Adams and Airy did not run smoothly, and no search was mounted from Britain. When the Uranus-disturbing planet, Neptune, was located on 1846 September 23, it was by Johann galle and Henrich d'arrest, observing from Berlin and guided by a position calculated independently by Urbain le verrier.

Adams was a brilliant scientist, but shy and rather retiring, and he refused a knighthood in 1847. He returned to Cambridge as Lowndean Professor in 1858, becoming director of the Cambridge Observatory in 1860. Adams' subsequent researches on the lunar parallax and other small motions, and the celestial mechanics of meteor streams following the 1866 Leonid storm, won him numerous honours. In spite of the Neptune affair, which led to arguments over the conduct of British science and a souring of Anglo-French scientific relations, Adams enjoyed the friendship of Airy, Challis and Le Verrier.

Adams, Walter Sydney, Jr (1876-1956) American astronomer, born in Syria to missionary parents, who succeeded George Ellery hale as director of Mount Wilson Observatory (1923-46). At Yerkes Observatory (1898-1904), Adams became an expert at using spectroscopic techniques to determine stellar radial velocities. He followed Hale to Mount Wilson, where a great new observatory specializing in solar astronomy was being built. Adams and Hale obtained solar spectra showing that sunspots were cooler than the rest of the Sun's surface, and by measuring Doppler shifts in solar spectra he was able accurately to measure our star's differential rotation, which varies with latitude. In 1914 he began studying the intensity of spectral lines of stars beyond the Sun, which could be used to calculate the stars' absolute magnitudes; during his Mount Wilson years, Adams computed and catalogued the radial velocities of 7000 stars, determining the absolute magnitudes of another 6000.

Adams discovered that the intensities differed for main-sequence, giant and dwarf stars, and used this knowledge to identify Sirius B as the first example (1915) of a white dwarf. His calculations showed that Sirius B is an extremely hot, compact star containing 80% of the Sun's mass packed into a volume roughly equal to that of the Earth. Ten years later he was able to measure a Doppler shift of 21 km/s (13 mi/s) for Sirius B, a result predicted by Arthur eddington's model of white dwarfs, which, because they are very dense, produce powerful localized gravitational fields manifested injust such a spectral redshift. Adams' discovery was therefore regarded as an astrophysical confirmation of Albert einstein's theory of general relativity. In 1932 Adams found that the atmosphere of Venus is largely composed of CO2; he also discovered that the interstellar medium contained the molecules CN and CH. The climax of Adams' career was his role in the design and building of Mount Palo-mar's 200-inch (5-m) Hale Telescope.

adaptive optics Technique that compensates for distortion caused in astronomical images by the effects of atmospheric turbulence, or poor seeing.

adaptive optics Telescopes using adaptive optics, such as the Very Large Telescope (VLT), have far better resolving power than earlier ground-based telescopes. Here the light from a close binary pair with a separation of only 0.03 has been reflected from the primary mirror on to a subsidiary mirror, which is continually adjusted to compensate for variations in the Earth’s atmosphere; it is then computer processed.

Adaptive optic technology uses a very thin, deformable mirror to correct for the distorting effects of atmospheric turbulence. It operates by sampling the light using an instrument called a wavefront sensor. This takes a 'snapshot' of the image from a star or galaxy many times a second and sends a signal back to the deformable mirror, which is placed just in front of the focus of the telescope. The mirror is very thin and can be flexed in a controlled fashion hundreds of times a second, compensating for the varying distortion and producing an image almost as sharp as if the telescope were in space. The control signals must be sent from the wavefront detector to the mirror fast enough so that the turbulence has not changed significantly between sensing and correction. See also active optics; speckle interferometry

ADC Abbreviation of astronomical data center

Adhara The star e Canis Majoris, visual mag. 1.50, distance 431 l.y., spectral type B2 II. It has a 7th-magnitude companion, which is difficult to see in very small telescopes as it is drowned by Adhara's light. The name comes from an Arabic phrase meaning 'the virgins', given to an asterism of four of five stars of which Adhara was the brightest.

adiabatic Process in thermodynamics in which a change in a system occurs without transfer of heat to or from the environment. Material within the convective regions of stars moves sufficiently rapidly that there is little exchange of energy except at the top and bottom of the region. The material therefore undergoes adiabatic changes, and this leads to a simple pressure law of the form: P = kp5/3 where P is the pressure, p the density, and k a constant. Such a pressure law is called polytropic, and it enables the region to be modelled very simply.

Adonis Second apollo asteroid to be discovered, in 1936. It was lost but became numbered as 2101 after its recovery in 1977. Because of its low inclination orbit, Adonis makes frequent close approaches to the Earth. It has been suggested to be the parent of a minor meteor shower, the Capricornid-Sagittariids, and may, therefore, have originated as a cometary nucleus. See table at near-earth asteroid

Adrastea One of the inner moons of jupiter, discovered in 1979 by David Jewitt (1958- ) and Edward Danielson in images obtained by the voyager project. It is irregular in shape, measuring about 25 X 20 X 15 km (16 X 12 X 9 mi). It orbits near the outer edge of Jupiter's main ring, 129,000 km (80,000 mi) from the planet's centre, taking 0.298 days to complete one of its near-circular equatorial orbits. See also metis

Advanced Composition Explorer (ACE) NASA spacecraft launched in 1997 August. It is equipped with nine instruments to determine and compare the isotopic and elemental composition of several distinct samples of matter, including the solar corona, interplanetary medium, interstellar medium and galactic matter. The craft was placed into the Earth-Sun Lagrangian point, or L1, 1.5 million km (940,000 mi) from Earth, where it remains in a relatively constant position with respect to the Earth and the Sun.

Advanced Composition Explorer ACE’s nine instruments sample a wide range of accelerated particles from the Sun and interstellar and galactic sources. One of their main functions is to give warning of geomagnetic storms that might endanger astronauts and disrupt power supplies and communications on Earth.

aerolite Obsolete name for stony meteorite.

aeronomy Study of the physics and chemistry of the upper atmosphere of the Earth and other planets. On Earth, this region is rather inaccessible, being generally above the height that meteorological balloons can reach, so research techniques rely heavily on the use of rockets and satellites together with remote sensing by radio waves and optical techniques.

The primary source of energy for the processes investigated is incident solar energy absorbed before it reaches the surface of the planet. This energy may ionize the upper atmosphere to form ionospheric plasma or may cause chemical changes, such as the photodissociation of molecules to form atoms or the production of exotic molecules such as ozone and nitrous oxide. Some minor constituents have an important catalytic role in the chemistry of the upper atmosphere, hence, for example, the significant influence of chlorine compounds on the ozone concentration in the stratosphere.

Consideration of the atmosphere as a fluid leads to an understanding of the various winds and circulation patterns. Fluid oscillations include atmospheric tides, internal gravity waves and disturbances that propagate because of buoyancy forces. The tides are predominantly caused by solar heating, rather than by gravitational forces, and, on Earth, are the principal component driving the wind system at an altitude of about 100 km (about 60 mi). Under certain conditions the upper atmosphere may become turbulent, which leads to mixing and enhanced heat transport.

Optical phenomena include airglow, in which pho-toemission may be caused by a range of physical and chemical processes, and aurorae, where the visible emissions are produced by charged particles from the magnetosphere. Associated with aurorae are electric current systems, which create perturbations in the magnetic field. There are other currents in the upper atmosphere caused by tidally driven dynamos.

aether All-pervasive fluid through which electromagnetic waves were originally thought to propagate. Electromagnetic theory showed that light needed no such medium to propagate and experimental tests such as the michelson—morley experiment failed to detect signs of such a medium, so the idea of aether was dropped from physical theory.

Agena Alternative name for the star p Centauri. Also known as hadar

Agena One of the most successful US rockets. It was used extensively for rendezvous and docking manoeuvres in the manned gemini project, launching satellites and as a second stage for US lunar and planetary missions.

Agena The Agena target vehicle is seen here from Gemini 8 during the rendezvous. Testing docking procedures was vital to the success of the Apollo missions.

Aglaonike Ancient Greek, the first woman named in the recorded history of astronomy. She was said to have predicted eclipses, and some of her contemporaries regarded her as a 'sorceress' who could 'make the Moon disappear at will'.

AGN Abbreviation for active galactic nucleus airglow Ever-present faint, diffuse background of light in the night sky resulting from re-emission of energy by atmospheric atoms and molecules following excitation during daylight by solar radiation. Airglow emissions, which occur in the upper atmosphere, mean that Earth's night sky is never completely dark.

Prominent among airglow emissions is green light from excited oxygen, at 557.7 nm wavelength, which is found mainly in a roughly 10 km (6 mi) deep layer at around 100 km (60 mi) altitude. Red oxygen emissions at 630.0 nm and 636.4 nm occur higher in the atmosphere; together with those from sodium, these emissions become more prominent in the twilight airglow. The night-time airglow varies in brightness, probably in response to changing geomagnetic activity. The day-time airglow is about a thousand times more intense than that seen at night but is, of course, a great deal more difficult to study because of the bright sky background.

Airy, George Biddell (1801—92) English astronomer, the seventh astronomer royal. The son of an excise officer, he grew up in Suffolk and won a scholarship to Trinity College, Cambridge. Airy became a professor at the age of 26, and was offered the post of Astronomer Royal in 1835, having already refused a knighthood on the grounds of his relative poverty. (He turned down two further offers, before finally accepting a knighthood in 1872.) Academic astronomy in Airy's day was dominated by celestial mechanics. Astronomers across Europe, especially in Germany, were making meticulous observations of the meridional positions of the stars and planets for the construction of accurate tables. These tables provided the basis for all sorts of investigations in celestial mechanics to be able to take place. As a Cambridge astronomy professor and then as Astronomer Royal at Greenwich, Airy was to be involved in such research for 60 years.

In addition to such mathematical investigations, Airy was a very practical scientist, who used his mathematical knowledge to improve astronomical instrument design, data analysis, and civil and mechanical engineering. Upon assuming office as Astronomer Royal he began a fundamental reorganization of the Royal Observatory, Greenwich. He did little actual observing himself, but developed a highly organized staff to do the routine business, leaving him free for analytical, navigational and government scientific work. Airy was quick to seize the potential of new science-based techniques such as electric telegraphy, and by 1854, for instance, the Observatory was transmitting time signals over the expanding railway telegraph network.

It is sad that in the popular mind Airy is perhaps best remembered as the man who failed to enable John Couch adams to secure priority in the discovery of Neptune in 1846. Yet this stemmed in no small degree from Adams' own failure to communicate with Airy and to answer Airy's technical questions. Airy made no single great discovery, but he showed his generation how astronomy could be made to serve the public good.

Airy disk Central spot in the diffraction pattern of the image of a star at the focus of a telescope. In theory 84% of the star's light is concentrated into this disk, the remainder being distributed into the set of concentric circles around it. The size of the Airy disk is determined by the aperture of the telescope. It limits the resolution that can be achieved. The larger the aperture, the smaller the Airy disk and the higher the resolution that is possible amount of light reflected by a body to the total amount falling on it, albedo values range from 0 for a perfectly absorbing black surface, to 1 for a perfect reflector or white surface. Albedo is commonly used in astronomy to describe the fraction of sunlight reflected by planets, satellites and asteroids: rocky bodies have low values whereas those covered with clouds or comprising a high percentage of water-ice have high values. The average albedo of the moon, for example, is just 0.07 whereas venus, which is covered in dense clouds, has a value of 0.76, the highest in the Solar System. The albedo of an object can provide valuable information about the composition and structure of its surface, while the combination of an object's albedo, size and distance determines its overall brightness.

Albert One of the amor asteroids; number 719. It is c.3 km (c.2 mi) in size. Albert was an anomaly for many decades in that it was numbered and named after its discovery in 1911 but subsequently lost. Despite many attempts to recover it, Albert escaped repeated detection until the year 2000.

Aitken, Robert Grant (1864-1951) Leading American double-star observer and director of lick observatory (1930-35). His principal work was the New General Catalogue of Double Stars (1932), based largely on data he gathered at Lick beginning in 1895. It contains magnitudes and separations for more than 17,000 double stars, including many true binary systems. Aitken discovered more than 3000 doubles, and computed orbits for hundreds of binaries.

AI Velorum star Pulsating variable star,similar to the delta scuti type, with period shorter than 0.25 days and amplitude of 0.3-1.2 mag. AI Velorum stars belong to the disk population and are not found in star clusters. They are sometimes known as dwarf Cepheids.

AL Abbreviation of astronomical league

Alba Patera Low-profile shield volcano on mars (40°.5N 109°.9W). It is only 3 km (1.9 mi) high but some 1500 km (930 mi) across.

Albategnius Latinized name of al-battani

Albategnius Lunar walled plain (12°S 4°E), 129 km (80 mi) in diameter. Its walls are fairly high, 3000-4250 m (10,000-14,000 ft), and terraced; they are broken in the south-west by a large (32 km/20 mi) crater, Klein. Albategnius is an ancient impact site, and its eroded rims display landslips and valleys. The terrain surrounding this crater is cut by numerous valleys and deep trenches, evidence of the Mare imbrium impact event. A massive pyramid-shaped mountain and many bowl craters mark the central floor.

al-BattanT, Abu'Abdullah Muhammad ibn Jabir (Latinized as Albategnius) (c.858-929) Arab observational astronomer (born in what is now modern Turkey) who demonstrated that the Sun's distance from the Earth, and therefore its apparent angular size, varies, which explains why both total and annular solar eclipses are possible. He made the first truly accurate calculations of the solar (tropical) year (365.24056 days), the ecliptic's inclination to the celestial equator (23° 35') and the precession of the equinoxes (54".5 per year).

albedo Measure of the reflecting power of the surface of a non-luminous body.

albedo This albedo map of Mars was produced by NASA’s Mars Global Surveyor. Red areas are bright and show where there is dust while blue areas show where the underlying, darker rocks have been exposed.

Albireo The star p Cygni, visual mag. 3.05, distance 386 l.y. Albireo is a beautiful double star of contrasting colours. It comprises an orange giant (the brighter component, spectral type K3 II) twinned with a companion of mag. 5.1 and spectral type B9.5 V which appears greenish-blue. The two are so widely spaced, by about 34", that they can be seen separately through the smallest of telescopes, and even with good binoculars (if firmly mounted). The name Albireo is a medieval corruption, and is meaningless.

al-BuzjanT See abu'l-wafa' al bUzjAni, muhammad

Alcor The star 80 Ursae Majoris, visual mag. 3.99, distance 81 l.y., spectral type A5 V. Alcor is a spectroscopic binary, though no accurate data are known. The name may comes from an Arabic word meaning 'rider'. Alcor forms a naked-eye double with mizar; the two are not a genuine binary, but Alcor is part of the ursa major moving cluster.

Alcyone The star t\ Tauri, distance 368 l.y., spectral type B7 III. At visual mag. 2.85, it is the brightest member of the pleiades star cluster. In Greek mythology, Alcyone was one of the seven daughters of Atlas and Pleione.

Aldebaran The star a Tauri, visual mag. 0.87 (but slightly variable), distance 65 l.y. It is an orange-coloured giant of spectral type K5 III. It marks the eye of Taurus, the bull. Its true luminosity is about 150 times that of the Sun. Although Aldebaran appears to be a member of the V-shaped Hyades cluster, it is a foreground object at about half the distance, superimposed by chance. The name comes from the Arabic meaning 'the follower' - Aldebaran appears to follow the Pleiades cluster across the sky.

Alderamin The star a Cephei, visual mag. 2.45, distance 49 l.y., spectral type A7 V. Its name comes from an Arabic expression referring to a forearm.

Aldrin, Edwin Eugene ('Buzz'), Jr (1930-) American astronaut. After setting a record for space-walking during the Gemini 12 mission in 1966, Aldrin was assigned to Apollo 11 as Lunar Module pilot, and on 1969 July 20 he became the second man to walk on the Moon, after Neil armh6.

Aldrin, Edwin Eugene (‘Buzz’), Jr As well as piloting the lunar module of Apollo 11, ‘Buzz’ Aldrin also deployed and monitored experiments on the Moon’s surface. Here, he is seen with the Solar Wind Composition experiment.

Alfven, Hannes Olof Gosta (1908-95) Swedish physicist who developed much of the theory of magnetohydrodynamics, for which he was awarded the 1970 Nobel Prize for Physics. In 1942 he predicted the existence of what are now called alfven waves, which propagate through a plasma, and in 1950 he identified synchrotron radiation from cosmic sources, helping to establish radio astronomy.

Alfven waves Transverse magnetohydrodynamic waves that can occur in a region containing plasma and a magnetic field. The electrically conducting plasma is linked to and moves with the magnetic field. Sometimes this phenomenon is referred to as the plasma and magnetic field being 'frozen-in' to each other. The plasma follows the oscillations of the magnetic field and modifies those oscillations. Alfven waves may transfer energy out to the solar corona, and they are also found in the solar wind and the Earth's magnetosphere. They are named after Hannes alfven.

Algenib The star 7 Pegasi, visual mag. 2.83, spectral type B2 IV, distance 333 l.y. It is a beta cephei star - a pulsating variable that fluctuates by 0.1 mag. with a period of 3.6 hours. The name Algenib comes from the Arabic meaning 'the side'; it is also an alternative name for the star a Persei (see mirphak).

Algieba The star 7 Leonis, visual mag. 2.01, distance 126 l.y. Small telescopes show it to be a beautiful double star with golden-yellow components of mags. 2.6 and 3.5, spectral types K1 III and G7 III. The pair form a genuine binary with an orbital period of nearly 620 years. The name may come from the Arabic al-jabha, meaning 'the forehead', referring to its position in a much larger figure of a lion visualized by Arab astronomers in this region.

Algol Prototype of the algol stars, a subtype (EA) of eclipsing binary stars. It is now known, however, that Algol is somewhat atypical of its eponymous subtype.

The first recorded observation of Algol was made by Geminiano montanari in 1669. In 1782 John goodricke established that Algol's variability was periodic, with a sudden fade occurring every 2.867 days. Gradually, the concept of an eclipsing companion became accepted and was finally confirmed when, in 1889, Hermann vogel showed that the radial velocity of Algol varied with the same period as that of the eclipses.

By this time, Algol was known to be a triple system. In 1855 F.W.A. argelander had observed that the period between primary minima had shortened by six seconds since Goodricke's observations. Fourteen years later he noted that the period between the times of minima varied in a regular fashion with a period of about 680 days. This was attributed to the variation in the distance that the light from the system had to travel because of orbital motion around the common centre of gravity with a third star

(Algol C).

In 1906 the Russian astronomer Aristarkh Belopolski (1854-1934) confirmed the existence of Algol C by showing that radial-velocity variations in the spectral lines of Algol also had a period of 1.862 years superimposed on the period of 2.867 days for Algol AB. Several years later, Joel stebbins, a pioneer in stellar photometry, found that there was a secondary minimum of much smaller amplitude occurring exactly halfway between the primary eclipses. This showed for the first time that the companion was not dark at all, but merely much fainter than A. Photoelectric observations showed the depth of the secondary minimum to be 0.06 mag. and that the light from star A increased as the secondary minimum approached. This was interpreted as a reflection of light from the body of star B.

There are two stars, of spectral types B8 and G, rotating about each other. The B8 star is a dwarf and is the visible component. The fainter star (whose spectrum was only observed directly for the first time in 1978) is a sub-giant. The orbit is inclined to the line of sight by 82°, which results in mutual eclipses corresponding to a drop in light of 1.3 mag. when A is eclipsed by B. Hipparcos data give a distance to Algol of 93 l.y. This corresponds to luminosities of 100 and 3 for A and B respectively. From the length and depth of the eclipses, sizes of 2.89 and 3.53 solar radii have been derived for A and B respectively. The corresponding masses are 3.6 and 2.89 solar masses, and this apparent anomaly gives rise to what is known as the 'Algol paradox'.

In current theories of stellar evolution, stars advance in spectral type as they evolve, and the rate at which they do so is a function of their initial masses. Thus, if two stars form together from interstellar material, the more massive of the two should evolve more quickly. In Algol the more evolved star is the less massive and the cause seems to be mass transfer from B to A. A stream of material between the two stars has been detected in the radio observations, and the current transfer rate is thought to be at least 10~7 solar masses per year. Optical spectra have shown very faint lines, which are thought to be emitted by a faint ring of material surrounding star A.

Algol C was first resolved by speckle interferometry in 1974 and on several occasions since. Its angular separation has never exceeded 0".1, which explains why the star has never been seen by visual observers.

The real nature of the Algol system is still far from clear. Even after 200 years of continuous observation it still evokes considerable interest from astronomers.

Algol stars (EA) One of the three main subtypes of eclipsing binary. The light-curves of Algol stars exhibit distinct, well-separated primary minima. Secondary minima may be detectable, depending on the characteristics of the system. Outside eclipses, the light-curve is essentially flat, although it may exhibit a small, gradual increase and decrease around secondary minimum, which is caused by the reflection effect, where light from the bright (main sequence) primary irradiates the surface of the cooler secondary, thus raising its temperature and luminosity. The components of the binary system may be detached or, as in algol itself, one component may be semidetached. Systems in which the semidetached component is transferring mass to the non-evolved component are sometimes described as being 'Algol-type' or 'Algol-like' binaries.

In the Algol-type binaries, the detached component is a main-sequence star and its less-massive companion is a red subgiant that fills its roche lobe. Such systems are differentiated physically from the closely related beta lyrae stars by the fact that an accretion disk is never present around the detached component.

Some systems that begin as Algol stars (with detached components) may evolve into beta lyrae systems, with a high rate of mass transfer and massive accretion disks. When the mass-transfer rate drops, the accretion disks disappear to reveal the unevolved stars, and the systems display all the characteristics of an Algol-like binary.

The eventual fate of an Algol system depends on many factors, most notably on the stars' masses, which determine their rates of evolution. If the main-sequence star in an Algol system is comparatively massive, it will evolve rapidly and expand to fill its Roche lobe while the companion star is still filling its own Roche lobe. The result is a contact binary in which both stars share the same photosphere. These binaries are often called w ursae majoris stars after the prototype for this subtype.

If the main-sequence star in an Algol system evolves slowly, then its companion may become a white dwarf before the primary swells to fill its Roche lobe. When the primary finally does expand to become a red giant, gas flows across the inner Lagrangian point and goes into orbit about the white dwarf, forming an accretion disk. Such systems, called u geminorum stars or dwarf novae, exhibit rapid irregular flickering from the turbulent hot spot where the mass-transfer stream strikes the accretion disk. However, most of these systems are not eclipsing pairs. It is important to note that eclipsing variables only appear to fluctuate in light because of the angle from which they are observed.

Algonquin Radio Observatory (ARO) One of Canada's principal radio astronomy facilities, operated by the National Research Council and situated in Ontario's Algonquin Provincial Park, well away from local radio interference. Instruments include a 32-element array of 3-m (10-ft) dishes for solar observations and a 46-m (150-ft) fully steerable radio dish for studies of stars and galaxies. The ARO began work in 1959, and the 46-m dish was built in 1966.

ALH 84001 Abbreviation of allan hills 84001

Alhena The star y Geminorum, visual mag. 1.93, distance 105 l.y., spectral type A1 IV. The name comes from an Arabic term that is thought to refer to the neck of a camel, from a former constellation in this area.

Alioth The star e Ursae Majoris, visual mag. 1.76, distance 81 l.y. It is one of the so-called peculiar A stars, of spectral type A0p with prominent lines of chromium. It is, by a few hundredths of a magnitude, the brightest star in the plough (Big Dipper). Its name may be a corruption of the Arabic for 'tail'.

Alkaid (Benetnasch) The star i) Ursae Majoris, visual mag. 1.85, distance 101 l.y., spectral type B3 V. The name comes from an Arabic word meaning 'the leader'. Its alternative name, Benetnasch, is derived from an

Arabic phrase referring to a group of mourners accompanying a coffin formed by the quadrilateral of stars which is now known as the bowl of the plough (commonly referred to in the US as the Big Dipper).

Allan Hills 84001 (ALH 84001) meteorite that was found in Antarctica in 1984 and identified as a martian meteorite in 1994. It has a mass of c.1.93 kg. A complex igneous rock, it has suffered both thermal and shock processes. In composition, it is an orthopyroxenite rich in carbonates, which form patches up to c.0.5 mm across. Few hydrated minerals have been identified amongst the alteration products in ALH 84001, so it has been proposed that the carbonates were produced at the surface of Mars in a region of restricted water flow, such as an evaporating pool of brine. Tiny structures (c.200 nm in size) within the carbonates have been interpreted by some as fossilized Martian bacteria; however the claim is controversial, and it is subject to continued investigation.

Allegheny Observatory Observatory of the University of Pittsburgh, located 6 km (4 mi) north of Pittsburgh. The observatory, which dates from 1859, became part of the university in 1867. During the 1890s its director was James E. keeler, who used a 13-inch (330-mm) refractor to discover the particulate nature of Saturn's rings. Later, Allegheny was equipped with the 30-inch (76-cm) Thaw telescope (the third-largest refractor in the USA) and the 31-inch (0.79-m) Keeler reflector. The observatory now specializes in astrometric searches for extrasolar planets.

Allende meteorite that fell as a shower of stones in the state of Chihuahua, Mexico, on 1969 February 8. More than 2 tonnes of material is believed to have fallen. Allende is classified as a CV3 carbonaceous chondrite. Studies of components, such as CAIs and chondrules, within Allende have been instrumental in understanding the structure, chemistry and chronology of the pre-solar nebula. The first interstellar grains (nanometre-sized diamonds) to be identified in meteorites were isolated from Allende.

Allen Telescope Array (ATA) Large-area radio telescope - formerly called the One-hectare Telescope (1hT) - that will consist of 350 steerable parabolic antennae 6.1 m (20 ft) in diameter. The ATA is a joint undertaking of the SETI Institute and the University of California at Berkeley. When completed in 2005, it will permit the continuous scanning of up to 1 million nearby stars for seti purposes, and will serve as a prototype for the planned square kilometre array.

ALMA Abbreviation of atacama large millimetre array

Almaak The star y Andromedae, visual mag. 2.10, distance 355 l.y. It is a multiple star, the two brightest components of which, mags. 2.3 and 4.8, are divisible by small telescopes, forming a beautiful orange and blue pairing, spectral types K3 II and B9 V. The fainter star has a close 6th-magnitude blue companion that orbits it every 61 years. Its name comes from the Arabic referring to a caracal, a wild desert cat, and is also spelled Almach and Alamak.

Almagest Astronomical treatise composed in c.ad 140 by ptolemy. It summarizes the astronomy of the Graeco-Roman world and contains a star catalogue and rules for calculating future positions of the Moon and planets according to the ptolemaic system. The catalogue draws from that of hipparchus, though to what extent is a matter of controversy. In its various forms the Almagest was a standard astronomical textbook from late antiquity until the Renaissance. Its original name was Syntaxis ('[Mathematical] Collection'), but it became known as Megiste, meaning 'Greatest [Treatise]'. Around ad 700-800 it was translated into Arabic, acquiring the prefix Al- (meaning 'the'). It was subsequently lost to the West but was treasured in the Islamic world; it was reintroduced to European scholars via Moorish Spain in the form of a translation of the Arabic version into Latin completed in 1175 by Gerard of Cremona (c.1114-87). It remained of great importance until the end of the 16th century, when its ideas were supplanted by those of Nicholas copernicus, Tycho Brahe and Johannes Kepler.

almanac, astronomical Yearbook containing information such as times of sunrise and sunset, dates for phases of the Moon, predicted positions for Solar System objects and details of other celestial phenomena such as eclipses. For astronomical and navigational purposes the leading publication is The astronomical almanac.

Alnair The star a Gruis, visual mag. 1.73, distance 101 l.y., spectral type B7 V. Its name means 'bright one', from an Arabic expression meaning 'bright one from the fish's tail', given by an unknown Arab astronomer who visualized the tail of the southern fish, Piscis Austrinus, as extending into this area.

Alnath The star p Tauri, visual mag. 1.65, distance 131 l.y., spectral type B7 III. The name, which is also spelled Elnath, comes from the Arabic meaning 'the butting one' - it marks the tip of one of the horns of Taurus, the bull.

Alnilam The star e Orionis, visual mag. 1.69, distance about 1300 l.y. A blue-white supergiant, spectral type B0 Ia, it is the central star of the three that make up the belt of Orion and is marginally the brightest of them. Its name comes from an Arabic phrase meaning 'string of pearls', referring to the belt.

Alnitak The star Ј Orionis, visual mag. 1.74, distance 820 l.y. It is a binary, with individual components of mags. 1.9 and 4.0, spectral types O9.5 Ib and B0 III, and an orbital period of around 1500 years. A telescope of 75-mm (3-in.) aperture or more should show both stars. Alnitak is a member of the belt of Orion, and its name comes from the Arabic meaning 'belt'.

Alpes (Montes Alps) Cross-faulted lunar mountains that rise 1-3 km (3600-9800 ft) above the north-east margins.

Alpha Regio This Magellan of Mare imbrium. The Alps are 290 km (180 mi) long, radar image shows multiple and are traversed by the alpes vallis. The majority of Blanc, one of the Moon's greatest mountains, is nearly 3500 m (11,500 ft) tall.

Alpha Regio This Magellan radar image shows multiple volcanic domes in Alpha Regio on Venus.

Alpes Vallis (Alpine Valley) Darkened gap 200 km (120 mi) long that cuts through the lunar mountain range known as the Montes alpes. The Alpine Valley is a graben that developed as a result of Mare imbrium's tectonic adjustment. Varying in width from 7 to 18 km (4-11 mi), it irregularly tapers away from Mare Imbrium. Two delicate faults cut at right angles across the valley's floor, which has otherwise been smoothed by the lavas that have filled it. Running down the middle of the valley is a sinuous rille, which seems to originate in a vent crater, which may be a volcanic feature, probably a collapsed lava tube.

Alpha Unofficial name for the international space station

Alpha2 Canum Venaticorum star (ACV) Type of main-sequence variable star that exhibits photometric, magnetic and spectral fluctuations, primarily as a result of stellar rotation. Periods range from 0.5 to 150 days; amplitudes from 0.01 to 0.1 mag.; and spectra from B8p to A7p. The subtype ACV0 exhibits additional low-amplitude (c.0.1 mag.) non-radial pulsations, with periods of 0.003 to 0.1 days. See also spectrum variable

Alpha Capricornids Minor meteor shower, active from mid-July until mid-August and best seen from lower latitudes. Peak activity occurs around August 2, from a radiant a few degrees north-east of a Capricorni. Rates are low, about six meteors/hr at most, but the shower produces a high proportion of bright, flaring meteors with long paths. The meteor stream may be associated with the short-period (5.27 years) comet 45P/Honda-Mrkos-Pajdusakova; it has a low-inclination orbit close to the ecliptic. Spreading of stream meteoroids by planetary perturbations means that the radiant is rather diffuse.

Alpha Centauri (Rigil Kentaurus, Toliman) Closest naked-eye star to the Sun, 4.4 l.y. away, with a visual mag. of —0.28, making it the third-brightest star in the sky. Small telescopes reveal that it is a triple system. The two brightest components are of solar type, mags. —0.01 and 1.35, spectral types G2 V and K1 V, forming a binary with an orbital period of 79.9 years. The third member of the system is the red dwarf proxima centauri, which is the closest star of all to the Sun. Alpha Centauri is also known as Rigil Kentaurus (Rigil Kent for short), from the Arabic meaning 'centaur's foot'. An alternative name, Toliman, is derived from an Arabic term meaning 'ostriches', the figure visualized by Arab astronomers in the stars of this region.

Alpha Monocerotids Normally very minor meteor shower, active around November 21-22. The shower produced outbursts of more substantial activity in 1925, 1935, 1985 and 1995, suggesting a ten-year periodicity with several h6er displays having been missed. In 1995 rates of one or two meteors per minute were sustained for only a short interval. The shower is apparently associated with comet C/1943 W1 van Gent-Peltier-Daimaca.

alpha particle Helium nucleus, consisting of two protons and two neutrons, positively charged. Helium is the second-most abundant element (after Hydrogen), so alpha particles are found in most regions of plasma, such as inside stars, in diffuse gas around hot stars and in cosmic rays. Alpha particles are also produced by the radioactive decay of some elements. In the proton-proton chain of nuclear fusion reactions inside stars, four protons (hydrogen nuclei) are converted to one alpha particle (helium nucleus) with release of fusion energy, which powers stars. In the triple-a process, which is the dominant energy source in red giant stars, three alpha particles fuse to form a carbon nucleus with release of energy.

Alphard The star a Hydrae, visual mag. 1.99, distance 177 l.y., spectral type K3 II or III. Its name comes from an Arabic word meaning 'the solitary one', a reference to its position in an area of sky in which there are no other bright stars.

Alpha Regio Isolated highland massif on venus (25°.5S 0°.3E), showing complex structure; it is best described as a plateau encircled by groups of high volcanic domes. The circular central area has a mean elevation of 0.5 km (0.3 mi).

Alphekka (Gemma) The star a Coronae Borealis, visual mag. 2.22, distance 75 l.y., spectral type A0 IV. It is an algol star; its brightness drops by 0.1 mag. every 17.4 days as one star eclipses the other. Its name, which is also spelled Alphecca, comes from the name al-fakka, meaning 'coins', by which Arab astronomers knew the constellation Corona Borealis. More recently, the star has also become known as Gemma, since it shines like a jewel in the northern crown.

Alpheratz The star a Andromedae, visual mag. 2.07, distance 97 l.y. It has a peculiar spectrum, classified as B9p, which has prominent lines of mercury and magnesium. Its name is derived from the Arabic al-faras, meaning 'the horse', since it used to be regarded as being shared with neighbouring Pegasus (and was also designated 8 Pegasi); indeed, it still marks one corner of the square of pegasus. Its alternative name, Sirrah, is derived from the Arabic surrat al-faras, meaning 'horse's navel'.

Alphonso X (1221-84) King of Leon and Castile (part of modern Spain), known as Alphonso the Wise, a patron of learning and especially of astronomy. He commissioned a new edition of the highly successful Toledan Tables of the motions of the Sun, Moon and five naked-eye planets, prepared originally by al-zarqalI in Toledo a century before. The new Alphonsine Tables, incorporating ten years of revised observations and completed in 1272, were not superseded for almost 400 years.

Alphonsus Lunar crater (13°.5S 3°W), 117 km (72 mi) across. Its fault-dissected walls rise to over 3000 m (10,000 ft) above the floor. Running nearly north-south across the floor is a ridge system, which is 15 km (9 mi) wide and, at the point where it forms a prominent central peak, about 1000 m (3000 ft) high. Within

Alphonsus are a series of kilometre-sized elliptical features with haloes of dark material; they are oriented roughly parallel to the central ridge system and are considered by many planetary geologists to be of volcanic origin. In 1958 Soviet astrophysicist Nikolai Kozyrev (1908-83) obtained a spectrum showing blue emission lines, which he interpreted as proof of a gaseous emission from the crater's central peak, but these results have never been duplicated. The north wall of Alphonsus overlaps the south wall of ptolemaeus, indicating that Alphonsus formed following the Ptolemaeus impact event.

ALPO Abbreviation of association of lunar and planetary observers

Alrescha The star a Piscium, visual mag. 3.82, distance 139 l.y. It is a close binary with a calculated orbital period of around 930 years. The brighter component, of mag. 4.2, is a peculiar A star of spectral type A0p with h6 lines of silicon and strontium; the fainter companion, mag. 5.2, is a metallic-line A star, type A3m. The name Alrescha, sometimes also spelled Alrisha, comes from an Arabic word meaning 'the cord' al-SUff, Abu'l-Husain (Latinized as Azophi) (903-986) Arab astronomer (born in modern Iran) famous for his Kitab suwar al-kawakib al-thabita ('Book on the Constellations of the Fixed Stars'), a detailed revision, based upon his own observations, of ptolemy's star catalogue. In this work he identified the stars of each constellation by their Arab names, providing a table of revised magnitudes and positions as well as drawings of each constellation. Al-SufT was the first to describe the two brightest galaxies visible to the naked eye: the Andromeda Galaxy and the Large Magellanic Cloud, which he called the White Bull.

Altair The star a Aquilae, visual mag. 0.76, distance 16.8 l.y. It is a white main-sequence star of spectral type A7 V, with a luminosity 10 times that of the Sun. Altair is the 12th-brightest star and forms one corner of the summer triangle. Its name comes from an Arabic expression meaning 'flying eagle'.

Altai Rupes Range of lunar mountains (25°S 22°E) cut by four deep cross-faults. The Altais curve 505 km (315 mi) from the west wall of Piccolomini to the west side of the large formation catharina. They rise very steeply from the east to an average altitude of 1800 m (6000 ft), with highest peaks at 3500-4000 m (11,000-13,000 ft). The scarp is roughly concentric with the south-west margins of Mare nectaris. It may be the sole remnant of an outer ring of a much larger, multi-ring impact basin.

altazimuth mounting Telescope mounting that has one axis (altitude) perpendicular to the horizon, and the other (azimuth) parallel to the horizon. An altazimuth (short for 'altitude-azimuth') mounting is much lighter, cheaper and easier to construct than an equatorial mounting for the same size telescope, but is generally not capable of tracking the apparent motion of celestial objects caused by the Earth's rotation. Many amateur instruments with altazimuth mountings can therefore be used for general viewing, but are not suitable for long-exposure photography.

altazimuth mounting This simple form of telescope mount allows free movement in both horizontal and vertical axes, but is not suitable for use with motordrives, unless they are computer controlled.

Historically, large professional telescopes were invariably built with massive equatorial mountings, which often dwarfed the instrument they held. The lightweight and simple nature of altazimuth mountings, combined with high-speed computers, has led to almost all modern instruments being built with altazimuth mountings. On these telescopes, computers are used to control the complex three-axis motions needed for an altazimuth mount to track the stars. Both the altitude and azimuth axes are driven at continuously varying rates but, in addition, the field of view will rotate during a long photographic exposure, requiring an additional drive on the optical axis to counter field rotation. Some amateur instruments, especially dobsonian telescopes, are now being equipped with these three-axis drive systems, controlled by personal computers.

altitude Angular distance above an observer's horizon of a celestial body. The altitude of a particular object depends both on the location of the observer and the time the observation is made. It is measured vertically from 0° at the horizon, along the great circle passing through the object, to a maximum of 90° at the zenith. Any object below the observer's horizon is deemed to have a negative altitude. See also azimuth; celestial coordinates al-TUsI, NasTr al-DTn (Latinized as Nasireddin or Nasiruddin) (1201-74) Arab astronomer and mathematician from Khurasan (in modern Iran) who designed and built a well-equipped observatory at Maragha (in modern Iraq) in 1262. The observatory used several quadrants for measuring planet and star positions, the largest of which was 3.6 m (12 ft) in diameter. Twelve years of observations with these instruments allowed him to compile a table of precise planetary and stellar positions, titled ZTj-i ilkharii. Al-TusT's careful measurement of planetary positions convinced him that the Ptolemaic Earth-centred model of the Solar System was incorrect. His work may have influenced copernicus.

altitude The altitude of a celestial object relative to an observer is measured on a scale of 0–90° from the observer’s horizon to the zenith – the point directly overhead.

aluminizing Process of coating the optics of a reflecting telescope with a thin, highly reflecting layer of aluminium. The optical component to be aluminized is first thoroughly cleaned and placed in a vacuum chamber, together with pure aluminium wire, which is attached to tungsten heating elements. After removing the air from the chamber, the heating elements are switched on, vaporizing the aluminium, which then condenses on to the clean surface of the optical component. This forms an evenly distributed coating, usually just a few micrometres thick.

Alvan Clark & Sons American firm of opticians and telescope-makers whose 19th-century refracting telescopes include the largest in the world. After a career as a portrait painter and engraver, Alvan Clark (1804-87) started an optical workshop in 1846 under the family name with his sons, George Bassett Clark (1827-91) and Alvan Graham Clark (1832-97), the latter joining the firm in the 1850s. Alvan Graham Clark discovered over a dozen new double stars, including, in 1862, the 8th-magnitude Sirius B.

During the second half of the 19th century, Alvan Clark & Sons crafted the fine objective lenses for the largest refracting telescopes in the world, including the united states naval observatory's 26-inch (0.66-m) (1873), pulkovo observatory's 30-inch (0.76-m) (1878), Leander McCormick (Charlottesville, Virginia) Observatory's 28-inch (0.7-m) (1883), lick observatory's 36-inch (0.9-m) (1888), lowell observatory's 24-inch (0.6-m) (1896) and yerkes observatory's 40-inch (1-m) (1897). In addition to these large professional instruments, the firm made numerous smaller refractors, 4-6 inch (100-150 mm) in aperture, which are prized by today's collectors of antique telescopes.

Alvarez, Luis Walter (1911-88) American physicist who first identified the layer of clay enriched by the element iridium that appears in the strata separating the Cretaceous and Tertiary geological periods, known as the K/T boundary. Since meteorites contain much higher amounts of iridium than do terrestrial rocks and soil, Alvarez' discovery supported the hypothesis that a giant meteorite impact (see chicxulub) may have caused a mass extinction event on our planet 65 million years ago.

Alvarez, Luis Walter Many years after Alvarez first proposed that an anomaly in the iridium levels at the boundary between the Cretacious and Tertiary geological periods might have been caused by a meteor impact, geologists looking for oil found evidence of a massive impact centred near Chicxulub on Mexico’s Yucatan Peninsula. Shown here is a radar image of the impact site.

al-Zarqali, Abuishaq Ibrahim ibn Yahya (Latinized as Arzachel, and other variants) (1028-87) Arab astronomer who worked in Toledo, Spain, and prepared the famous Toledan Tables of planetary positions, which corrected and updated the work of Ptolemy and Muhammad ibn Muusau al-KhwaurizmuT (c.780-c.850). Al-ZarqauluT also accurately determined the annual rate of apparent motion of the Earth's aphelion relative to the stars as 12", remarkably close to the correct value of 11".8.

Amalthea Largest of jupiter's inner satellites. Amalthea was the fifth Jovian

moon to be found, in 1892 by E.E. barnard, and the first since the four much larger galilean satellites were discovered in 1610. The discovery was made visually, the last such discovery for a planetary satellite. Amalthea is irregular in shape, measuring about 270 X 165 X 150 km (168 X 103 X 93 mi). Amalthea orbits Jupiter at a distance of only 181,400 km (112,700 mi), under synchronous rotation with a period of 0.498 days such that it always keeps the same blunt end towards the planet. Its orbit is near-circular, inclined to the Jovian equator by only 0°.4. Amalthea is notable as being the reddest object in the Solar System, possibly because of the accumulation of a surface covering of sulphur derived from the ejecta of io's volcanoes. Amalthea has considerable surface relief, with two large craters, called Pan and Gaea, and two mountains, named Mons Ida and Mons Lyctos. Some sloping regions appear very bright and green, the cause of this phenomenon being unknown.

Amalthea The bright streak to the left on Amalthea’s surface is about 50 km (30 mi) long. It is not clear whether this feature (called Ida) is the crest of a ridge or material ejected from the crater to its right.

amateur astronomy, history of From at least as early as the 17th century until around 1890, astronomical research in Britain was invariably undertaken by those who worked for love and considered themselves 'amateurs' (from Latin amat, 'he loves'). The reasons were political and economic, as successive governments operated low-taxation, low-state-spending policies that encouraged private rather than public initiatives. Amateur astronomy, while it existed on Continental Europe, was less innovative, largely because the governments of France, Germany and Russia taxed more heavily and invested in professional science as an expression of state power. The United States had a mixed astronomical research tradition, with outstanding amateurs, such as the spectroscopist Henry draper, engaged in front-rank research, and major professional observatories financed by millionaire benefactors.

Although the British astronomical tradition was predominantly amateur, its leading figures were 'Grand Amateurs' in so far as fundamental research was their dominant concern. In the Victorian age, wealthy gentleman scientists were willing to spend huge sums of money to pursue new lines of research and commission ground-breaking technologies, such as big reflecting telescopes. The quality of Grand Amateur research enjoyed peer recognition from European and American professionals, while its own esprit de corps was expressed through membership of the ROYAL ASTRONOMICAL SOCIETY and the Royal Society in London, academic honours and a clearly defined social network. This was, indeed, professional-quality research paid for by private individuals. Grand Amateurs pioneered work on the gravitation of double star systems, cosmology, planetary studies, selenography, photography and spectroscopy, and included between 1820 and 1900 such figures as John HERSCHEL, William DAWES, Lord ROSSE, Admiral William SMYTH, William LASSELL, William HUGGINS, Norman LOCKYER and the master-builder and astrophotographer Isaac Roberts (1829-1904). The results of their researches transformed our understanding of the Universe.

Yet Victorian Britain also saw a fascination with astronomy spreading to the less well-off middle and even working classes. School teachers, modest lawyers, clergymen and even artisans took up astronomy; the self-educated telescope-maker John Jones worked for a few shillings per week as a labourer on Bangor docks, Wales. People with modest and often home-made instruments (especially after the silvered glass mirror replaced speculum in the 1860s) did not expect, like the Grand Amateurs, to change the course of astronomy, but enjoyed practical observation as a serious and instructive hobby. The Reverend Thomas WEBB's celebrated Celestial Objects for Common Telescopes (1859) became the 'bible' for these serious amateurs.

The big-city astronomical societies of Leeds (1859, 1892), Liverpool (1881), Cardiff(1894), Belfast ( c.1895) and others became the foci for these observing amateurs, with their lectures, meetings and journals. In 1890 the BRITISH ASTRONOMICAL ASSOCIATION (BAA) became the national organizing body for British amateurs, with branches in Manchester (1892, 1903) and elsewhere, many of which later became independent societies. Unlike the Royal Astronomical Society, they all admitted women as members. These societies, which dominated amateur astronomy well into the 20th century, remained predominantly middle-class, and it was not until the major social and economic changes in Britain following 1945 that the demographic base of British amateur astronomical societies began to widen significantly. The BAA established a system whereby amateurs would send their observations to a central clearing house where they would be synthesized by an expert and the collective results published. The great majority of amateurs who contribute observations on behalf of science continue to operate within such systems (see also the astronomer).

Amateur astronomy changed dramatically after World War II, and much of the emphasis moved across the Atlantic. Before the war, in the 1920s, the ranks of active amateur observers were swelled with the founding of the amateur telescope-making (ATM) movement by Russell PORTER and Albert G. Ingalls (1888-1958). Now that inexpensive war-surplus optical equipment was widely available, it was no longer essential for an amateur astronomer to build a telescope from the ground up as a rite of passage. By the mid-1950s, a wide variety of commercial instruments had entered the marketplace; later designs, such as the SCHMIDT-CASSEGRAIN and DOBSONIAN TELESCOPES, owed much to amateur observers and remain extremely popular. The numbers of amateur observers grew rapidly, particularly in the United States. It is no coincidence that the ASTRONOMICAL LEAGUE (1946) and the ASSOCIATION OF LUNAR AND PLANETARY OBSERVERS (1947) were formed at this time. A watershed for professional-amateur collaboration came in 1956 with the establishment of the Moonwatch programme, in anticipation of satellite launches for the International Geophysical Year (1957-59). Energized by the Soviet Union's launch of Sputnik 1, and guided by astronomers at the Smithsonian Astrophysical Observatory, Moonwatch galvanized amateurs around the world in a unique and grand pro-am effort.

The appearance of affordable charge-coupled devices (CCDs) in the final decade of the 20th century had an even greater impact on amateurs than had the war-surplus items of two generations before. Digital data, exponentially increasing computing power, and ever more sophisticated commercial SOFTWARE together created a revolution. They allowed amateurs to become competitive with ground-based professionals in the quality of data obtained in such areas as astrometry, photometry and the imaging of Solar System objects.

New organizations with new ideas sprang up. The INTERNATIONAL AMATEUR-PROFESSIONAL PHOTOELECTRIC PHOTOMETRY group, founded in 1980, is the prototype organization representing this new era. It encourages joint amateur-professional authorship of technical papers. Similar, though focused on campaigns to study cataclysmic variable stars, is the Center for Backyard Astrophysics. One of the latest groups to form is The Amateur Sky Survey, a bold venture to develop the hardware and software needed to patrol automatically the sky in search of objects that change in brightness or move. Other groups, such as the INTERNATIONAL OCCULTATION TIMING ASSOCIATION, have graduated from visual observations of lunar events to video recordings that determine the profiles of asteroids. The INTERNATIONAL DARK-SKY ASSOCIATION campaigns on an issue of concern to professionals and amateurs alike.

As the present era of mammoth all-sky surveys from Earth and space culminates, the need for follow-up observations - particularly continuous monitoring of selected objects - will grow dramatically. In a traditional sense, because of their numbers and worldwide distribution, sophisticated amateurs are ideally suited for such tasks, not as minions but as true partners with professionals. And, in the era of the Internet, amateurs should be able to plumb online sky-survey DATABASES just as readily as professionals can. The challenge facing the entire astronomical community today is to educate both camps about rewarding possibilities through mutual cooperation.

Ambartsumian, Viktor Amazaspovich (1908-96) Armenian astronomer who became an expert on stellar evolution and founded Byurakan Astrophysical Observatory. His development of the theory of radiative transfer allowed him to show that T Tauri stars are extremely young. He greatly advanced the understanding of the dynamically unstable stellar associations and extended principles of stellar evolution to the galaxies, where he found much evidence of violent processes in active galactic nuclei.

AM Canum Venaticorum Unique blue variable star with fluctuating period of about two minutes. It has primary and secondary minima, the latter sometimes disappearing. It is probably a semidetached binary of two white dwarfs, an accretion disk and a hot spot.

American Association of Variable Star Observers (AAVSO) Organization of amateur and professional astronomers, based in the USA but with an international membership. Founded in 1911, it originally collected mainly visual estimates of the changing brightnesses of mainly long-period variable stars, but its programme now encompasses all manner of variable objects, from pulsating rr lyrae stars and eclipsing binaries to exotic gamma-ray bursters. The AAVSO continues to provide timely data to researchers, including those using instruments on board spacecraft such as hipparcos and High-Energy Transient Explorer 2. By 2001 the AAVSO International Database contained more than 9million observations.

Ancient Beijing Observatory This engraving shows the Imperial Astronomical Observatory at Beijing in the late 17th century The instruments were used for mapping the skies extremely accurately.

Ames Research Center national aeronautics and space administration (NASA) research institute located at Moffett Field, California, in the heart of 'Silicon Valley'. It is NASA's centre of excellence for information technology and its lead centre in Aeronautics for Aviation Operations Systems. Ames also develops science and technology requirements for current and future flight missions relevant to astrobiology. Moffett Field has been a government airfield since 1933, but was closed as a military base in 1994. It is now a shared facility known as Moffett Federal Airfield.

AM Herculis star (AM) Binary system, with period in the range 1 to 3 hours, that shows h6ly variable linear and circular polarization and also eclipses. AM Herculis stars are h6ly variable X-ray sources and their light-curves change from orbit to orbit. They also show changes in brightness and in variability with time scales of decades. The total range of light variations may reach 4-5 magnitude V. AM Herculis stars seem to be related to dwarf novae, in that one component is a K-M-type dwarf and the other a compact object, but they differ in that the magnetic field of the compact component is sufficiently h6 to dominate the mass flow and thus cause the effects observed. Am Herculis stars are also known as Polars. See also cataclysmic variable

AMiBA Abbreviation of array for microwave background anisotropy

Amor asteroid Any member of the class of asteroids that approach, but do not cross, the orbit of the Earth; their perihelion distances range from the terrestrial aphelion at 1.0167 AU to an arbitrary cut-off at 1.3 AU. Like the other mars-crossing asteroids, Amor asteroids have limited lifetimes because of the chance of a collision with that planet. Over extended periods of time many Amors will evolve to become apollo asteroids, reducing their lifetimes further because of the greater chance of an impact on the Earth, or one of the other terrestrial planets. The disparity of compositional types observed indicates that Amors derive from various sources, including extinct cometary nuclei, the kirkwood gaps (through jovian perturbations) and the inner main belt (through perturbations imposed by Mars).

The first Amor-type asteroid to be discovered was eros, in 1898, but the archetype giving them their collective name is (1221) Amor. That object was found in 1932, the same year as the first Apollo asteroid. Over 760 Amors had been discovered by late 2001. Notable examples listed in the near-earth asteroid table include (719) albert, (887) Alinda, (1036) ganymed, (1580) Betulia, (1627) Ivar, (1915) Quetzalcoatl, (3552) Don Quixote and (4954) Eric.

amplitude In the study of variable stars, the overall range in magnitude of a variable, from maximum to minimum. This definition is in contrast to the normal usage in physics, where the term is applied to half of the peak-to-peak value assumed by any parameter.

Am star Metallic-line class astarwith high abundances of particular metals. These class 1 chemically peculiar stars (CP1) extend to class Fm. Am stars are enriched by factors of 10 or so in copper, zinc, strontium, zirconium, barium and the rare earths, but are depleted in calcium and scandium. As slow rotators that lack outer convection layers, these stars are apparently braked by the gravitational effects of close companions. In the quiet atmospheres, some atoms fall under the action of gravity, while others rise by means of radiation pressure. Sirius is an Am star.

amu Abbreviation of atomic mass unit

analemma Long, thin figure-of-eight shape obtained by plotting (or photographing) the position of the Sun on the sky at the same time of day at regular intervals throughout the year. The elongated north-south variation is due to the inclination of the Earth's equator to its orbit, and the much shorter east-west variation is due to the eccentricity of the Earth's orbit. A considerable degree of patience and technical skill is required to record the analemma photographically.

analemma A plot of the Sun’s apparent position from 52°N, looking south at midday, at 5-day intervals throughout the course of a year. The Sun is at the top of the figure 8 at the summer solstice and at the bottom at the winter solstice.

Ananke One of JUPITER's outer moons, c.30 km (c.20 mi) in size. All members of this group, which includes Carme, Pasiphae and Sinope, are in RETROGRADE MOTION (Ananke's inclination is 149°). They are thought to be fragments of a captured asteroid that subsequently broke apart. Ananke was discovered in 1951 by Seth Nicholson. It takes 631 days to orbit Jupiter at an average distance of 21.28 million km (13.22 million mi) in an orbit of eccentricity 0.244. The population of known outer satellites of Jupiter is increasing rapidly, with eleven more having been discovered since 1999.

anastigmat Compound lens designed to be free of ASTIGMATISM. In practice the astigmatism will only be eliminated in some areas of the lens but other ABERRATIONS will be sufficiently well corrected to give excellent definition across the whole field of view.

Anaxagoras Comparatively young crater (75°N 10°W), 51 km (32 mi) in diameter, near the Moon's north pole. Like other freshly formed impact sites, Anaxagoras is the centre of a bright system of rays and steep, finely terraced walls. Its rays extend south to Plato. The rims rise to a height of 3000 m (10,000 ft) above the floor. Anaxagoras has a very bright, 300-m (1000-ft) high central peak, which is part of a larger range that crosses the crater's floor. To the east, Anaxagoras overlaps Goldschmidt, a degraded ring, 80 km (50 mi) in diameter .

Anaxagoras of Clazomenae (c.499-128 bc) Greek philosopher (born in what is now modern Turkey) whose theory of the origin and evolution of the Solar System is, in terms of today's 'standard model', correct in its basic premise. He believed it originated as a disk whose rotation caused the matter in it to separate according to its density, the densest materials settling at the centre and the more rarefied materials spreading out towards the periphery. He was imprisoned for teaching that the Sun was not a deity but a red-hot stone, and that the Moon, the phases of which he correctly explained, shone by reflected sunlight.

Anaximander of Miletus (c.611-547 bc) Greek philosopher (born in what is now modern Turkey) who believed the Earth to be one of many existing worlds, and the Sun and Moon rings of fire. He taught that Earth moves freely in space - not fixed upon anything solid. In his cosmogony, the Universe came into existence from an 'eternal reservoir', rotation having spread fire (the stars) to outer regions, leaving heavy matter (Earth) at the centre. He was said to have discovered the equinoxes and the obliquity of the ecliptic, but there is little evidence for this.

Ancient Beijing Observatory Astronomical observatory founded in 1442, situated in central Beijing on an elevated platform 14 m (46 ft) above street level. In about 1670, the Flemish Jesuit missionary Ferdinand Verbiest (1623-88) began re-equipping the observatory, and six of the eight large bronze instruments remaining on the site date from 1673. The other two were built in 1715 and 1744. It is not known why Verbiest based his instruments on outmoded designs by Tycho BRAHE well into the era of telescopic astronomy.

Anderson, John August (1876-1959) American astronomer who, with Francis PEASE, used the Michelson stellar interferometer at the prime focus of Mount Wilson Observatory's 100-inch (2.5-m) Hooker Telescope to measure the diameter of the red giant star Betelgeuse. Using this arrangement, Anderson was also able to separate very close double stars. He supervised the grinding and polishing of the primary mirror for Mount Palomar Observatory's 200-inch (5-m) Hale Telescope.

Andromeda See feature article

Andromeda Galaxy (M31, NGC 224) One of the two giant spiral galaxies in the LOCAL GROUP of galaxies, the other being our Galaxy, the Milky Way. M31 is the nearest spiral to the Milky Way, some 2.4 million l.y. away. Its proximity has led to intensive studies by astronomers, yielding fundamental advances in such diverse fields as star formation, stellar evolution and nucleosynthesis, dark matter, and the distance scale and evolution of the Universe.

The Andromeda spiral is visible to the naked eye. Found close to the 4th-magnitude star v Andromedae, M31 (RA 00h 42m.7 dec. +41°16') appears as a faint patch of light, best seen on a transparent, moonless night from a dark site. It was recorded by the 10th-century Persian astronomer AL SUFI as a 'little cloud'. Binoculars and small telescopes show the central regions as an elongated haze; long-exposure imaging with large instruments is required to show the galaxy's spiral structure.

The Andromeda Galaxy played an important role in the 'GREAT DEBATE' among astronomers in the 1920s regarding the nature of the spiral nebulae: were these 'island universes' - complete star systems outside our own - as proposed by the 18th-century philosopher Immanuel KANT, or were they gas clouds within the Milky Way collapsing to form stars? Photographs taken in 1888 by Isaac Roberts (1829-1904) using a 20-inch (0.5-m) telescope revealed M31's spiral nature, but it was not until the 1920s that the most important clues were uncovered by Edwin HUBBLE. In 1923-24, using the 100-inch (2.5-m) Hooker Telescope at Mount Wilson, California, Hubble was able to image individual CEPHEID VARIABLES in the Andromeda spiral. Applying the PERIOD-LUMINOSITY RULE to the derived light-curves showed that the spiral was a galaxy in its own right beyond our own.

The next important stage in the study of M31 came between 1940 and 1955, with the painstaking observations of Walter BAADE from Mount Wilson during the wartime blackout, and later with the 200-inch (5-m) Hale Reflector at the Palomar Observatory, California. Baade succeeded in resolving stars in the Andromeda Galaxy's central bulge; they appeared to be mainly old and red, substantially fainter than the bright blue stars of the outer regions, and apparently similar to those in globular clusters. Baade referred to the bulge stars as Population II, labelling the hot disk stars as Population I (see populations, stellar). This distinction remains in current use and is an essential feature of accepted theories of star formation, and stellar and galaxy evolution.

The discovery of the two stellar populations led in turn to a crucial finding for cosmology. The Cepheid variables turned out to be of two subsets, one belonging to each population, obeying different period-luminosity rules. Since the Cepheids observed by Hubble were of Population I, the derived distance of M31 had to be revised upwards by a factor of two - as, were all other distances to galaxies, which had used the Andromeda Galaxy as a 'stepping stone'.

The neutral hydrogen (HI) distribution in M31 has been extensively studied by radio astronomers, observing the twenty-one centimetre emission line. Neutral hydrogen is a constituent of the galaxy's gas and is distributed like other Population I components. The gas shows a zone-of-avoidance near the galactic centre, which is where Population II stars dominate. The gas is distributed in a torus, the innermost parts of which seem to be falling towards the nucleus.

Radial velocities of hot gas clouds across the galaxy have been mapped. Together with HI observations, these measurements allow a rotation curve to be constructed as a function of galactic radius. HI measurements, particularly, suggest that the outer regions of M31 contain substantial amounts of unseen additional mass. Such halos of dark matter are crucial to current theories of galaxy formation and clustering, and cosmology.

Observations with the hubble space telescope in 1993 showed the nucleus of M31 to be double, with its components separated by about 5 l.y. This may be the result of a comparatively recent merger between the Andromeda Galaxy and a dwarf companion. Several small satellite galaxies surround M31, the most prominent being M32 (NGC 221) and M110 (NGC 205).

The disk of M31 shows a number of star clouds, the most obvious being NGC 206, which covers an area of 2900 X 1400 l.y. About 30 novae can be detected in M31 each year by large telescopes. M31 was the site, in 1885 August, of the first supernova to be observed beyond the Milky Way: it was designated S Andromedae and reached a peak apparent magnitude + 6.

It might be expected that the proximity of M31 would mean that it could make a substantial contribution to theories for the development of spiral structure. Instead, it has contributed controversy, partly because the galaxy is so close to edge-on that details of the spiral structure are hard to delineate. Indeed, it is not even known how many spiral arms there are. Halton arp has proposed two trailing spiral arms, one of these disturbed by the gravitational pull of M32. A. Kalnajs proposes instead a single leading spiral arm, set up via gravitational resonance with M32. The dust clouds do not help in deciding between these two models. Resolution of the debate will ultimately advance our understanding of the mechanism generating spiral structure (see density wave theory).

M31 is surrounded by a halo of globular clusters, which is some three times more extensive than the halo around our Galaxy. The stars in these clusters show a generally higher metallicity than is found in our own Galaxy's globu-lars. The great spread in element abundances in the M31 globular clusters suggests slower and more irregular evolution than has occurred in the Milky Way.

Andromeda Galaxy The Andromeda Galaxy, M31, is the largest member of the Local Group and is the farthest object that can be seen with the naked eye. Many of the star-like points in this image are in fact globular clusters within its galactic halo.

Nearly every galaxy in the Universe shows a redshift, indicative of recession from the Milky Way. The spectrum of M31, however, shows it to be approaching at a velocity of about 35 km/s (22 mi/s). In some 3 billion years, M31 and the Milky Way will collide and merge eventually to form a giant elliptical galaxy.

M31 is our sister galaxy, the nearest spiral galaxy that is similar in most attributes to the Milky Way. Much of our home Galaxy is hidden from our perspective by massive dust clouds; we rely on the Andromeda Galaxy for an understanding of our own Galaxy, as well as of the rest of the Universe.

Andromedids (Bielids) meteor shower associated with comet 3D/biela. The parent comet split into two fragments in 1845 and has not been definitely seen since 1852; it is now considered defunct. Swarms of meteoroids released from the comet have given rise to spectacular meteor showers. Its name derives from its radiant position, near y Andromedae.

The shower's first recorded appearance was in 1741, when modest activity was observed. Further displays were seen in 1798, 1830, 1838 and 1847, in each case during the first week of December. The 1798 and 1838 displays produced rates of over 100 meteors/hr. When seen in 1867 the Andromedids appeared on the last day of November. The node, where the orbit of the mete-oroid swarm and the orbit of the Earth intersect, is subject to change as a result of gravitational perturbations by the planets. The Andromedid node is moved earlier (regresses) by two or three weeks per century.

In 1867 the association between a meteor shower and a comet was demonstrated by Giovanni schiaparelli in the case of the perseids; other such connections were sought. It was known that the orbit of Biela's comet approached that of the Earth very closely, so that its debris could conceivably give rise to a meteor shower, and when the radiant was calculated it was found to agree closely with that of the meteor showers previously seen to emanate from Andromeda.

Biela's comet, if it still existed, would have been in the vicinity of the Earth in 1867, and since the meteoroid swarm would not be far displaced from its progenitor, a display could be expected. A good, though not spectacular, Andromedid shower was seen on November 30, confirming the prediction. Since the orbital period was about 6.5 years, Edmund Weiss (1837-1917), Heinrich d'arrest and Johann galle, who had made the first calculations, predicted another display for 1872 November 28.

Soon after sunset on 1872 November 27, a day earlier than expected, western European observers were treated to an awesome spectacle; meteors rained from the sky at the rate of 6000 per hour. The event caused less alarm and terror among the general population than it might have done, there having occurred only six years previously an equally dramatic leonid display, which had caused no harm. Although the Andromedids were about as numerous as the 1866 Leonids, they were less brilliant due to their lower atmospheric velocity, at 19 km/s (12 mi/s) compared with the Leonids' 70 km/s (43 mi/s).

The shower of 1872 led, incidentally, to a mysterious episode in astronomical history. E.F. Wilhelm Klinkerfues (1827-84), at Gottingen Observatory, reasoning that it should presumably be ahead of the meteoroid swarm, suggested that the comet should be visible in the opposite part of the sky direction to the Andromedid radiant. He accordingly cabled to Norman pogson, an astronomer at Madras (the comet would not be visible from high northern latitudes): 'Biela touched Earth 27 November - search near Theta Centauri.' On December 2, Pogson's search found a comet near the indicated position.

The object was observed again the following night, but then clouds intervened and when a clearance finally came, there was no sign of it. The observations were inadequate for the calculation of an orbit and the prediction of future positions, so the comet, if such it was, was lost. If Biela's comet still existed and was pursuing its original orbit, it would have passed the position indicated by Klinkerfues some months previously. We must conclude that if Pog-son, who was an experienced observer, did see a comet, it was not 3D/Biela, and was either a fragment of that object or another comet that just happened to be there at the time. Both alternatives are hard to believe, and the question remains open.

The next encounter was badly timed, and no shower was seen. In 1885, two revolutions after the 1872 event, European observers were delighted and thrilled by an even greater meteor storm on November 27, during which Andromedid rates were estimated (counting was virtually impossible) at 75,000 per hour. This rate compares with that of 140,000 per hour estimated for the 1966 Leonid peak. The most intense activity in the 1885 Andromedids was over in about six hours (such meteor storms are invariably short-lived), though lower rates were detected for a few days to either side. This suggests that the core filament in the Andromedid meteor stream in 1885 had a width of about 160,000 km (100,000 mi).

The fall of an iron meteorite at Mazapil, Mexico, during the 1885 Andromedid display can be dismissed as coincidence. Meteoroids shed by comets are generally small, have a dusty composition, and never survive ablation in Earth's atmosphere.

Since 1885 the Andromedids have been quite undistinguished, and the shower is now to all intents and purposes defunct. Planetary perturbations have shifted the orbit of the meteoroid swarm so that it does not at present meet that of the Earth. A fairly h6 display occurred on 1892 November 23, while on 1899 November 24 about 200 Andromedids per hour were seen. W.F. denning recorded 20 Andromedids per hour in 1904, and visual observations in 1940 yielded rates of about 30 per hour on November 15. A few individual shower members were caught by the Harvard Super-Schmidt meteor cameras in the USA in 1952 and 1953. A computer model of the Andromedid stream by David Hughes (1941- ), of the University of Sheffield, UK, suggests that further planetary gravitational perturbations will bring the shower back to encounter the Earth around 2120.

Angara New fleet of Russian satellite launch vehicles to be operational in about 2003 to replace the proton launch vehicle. The Angara will be based on a first stage, which forms the basic vehicle for flights to low Earth orbit. This stage can be clustered together with two types of upper stage, forming a more powerful first stage, for flights to geostationary transfer orbit (GTO). The largest Angara, with five core first stages and a high-energy KVRD upper stage, will be able to place 6.8 tonnes into GTO.

Anglo-Australian Observatory (AAO) Australia's principal government-sponsored organization for optical astronomy, funded jointly by Australia and the UK. The AAO operates the 3.9-m (153-in.) anglo-australian telescope (AAT) and the 1.2-m (48-in.) united kingdom schmidt telescope (UKST) at siding spring observatory near Coonabarabran in New South Wales. A laboratory in the Sydney suburb of Epping, 350 km (200 mi) from the telescopes, houses the AAO's scientific, technical and administrative staff; while the operations staff are based at Coonabarabran. Of increasing importance within the AAO is its External Projects Division, which contracts to build large-scale instrumentation for, among others, the gemini telescopes, the subaru telescope and the very large telescope.

Anglo-Australian Telescope (AAT) Optical telescope funded jointly by the British and Australian governments, located at siding spring observatory in New South Wales. It is operated by the anglo-australian observatory. Inaugurated in 1974, the 3.9-m (153-in.) AAT remains the largest optical telescope in Australia, although both partner countries now have access to larger southern-hemisphere instruments. Its IRIS2 infrared imager and 2dF (two-degree field) system, which allows the spectra of 400 target objects to be obtained simultaneously, provide unique facilities for wide-field observations.

angrite Subgroup of the achondrite meteorites. Angrites are medium- to coarse-grained basaltic igneous rocks. Although the angrites have similar oxygen isotopic compositions to the howardite-eucrite-diogenite association (HEDs) meteorites, they are unrelated.

Angstrom, Anders Jonas (1814-74) Swedish physicist who helped prepare the ground for the application of spectroscopy in astronomy. He used diffraction gratings to make high-precision measurements of the Sun's spectral lines, and in 1868 he published an atlas of the solar spectrum. It contained measurements of over a thousand lines, expressed in units of 10-7 mm, a quantity later named the angstrom unit in his honour.

angstrom unit (symbol A) Unit of length, formerly used to express the wavelength of light, particularly in spectroscopy; it is equal to 10~10 m. It is named after Anders Jonas angstrom, who first used it in his atlas of the solar spectrum in 1868. The angstrom has now been replaced by the SI measurement the nanometre (nm).

angular measure Measure of the apparent diameter of a celestial object, or the distance between two objects, expressed as an angle, usually in degrees, arcminutes or arcseconds. The angle subtended by an object is determined by its true diameter and its distance from the observer; if the distance to an object is known, its true diameter may be calculated by measuring its apparent diameter.

angular momentum Property of rotating or orbiting bodies that is the rotational equivalent of the momentum of an object moving in a straight line. It is the product of the angular velocity and the moment of inertia (I). The moment of inertia is the rotational equivalent of mass, and for a small particle it is given by the mass of that particle multiplied by the square of its distance from the rotational axis. For large objects the overall moment of inertia must be found by adding together the individual moments of inertia of its constituent particles. A planet orbiting the Sun may be regarded as a small particle and so has a moment of inertia of I = Mp.Ro2, where Mp is the mass of the planet and Ro the distance of the planet from the Sun. A spherical, uniform, rotating planet will have I =0.4 MpRp2 where Rp is the radius of the planet. Angular momentum is always conserved (that is, its total value for the system remains constant) during any changes.

T annular eclipse The bright photosphere of the Sun can be seen surrounding the Moon during an annular eclipse. This eclipse was photographed on 1994 May 10 from Mexico.

The rotational kinetic energy is given by 7co2/2, where co is the angular velocity (compare this with the kinetic energy of an object moving in a straight line: M^2/2).

For a planet in an elliptical orbit, Ro is smaller at perihelion than at aphelion; conservation of angular momentum therefore means that the orbital angular velocity, coo, must decrease from perihelion to aphelion in order that the product, Rocoo, remains constant. Kepler's second law of planetary motion is thus the result of conservation of angular momentum. Similarly conservation of angular momentum results in accretion disks forming in close binary stars where mass is being exchanged, and so is linked to many types of variable star including novae and Type I supernovae.

Ann Arbor Observatory (University of Michigan Detroit Observatory) Historic institution in Ann Arbor, 58 km (36 mi) west of Detroit. It was founded (c.1854) as part of the university's bid for pre-eminence in scientific teaching and research. It is equipped with a 12i-inch (320-mm) refractor dating from 1857. Today, the observatory is used largely for public outreach, having been restored in 1999 as a centre for 19th-century science, technology and culture.

annual parallax (heliocentric parallax) Difference in the apparent position of a star that would be measured by hypothetical observations made from the centre of the Earth and the centre of the Sun.

Due to the Earth's movement in its orbit around the Sun, nearby stars appear to shift their position relative to more distant, background stars, over the course of a year, describing a path known as the parallactic ellipse. When observed six months apart from opposite sides of the Earth's orbit, the position of a nearby star will appear to have shifted by an angle A0. Half of this angle, it, gives the annual parallax, which is equal to the angle subtended at the observed star by the semimajor axis of the Earth's orbit. The reciprocal of annual parallax in arcseconds gives the distance to the star in parsecs. See also diurnal parallax; trigonometric parallax

annular eclipse Special instance of a solar eclipse, occurring when the Moon is close to apogee, such that the Sun has a significantly greater apparent diameter than the Moon. As a result, the Moon's central passage across the solar disk leaves a ring - or annulus - of bright sunlight visible at mid-eclipse. Such events, while interesting in their own right, are not generally considered as dramatic as a total solar eclipse, since they do not allow the solar corona and prominences to become visible, or cause a noticeable darkening even at their maximum extent. Under favourable circumstances, the annular phase, with the dark body of the Moon silhouetted against the Sun's brilliant photosphere, can last up to 12m 30s.

anomalistic month Time taken for the Moon to complete a single orbit around the Earth, measured from perigee to perigee. An anomalistic month is shorter than the more commonly used synodic month, being equivalent to 27.55455 days of mean solar time. See also draconic month; month; sidereal month; tropical month anomalistic year Period of a single orbit of the Earth around the Sun, measured from perihelion to perihelion. Equivalent to 365.25964 days of mean solar time, an anomalistic year is about 4m 43s.5 longer than a sidereal year because of the gradual eastward movement of the point of perihelion. See also tropical year .

anomaly Angular measurement used for describing the position of a body in an elliptical orbit, measured around the orbit in the direction of motion from the pericentre. It can be defined as a true, eccentric or mean anomaly. In the diagram the point X represents the position of the body, and the angle PSX is the true anomaly. The mean anomaly is measured similarly, but to the position of an imaginary body that orbits at constant angular speed with the same period as the real body. It cannot be indicated by a simple geometrical construction. The eccentric anomaly is the angle PCX', where X' is the position of the body projected on to a circumscribing circle. The eccentric and mean anomalies are intermediate angles used in the calculation of the position of the object in its orbit at any time. The difference between the true and mean anomaly is the equation of the centre. See also ellipse.

anomaly The position of a body (X) in an elliptical orbit can be described in terms of its angular distance in the direction of motion from the pericentre. The true anomaly is described by the angle PSX in the diagram here. The eccentric anomaly, PCX’, is given by projecting the body’s position on to a circumscribing circle, and can be used as a matter of convenience in orbital calculations.

anorthosite Type of basaltic rock found in the lunar highland crust. Highland basalts are richer in aluminium and calcium, and poorer in iron, magnesium and titanium, than basalts found in the lunar maria (low-lying plains).

ansae Term applied by early telescopic observers to define the opposite extremities of Saturn's ring system, which, when viewed foreshortened from Earth, resemble handles to the planet.

antapex Point on the celestial sphere from which the Sun and the entire Solar System appears to be moving away, at a velocity of around 19-20 km/s (c.12 mi/s), relative to nearby stars. The antapex lies in the direction of the constellation Columba at around RA 6h dec. —30°. It is diametrically opposite on the celestial sphere to the apex, the direction towards which the Sun and Solar System appear to be moving.

Antarctic astronomy Collective term for the astronomical activities conducted in the special conditions of the Earth's south polar continent. The Amundsen-Scott South Pole Station sits atop a 2800-m (9200-ft) cap of ice - the coldest, driest place on Earth. This venue is ideal for astronomers who want to work at infrared, submillimetre and millimetre wavelengths. These portions of the electromagnetic spectrum are compromised by water vapour, which is pervasive at most other locations on Earth and makes observations at these long wavelengths difficult or impossible. But at the pole, the vapour is frozen out, leading to a dark, transparent sky ideal for investigating star-formation processes in molecular clouds and the evolution of protostars and other young objects. Similarly, distant, primeval galaxies can be effectively studied because their visible light has been redshifted to long wavelengths. The pole is also a premier site for assaying variations in the so-called cosmic microwave background (CMB), subtle differences that reflect the large-scale distribution of matter and energy in the very early Universe.

Antarctic astronomy A plot of a neutrino ‘event’ at the AMANDA-B detectors. The impacting neutrino decayed to a muon (the turquoise line is the latter’s projected track), which then decayed into other subatomic particles. The purple circles indicate which detectors were hit by radiation and the number beside each shows the time elapsed in nanoseconds from the first detector being hit.

In addition, of course, a polar site features a totally dark sky 24 hours a day during midwinter, which allows continual monitoring of targets. And during midsummer the Sun can also be watched continuously, which was crucial in the 1980s, during the early days of helioseismology, when we got the first glimpse of the interior of our star. Antarctic astronomy was born around 1980, spurred by Martin Pomerantz (1916- ) of the Bartol Research Institute. The principal organization is the Center for Astrophysical Research in Antarctica. Among other facilities, it supports research with the Antarctic Submil-limeter Telescope/Remote Observatory (AST/RO), a 1.7-m (67-in.) instrument that focuses on atomic and molecular gas in our Galaxy and others nearby, and the 13-element Degree Angular Scale Interferometer (DASI) for studies of the CMB.

One 'telescope' at the south pole looks down rather than up - the Antarctic Muon and Neutrino Detector Array (AMANDA). Its mission is to count ultra-high-energy neutrinos from our Galaxy and beyond that have passed through the Earth, which screens out 'noise'. AMANDA detects the light that is emitted when a neutrino interacts with an atom in the Antarctic icecap. Sources of extremely high-energy neutrinos include active galactic nuclei, black holes, supernovae remnants and neutron stars.

There is much more to Antarctic astronomy than what happens at the pole. This continent provides one of our best 'laboratories' to prepare for the search for exotic life forms elsewhere in the Solar System. On the polar plateau in East Antarctica lies Lake Vostok. The size of Lake Ontario in North America, it has been buried under thousands of metres of ice for millions of years. Although unexplored, Lake Vostok might hold extremophiles that could help us determine how organisms might survive in the putative ocean under the ice mantle of Jupiter's satellite Europa.

Off the plateau and within a helicopter flight of McMur-do Station, the 'capital' of Antarctica, are the Dry Valleys, frozen landscapes that, though more benign, nevertheless mimic those of Mars. Yet life goes on in these valleys, sometimes hidden inside rocks that protect cyanobacteria from desiccation, thermal extremes and overexposure to ultraviolet radiation. Perhaps this strategy also operates, or once did, on Mars and elsewhere in the Solar System.

Antarctica is also conducive for launching experiments aboard long-duration balloons that ascend 40 km (25 mi) into the upper stratosphere. The prevailing westerly winds carry such balloons around the continent and return them to near the launch site at McMurdo Station in about 10 days. Gamma-rays, X-rays and CMB radiation have all been sampled during such balloon flights.

One of the most surprising discoveries in Antarctica took place in 1969, when nine meteorites were found near the Yamato Mountains. Since then concerted searches have uncovered some 20,000 rocks from space - more than half of all known meteoritic specimens. Antarctica is not favoured by falling meteorites. The abundance occurs because when meteorites land they become embedded in flowing ice - as if on a conveyor belt - and carried to sites where they become exposed by wind erosion. A few have been identified as pieces of the Moon and Mars. An Antarctic meteorite known as ALH 84001 became famous in 1996 when scientists announced that it contains fossil evidence for life on Mars. See also balloon astronomy; life in the universe

Antares The star a Scorpii, marking the heart of Scorpius, the scorpion, distance 604 l.y. An irregular variable, it fluctuates between visual mags. 0.9 and 1.2 and is the brightest member of the Scorpius-Centaurus Association, the nearest ob association. Antares is a red supergiant of spectral type M1 Ib, about 400 times the Sun's diameter and more than 10,000 times as luminous. It has a much smaller and hotter companion, mag. 5.4 and spectral type B2.5 V, which orbits it in about 900 years; this companion can be seen through telescopes with apertures of 75 mm (3 in.) or more. The name Antares reflects its pronounced red colour: it comes from a Greek expression that can be translated either as 'like Mars' or 'rival of Mars'.

antenna Metal wire, rod, dish or other structure used to transmit and receive radio waves, whereas a normal radio telescope only receives radio waves. radar astronomy uses antennae (or aerials) to make either continuous wave or pulsed transmissions to bodies in the Solar System, either from Earth or from space satellites. Amateur astronomers can study meteors by using low-power transmitters in the 10-20 MHz band and observing the reflections off the ionized trails made by the meteors in the upper atmosphere. Other work requires powerful transmitters and high-gain antenna systems. Transmissions have been made to the Moon, planets and comets with large radio telescopes receiving the reflections, for example jodrell bank and arecibo. Arecibo has acted as an antenna transmitting signals for Project Ozma. The Haystack antenna (of Lincoln Laboratory) was used to test the theory of general relativity in 1967. For a radar pulse passing near the Sun, and reflected from a planet, there should be a small time delay of 2 X 10-4 s, equivalent to 60 km (40 mi) difference in the distance of the planet. Mars, Mercury and Venus were used by Haystack, and Arecibo used Mercury and Venus.

Antennae (NGC 4038 & 4039) Pair ofinteracting galaxies - a spiral and a lenticular - in the constellation Corvus (RA 12h 01m.9 dec. -18°52'). Each has an apparent mag. +10.5 and maximum diameter 5'. Two long curved tails, comprising stars and gas ejected by the collision, extend away for an apparent distance of 20'. The pair lie at a distance of 60 million l.y. They are sometimes known as the Ring-Tail Galaxy.

anthropic principle Idea that the existence of the Universe is intimately related to the presence of life. The principle exists in two distinct forms, known as the weak and h6 versions.

The weak anthropic principle (WAP) arises from the notion that any observations made by astronomers will be biased by selection effects that arise from their own existence. Characteristics of the Universe that appear to be quite improbable may merely arise from the fact that certain properties are necessary for life to exist. Cosmologist John D. Barrow (1952- ) has given this definition of the WAP: the observed values of all physical and cosmological quantities are not equally probable, but take on values restricted by the requirements that (1) there exist sites where carbon-based life can evolve and (2) the Universe be old enough for it to have already done so.

The h6 anthropic principle (SAP) goes further, stating that the Universe must have fundamental properties that allow life to develop within it at some stage in its history. This implies that the constants and laws of nature must be such that life can exist. A number of quite distinct interpretations of the SAP are possible, including the suggestion that there exists only one possible Universe 'designed' with the goal of generating and sustaining 'observers' - life. The American mathematician John Archibald Wheeler (1911- ) has pointed out that this argument can be interpreted as implying that observers are necessary to bring the Universe into being, an idea he calls the 'participatory anthropic principle' (PAP). A third possible interpretation of the SAP is that our Universe is just one of an ensemble of many different universes, and that by chance its properties are optimized for the existence of life. This idea is consistent with the 'many-worlds' or 'sum-over-histories' approach of quantum cosmology (see QUANTUM GRAVITY), which requires the existence of many possible real 'other universes'.

antimatter Matter consisting of particles with opposite quantum numbers to normal particles. The marriage of the QUANTUM THEORY with SPECIAL RELATIVITY led Paul Dirac (1902-84) in 1929 to propose the existence of electrons with positive charge and spin opposite that of normal electrons. This anti-electron, commonly known as a positron, was the first hint that antimatter existed. The tracks characteristic of positrons were subsequently found in COSMIC RAY cloud chamber experiments by Carl Anderson (1905-91) in 1929, proving the existence of these particles and of antimatter in general. Consequently, quantum theorists now associate every known particle with a complementary antiparticle and most have been seen in high-energy physics experiments. If the particle and its antimatter partner interact, they mutually annihilate and the rest mass is converted into electromagnetic energy via Einstein's formula E = mc2. Antimatter has played important roles in cosmology and extragalactic astronomy. The question of why the Universe consists primarily of ordinary matter and very little antimatter has defied explanation. If entire galaxies of antimatter existed, the effects of these galaxies should be seen. These effects have not been observed.

It was proposed that quasars might actually be galaxies of ordinary matter and antimatter galaxies colliding, but this model quickly gave way to the more common idea of massive black holes in the centres of protogalax-ies. Antimatter is definitely present in astrophysical objects, but not in significant amounts. For instance, pair production is the creation of an electron-positron pair from a gamma ray scattering off a baryon. This is thought to be a prominent process in both pulsars and quasars. The opposite process, pair annihilation, forms a gamma-ray photon, which has energy of 0.511 MeV (million electron volts); this spectral line is seen in the spectra of ACTIVE GALACTIC NUCLEI (AGN).

Antiope MAIN-BELT ASTEROID, number 90, which in 2000 was found to be binary in form. The two components of Antiope are each c.80 km (c.50 mi) in size, and they orbit their mutual centre of gravity with a separation of c.160 km (c.100 mi).

antitail COMET tail that appears to point towards the Sun. Solar radiation pressure drives the dust tails of comets away from the Sun. Under certain circumstances, however, active comets may be observed to have a sunward-pointing 'spike' or fan. Such antitails result from ejection of dust from the comet nucleus in thin sheets, which, when viewed in the plane of the comet's orbit, can appear to point towards the Sun as a result of perspective. Perhaps the best-known example is the antitail sported by Comet AREND-ROLAND in 1957; antitails were also shown by comets KOHOUTEK and HALE-BOPP.

Antlia See feature article

Antoniadi, Eugene Michael (1870-1944) French astronomer, born in Turkey of Greek parents, who became an expert observer of planetary features. As a

ANTLIA (gen. antliae, abbr. ant) mall, faint southern constellation representing an air pump; it lies between Hydra and Vela. Antlia was introduced by Lacaille in the 18th century. Its brightest star, a Ant, is mag. 4.3; Ј Ant is a wide double with components of mags. 5.9 and 6.2. Antlia's brightest deep-sky object is NGC 2997, a 10th-magnitude spiral galaxy. young man, Antoniadi made many fine drawings of sunspots and the planets Mars, Jupiter and Saturn using 3- and 4i-inch (75- and 110-mm) telescopes. This work attracted the attention of the French astronomer and writer Camille FLAMMARION, who had built a fine private observatory at Juvisy, France. From 1894 to 1902 Antoniadi used Juvisy's 9 -inch (240-mm) Bardou refracting telescope to make detailed studies of the planets, especially Mars. From 1896 to 1917 he directed the British Astronomical Association's Mars Section, publishing ten Memoirs that collected and discussed the observations of the section's members; many of the observations were by Antoniadi himself, made with the Juvisy telescope and the great 33-inch (0.83-m) refractor at Meudon, France.

At a time when the 'canals' supposedly seen on Mars by Giovanni SCHIAPARELLI and Percival LOWELL were generally accepted as real features by many astronomers, Anto-niadi interpreted these linear markings as optical illusions produced by the tendency of human vision to transform disconnected features into continuous lines. His observations of Mars during the 1909 opposition convinced him that the canals were natural features similar to Earth's valleys; two years later he correctly described the clouds obscuring the planet's surface as due to windblown sand and dust from the Martian deserts. In 1924 he made one of the first sightings of the Tharsis volcanoes. Antoniadi summarized both his own and historical observations of Mars in the classic book La Planete Mars (1930). Four years later he wrote another classic work, La Planete Mer-cure, which remained a standard for decades. Antoniadi was also an expert on ancient languages, a skill he used to research and write LAstronomie Egyptienne (1940).

Antoniadi scale Standard scale of SEEING conditions devised in the early 1900s by Eugene ANTONIADI to describe the conditions under which lunar and planetary observations are made. The five gradations on this scale are: (I) perfect seeing, without a quiver; (II) slight undulations, with periods of calm lasting several seconds; (III) moderate seeing, with larger tremors; (IV) poor seeing, with constant troublesome undulations; and (V) very bad seeing, scarcely allowing a rough sketch to be made.

Apache Point Observatory (APO) Major optical/infrared observatory owned and operated by the Astrophysical Research Consortium (ARC) of several prominent US universities. Located at an elevation of 2790 m (9150 ft) in the Sacramento Mountains of New Mexico approximately 225 km (140 mi) south-east of Albuquerque, this world-class observatory site is managed by the New Mexico State University on the ARC's behalf. Its principal instrument is a 3.5-m (138-in.) reflector, a general-purpose telescope with a lightweight spun-cast mirror. The APO is also home to the 2.5-m (98-in.) telescope of the SLOAN DIGITAL SKY SURVEY.

ap-, apo- Prefixes referring to the farthest point of an orbit from the primary body, as in APASTRON, APHELION, APOGEE, apocentre, apoapse. See also APSIDES

apastron Point in an orbit around a star that is farthest from that star. The term is usually used to describe the positions of the two components in a binary star system, moving around their common centre of mass in an elliptical orbit, when they are farthest away from one another. See also APSIDES

Apennines, Montes (Apennine Mountains) Most impressive lunar mountain range, 1400 km (870 mi) long; it rises to 4 km (2.5 mi) above the lava-filled southeast 'shores' of Mare IMBRIUM. The Apennines' highest peak is Mount Huygens (at 5600 m/18,400 ft), followed by Mount Bradley (at 5000 m/16,400 ft),

Mount Hadley (4500 m/14,800 ft) and Mount Wolff (3500 m/11,500 ft). The numerous valleys and gorges that cut through the range are probably shock fractures from the impact that created Mare Imbrium.

aperture Clear diameter of the primary lens or mirror of a telescope. The aperture is the single most important factor in determining both the resolving power and light-gathering ability of the instrument.

aperture synthesis array of radio telescopes that work together to observe the same astronomical object and to produce an image with higher resolution than the telescopes individually could achieve. Resolution approaching that of Earth-bound optical telescopes -say, one arcsecond - would require a single radio dish several kilometres in diameter. To construct such a radio telescope, and make it steerable to observe anywhere in the sky, has been impossible in the past, so aperture synthesis has been a most valuable tool for mapping in detail small-scale structure. The individual radio telescopes play the role of different parts of the surface of a hypothetical 'super-telescope', which is synthesized by correctly combining their outputs. The technique is further enhanced by using Earth's rotation to fill in missing parts of the super-telescope via the movement of the telescopes relative to the astronomical target. This technique of using Earth's rotation was developed by Martin ryle. Usually some parts of the super-telescope surface are missing, that is, it is not 'fully filled', causing defects in the map such as bands or rings of spurious emission. The problem can be overcome to some extent either by complementing the map with an observation made by a single large radio telescope, or by computing techniques such as maximum-entropy methods which clean up the defects. The Cambridge One-mile telescope and the very large array are examples of aperture synthesis telescopes.

apex (solar apex) Point on the celestial sphere towards which the Sun and the entire Solar System appear to be moving, at a velocity of around 19-20 km/s (c.12 mi/s) relative to nearby stars. It lies in the direction of the constellation Hercules at around RA 18h dec. +30°. It is diametrically opposite on the celestial sphere to the antapex, the direction from which the Sun and Solar System appear to be moving.

The apex is also the point on the celestial sphere towards which the Earth appears to be moving at any given time, due to its orbital motion around the Sun.

aphelion Point at which a Solar System body, such as a planet, asteroid or comet, moving in an elliptical orbit, is at its greatest distance from the Sun. For the Earth, this occurs around July 4, when it lies 152 million km (94 million mi) from the Sun. See also apsides; perihelion.

aphelion Halley’s Comet has a highly eccentric orbit: its aphelion – its greatest distance from the Sun – is 35.295 AU (roughly halfway between the mean orbits of Uranus and Pluto), while its perihelion – its closest approach to the Sun – is just 0.587 AU, well within Venus’ orbit.

Aphrodite Terra Largest and most extensive of the continent-like highland regions on venus. About the size of Africa, Aphrodite Terra exhibits considerable variation in physiography. It extends for 10,000 km (6200 mi) from 45° to 210°E in a roughly east-west direction along and south of Venus' equator. The western and central mountainous regions, known respectively as Ovda Regio and Thetis Regio, rise to between 3 and 4 km (1.9-2.5 mi) above the mean planetary radius. East of the central massif is a complex regional fracture system with east-north-east-trending ridges and linear troughs, the most prominent of which are Dali and Diana Chasmata, 2077 km (1291 mi) and 938 km (583 mi) in length respectively. Each trough or rift valley is at least 1 km (0.6 mi) deep. Diana Chasma may be tectonic in origin. To the south is the semicircular Artemis Chasma, thought to be a massive corona-type structure (a concentric arrangement of ridges and grooves). In the east the broad upland dome of Atla Regio, with its fissured surface and volcanic centres, is somewhat higher than the western and central mountainous districts, but is itself overshadowed by the imposing volcanic peaks Ozza Mons (6 km/4 mi high and 500 km/310 mi across) and Maat Mons (9 km/6 mi high and 395 km/245 mi across). The absence of craters suggests the whole region is of comparatively recent origin.

Aphrodite Terra The surface of this upland region of Venus is extremely varied, containing deep troughs and high ridges as well as volcanoes. The area to the left of the ridge shown in this Magellan radar image was first interpreted as a landslide but is now thought to be simply an effect of the image-processing methods used.

apochromat Composite lens designed to be free of chromatic aberration. Modern materials and design techniques make it possible to produce lenses that introduce no discernible false colour into the image. Apochromatic refractors used in astronomy usually have an objective made of three separate pieces of glass, each of different materials with different characteristics. Some or all of the materials may be highly specialized and expensive, but the overall effect is to produce some of the best optical systems that are currently available to amateur astronomers.

Telescopes incorporating apochromatic objectives are sometimes referred to as 'fluorites' or 'ED' telescopes, the names being derived from the materials used.

Apochromats may also be incorporated into eyepieces and barlow lenses. The smaller sized lenses make these more common and affordable than apochromatic objectives. They are particularly useful with reflecting telescopes that are inherently free of chromatic aberration. See also achromat

apogee Point in the orbit of the Moon or an artificial satellite that is farthest from the Earth. See also apsides.

Apollo asteroid Any member of the class of asteroids with orbits that cross that of the Earth (requiring a perihelion distance less than 1.0167 AU and an aphelion distance greater than 0.9833 AU), and with orbital periods longer than one year. The archetype, (1862) Apollo, was discovered in 1932 as the first Earth-crossing asteroid. By late 2001 more than 740 Apollo asteroids of varying sizes had been discovered, mostly since 1990. Because these asteroids collide with the Earth or other terrestrial planets on time scales of order ten million years, or may be lost through other dynamical paths, they cannot have been in place since the formation of the Solar System. They must, therefore, be delivered to their present orbits on a continual basis. At one time it was thought that they might mostly be extinct cometary nuclei, but it has been shown that they may also be derived from main-belt asteroids pumped by Jupiter's gravity out of the kirkwood gaps and into high-eccentricity orbits in the inner Solar System.

Some characteristics of the following noteworthy Apollo asteroids are shown in the near-earth asteroid table: (1566) Icarus, (1620) Geographos, (1685) Toro, (1862) Apollo, (1863) Antinous, (1866) Sisyphus, (2063) Bacchus, (2101) Adonis, (2102) Tantalus, (2201) Oljato, (2212) Hephaistos, (3200) Phaethon, (4015) Wilson-Harrington, (4179) Toutatis, (4183) Cuno, (4581) Asclepius, (4660) Nereus, (4789) Castalia, (6489) Golevka and 1937 UB Hermes. See also aten asteroid; amor asteroid

Apollonius of Perga (262-190 bc) Greek mathematician and astronomer, known as the Great Geometer. His most famous published work was the eight-volume Conics, which deals with the geometry of conic sections - the circle, ellipse, parabola and hyperbola, all of which are important in describing the motions of celestial bodies. Although Apollonius' theory of planetary motions was based upon the erroneous geometrical model of epicycles and eccentrics promoted by ptolemy, he correctly observed and described the retrograde motion of Mars and other planets.

Apollo programme Name given to the US programme to land men on the Moon and return them safely to Earth. The cost of the national aeronautic and space administration's (NASA's) triumph in the Moon race with the Soviet Union was US$25 billion. At its peak, the programme employed some 500,000 people.

Apollo programme The Saturn V rocket, carrying the Apollo 11, is launched. The first manned mission to the Moon, it was launched on 1969 July 16.

The first public announcement of the intention of the USA to achieve the first manned landing on another world was made by President Kennedy in 1961. However, NASA was already preparing three series of robotic missions, namely ranger, surveyor and lunar orbiter, to investigate the feasibility of manned missions and to search for suitable landing sites.

The Apollo spacecraft consisted of four units in two pairs: the Command and Service Module (CSM), together with the descent and ascent stages of the lunar module (LM). The Command Module (CM) housed the astronauts during the journeys to the Moon and back. It remained attached to the Service Module (SM), which contained the rocket engines, fuel and electrical supply, until shortly before re-entering the Earth's atmosphere.

Apollo programme The Lunar Roving Vehicles allowed the astronauts on the final three Apollo lunar missions to range farther from the landers and so collect more varied samples from the Moon’s surface.

After achieving lunar orbit, the LM, containing two astronauts, undocked from the CSM, decelerated, and landed at the pre-selected location on the Moon's surface. The third astronaut remained orbiting the Moon in the CSM. At the completion of extra-vehicular activities (EVAs) on the surface, the descent stage served as a launch platform for the ascent stage, which then redocked with the orbiting CSM. Following the transfer of the two astronauts, film cassettes and Moon rocks from the ascent stage to the CM, the former was jettisoned. It was usually targeted to impact the lunar surface, thereby providing a seismic signal of known energy to calibrate seismometers deployed on the surface.

Apollo 1 should have been the first Earth orbital test flight of the CM, but a fire in the CM during ground testing on 1967 January 27 killed astronauts Roger Chaffee (b.1935), Virgil Grissom (b.1926) and Edward White (b.1930) and prompted numerous design changes. This was followed by Apollo 4, 5 and 6, all unmanned flights to test the saturn rocket.

The manned flights with three astronauts aboard - the Commander (CDR), the Lunar Module Pilot (LMP) and the Command Module Pilot (CMP) - began with Apollo 7. Originally 14 manned Apollo flights were planned but funding cuts later reduced this to 11. As confidence was gained, each mission gathered more scientific data than its predecessors. As durations of EVAs and distances covered increased, more comprehensive selections of rock and soil samples were obtained. More experiments were also deployed or conducted on the lunar surface, and the later orbiting CSMs obtained hundreds of high-resolution stereophotographs and other measurements of the surface.

Within the constraints dictated by spacecraft design and landing safety, the landing sites were chosen to give a reasonably representative selection of different surface types, as determined from earlier ground-based or spacecraft studies. In the case of Apollo 11, a near-equatorial site provided the simplest landing and redocking conditions, with a relatively smooth and level descent path and landing point. A location in Mare Tranquillitatis was chosen, not too far from the impact point of Ranger 8 and the landing point of Surveyor 5. Neil armh6 and 'Buzz' aldrin became the first men to step on the Moon. Apollo 12 was again targeted for an equatorial site on Oceanus Procellarum, very close to the earlier landing site of Surveyor 3. This enabled the astronauts to inspect and return spacecraft samples that had been exposed to the lunar environment for two and a half years.

The ill-fated Apollo 13 was targeted for Fra Mauro, a near-equatorial site in the lunar highlands that was thought to be covered by ejecta from the impact event that produced the Imbrium Basin. Unfortunately, an oxygen tank in the SM exploded halfway to the Moon, disabling most of the spacecraft systems. Despite many hardships, the astronauts returned safely. Apollo 14 was targeted for the same landing location and was completely successful. The exploration by astronauts Alan shepard and Stuart Roosa (1933- ) was aided by a wheeled trolley known as the Modular Equipment Transporter.

The three remaining missions touched down well away from the equator and had the benefit of a battery-powered roving vehicle that widened the field of exploration. The Apollo 15 landing site combined a mare region (Palus Putredinis), a nearby sinuous valley (Hadley Rille) and very high mountains (the Apennines). Apollo 16 landed in a highlands area, on the so-called Cayley Formation - the loose material that fills in craters and covers the lower slopes here - but very close to the Descartes Formation, which was thought to represent possible highlands volcanism.

The choice of the Taurus-Littrow site for the final mission resulted largely from the Apollo 15 observations of small, dark-haloed craters, identified as possible volcanic vents, in the area. Additionally, the valley floor was one of the darkest mare surfaces on the Moon. Geologist-astronaut Harrison Schmitt (1935- ) was the only scientist to set foot on the Moon.

The quantity of data resulting from the Apollo programme was overwhelming. The 380 kg of rock and soil samples have attracted particular attention, having been subjected to every kind of test and analysis imaginable. However, many of the stored samples have yet to be examined.

Another major archive is the collection of many thousands of photographs taken during the transit and orbital phases of the missions. In particular, the metric and panoramic cameras used on the last three missions produced high-resolution, wide-angle, stereoscopic coverage of about 20% of the lunar surface. Of the 16 or so other experiments conducted from orbit, ten were concerned with 'remote sensing', allowing a comparison with data from the landing sites.

About 25 different types of experiment were carried out or deployed on the surface. These included seismometers, magnetometers and heat-flow experiments to investigate the subsurface structure and properties. Laser reflectors helped to refine the Moon-Earth distance.

apparent magnitude Apparent brightness of a star (a measure of the light received at Earth) measured by the stellar magnitude system. Because stars are at different distances from us and the interstellar medium has a variable absorption, apparent magnitude is not a reliable key to a star's real (intrinsic) luminosity apparent solar time Local time, based on the position of the Sun in the sky, and as shown by a sundial. Apparent noon occurs when the Sun crosses the local meridian at its maximum altitude. The length of a solar day measured this way is not uniform. It varies throughout the year by as much as 16 minutes because of the elliptical orbit of the Earth and the fact that the Sun appears to move across the sky along the ecliptic, rather than the celestial equator. Clock time is therefore based on the uniform movement of a fictitious mean Sun. The difference between apparent and mean solar time is called the equation of time.

apparition Period of time during which it is possible to observe a celestial body that is only visible periodically. The term is usually used to describe a particular appearance of a comet, such as the 1985-86 apparition of Comet halley.

appulse Close approach in apparent position in the sky between two celestial bodies. Unlike an occultation, when one body passes directly in front of another, such as a planet occulting a star, an appulse occurs when their directions of motion on the celestial sphere converge, the two just appearing to touch. The impression is caused by a line-of-sight effect, the two bodies actually lying at greatly differing distances from the observer. See also conjunction

apsides The two points in an elliptical orbit that are nearest to and farthest away from the primary body. The line joining these two points is the line of apsides. For an object orbiting the Sun the nearest apse is termed the perihelion and the farthest apse is the aphelion. For the Moon or an artificial satellite orbiting the Earth the apsides are the perigee and apogee. The components of a binary star system are at periastron

APUS (gen. apodis, abbr. aps) Small, faint southern constellation, representing a bird of paradise, between Triangulum Australis and the south celestial pole. It was introduced by Keyser and de Houtman at the end of the 16th century. The brightest star, a Aps, is mag. 3.8; 8 Aps is a wide double, with red and orange components, mags. 4.7 and 5.3 when they are closest together and at apastron when farthest apart. For objects orbiting, say, the Moon or Jupiter, one sometimes sees terms such as periselenium or perijove. Such terms are rather cumbersome, however, and it has become more usual to use the general terms pericentre and apocentre (or less commonly periapse and apoapse) for all cases where the primary is not the Sun, the Earth or a star, or when discussing elliptic motion in general.

Ap star Class astarwith peculiar chemical composition. Ap stars extend to types Fp and Bp. In these class 2 chemically peculiar stars (CP2), elements such as silicon, chromium, strontium and the rare earths are enhanced (europium by a factor of 1000 or more). The odd compositions derive from diffusion, in which some atoms sink in the quiet atmospheres of slowly rotating stars, whereas others rise. The strong, tilted magnetic fields of Ap stars concentrate element enhancements near the magnetic poles. As the stars rotate, both the fields and spectra vary.

Apus See feature article

Aquarius See feature article

Aquila See feature article

Ara See feature article

Arab astronomy See islamic astronomy

Arago, (Dominique) Frangois (Jean) (1786-1853) French scientist and statesman, director of Paris Observatory from 1830, who pioneered the application of the photometer and polarimeter to astrophysics. He discovered that two beams of light polarized at right angles do not interfere with each other, allowing him to develop the transverse theory of light waves. Arago supervised the construction in 1845 of Paris Observatory's 380-mm (15-in.) refractor, at the time one of the world's largest telescopes.

Aratus of Soli (315-245 bc) Ancient Greek poet (born in what is now modern Turkey) whose Phaenomena elaborated upon the descriptions of the 'classical' constellations given a century before by eudoxus. Aratus' patron, King Antigonus of Macedonia, was so inspired by the Phaenomena that he had the constellation figures described in the epic poem painted on the concave ceilings of the royal palace, thus creating what was probably the earliest celestial atlas.

Arcetri Astrophysical Observatory Observatory of the University of Florence, situated at Arcetri, south of the city. The present observatory was built by Giovanni donati in 1872, replacing a much older one in the city centre. A solar tower was added in 1924. Arcetri is now a major centre of Italian astrophysics, and its scientists use both national (galileo national telescope) and international (mainly european southern observatory) facilities. It has a major role in the development of the large binocular telescope.

archaeoastronomy Study of ancient, essentially prehistoric, astronomical theories and practices through evidence provided by archaeology, the interpretation of ancient artefacts, the written records (if available) and ethnological sources such as tribal legends (sometimes called ethnoastronomy).

Many ancient peoples - for example the Stone Age populations of the European countries and the Eastern Mediterranean - are known to have been fully aware of celestial phenomena, as manifested in the alignments of their megalithic monuments such as Stonehenge, New-grange and the dolmens of north-west France. Measurements of the Egyptian pyramids show that they are orientated within a small fraction of a degree in the north-south and east-west directions, and this must have required considerable practical observational skill (see egyptian astronomy).

Ancient astronomical observations were not limited to the Old World: for example, Meso-American peoples such as the Maya (see native american astronomy) and the Aztec have left codices and other evidence which reveal knowledge of celestial events and the determination of celestial cycles.

In some ancient civilizations, observation of the heliacal rising of Sirius provided a reference point that enabled their luni-solar calendar to be corrected to match the true solar year. This was essential to the civilisation if the calendar was to be used to decide the proper time for planting crops.

What may be called applied historical astronomy makes use of ancient records to derive data of value to modern astronomical research. For example, ancient observations of eclipses from Babylon, China, Europe and the Islamic world can be used to determine the variation in the Earth's rotational period over the past 2700 years. In addition, ancient records of the extremely rare outbursts of supernovae, often recorded as 'guest' or 'new' stars, are

ARA (gen. arae, abbr. ara) invaluable in the study of the development and present-day remnants of these events.

Archimedes Lunar crater (30°N 4°W), 82 km (51 mi) in diameter, the largest in Mare imbrium. Archimedes' terraced walls average only 1200 m (4000 ft) above its almost featureless floor: it is filled almost to the brim by Imbrium's lava flows. A few peaks are as high as 2250 m (7400 ft). Scattered bright streaks crossing the crater's floor are probably part of the Autolycus ray system. Archimedes' inner walls are steeper than its outer ramparts, which are about 11 km (7 mi) wide; these ramparts are highlighted by bright ejecta. To the south and south-east of Archimedes lies a group of rilles; these rilles are probably faults caused by the cumulative weight of Imbrium's massive lava flows. North of Archimedes are a very bright group of mountains and a sinuous wrinkle ridge. The Spitzbergen Mountains lie to the north-west.

archive Collection of records. The archives resulting from the space instruments like the Hubble Space Telescope or IUE are organized as large databases: they can be queried to get the observations matching various criteria, and the observed images can frequently be downloaded on the Internet. The large ground-based observatories are also installing similar large archives.

arcminute, arcsecond Small units of angular measure. An arcminute (symbol ') is 1/60 of a degree and an arcsecond (symbol ") 1/60 of an arcminute, or 1/360 of a degree. The units are widely used in astronomy, particularly as a measure of angular separation or diameter of celestial bodies. The resolving power of a telescope is also usually expressed in arcseconds.

Arcturus The star a Bootis, visual mag. —0.05 (but variable by a few hundredths of a magnitude), distance 37 l.y. Arcturus, the fourth-brightest star, is an orange-coloured giant, spectral type K2 III, estimated to be 25 times the Sun's diameter and 100 times as luminous. Its name comes from the Greek and means 'bear watcher' or 'bear guardian', from its proximity to Ursa Major, the great bear.

Arecibo Observatory Site of the world's largest single-dish radio telescope, and one of the most important facilities for radio astronomy, planetary radar and terrestrial atmospheric studies. The observatory is located in the Guarionex Mountains of north-western Puerto Rico about 12 km (8 mi) south of the city of Arecibo. A natural depression provided a bowl in which the 300-m (1000-ft) spherical reflector was built by Cornell University, beginning in 1960. The telescope was dedicated in 1963 November as the Arecibo Ionospheric Observatory, with radar studies of the Earth's atmosphere and the planets as its primary mission. In 1971, with radio astronomy investigations increasing, it became part of the National Astronomy and Ionospheric Centre (NAIC) operated by Cornell, and in 1974 the mirror's surface was upgraded to allow centimetre-wavelength observations to be made. This event was celebrated by the transmission of the coded Arecibo message towards the globular cluster M13, some 25,000 l.y. away, containing information about terrestrial civilization. The telescope was further upgraded in 1997.

Arecibo Observatory As well as its more famous role in the SETI project, the radio telescope is a powerful radar transmitter and receiver, and is used to map small bodies such as asteroids and to examine the Earth’s upper atmosphere.

The fixed, upward-pointing dish is 51 m (167 ft) deep, covers 8 hectares (20 acres) and is surfaced with almost 40,000 perforated aluminium panels supported by a network of steel cables underneath. Suspended 137 m (450 ft) above the dish is a 915-tonne platform in which the telescope's highly sensitive radio receivers are located. They can be moved along a rotating bow-shaped azimuth arm 100 m (330 ft) long to allow the telescope to point up to 20° from the vertical. The near-equatorial location of Arecibo (latitude 18°N) means that all the planets are accessible, together with many important galactic and extragalactic objects.

Small, rather inconspicuous southern constellation representing an altar (possibly that upon which the gods swore allegiance before their battle against the Titans in Greek mythology), between Scorpius and Apus. Its brightest stars, a Ara and p Ara, are both mag. 2.8. The constellation's deep-sky objects include NGC 6193, a 5th-magnitude open cluster, and NGC 6397, one of the closest globular clusters, just visible to the naked eye

Arend-Roland, Comet (C/1956 R1) Comet discovered by Silvain Arend (1902-92) and Georges Roland (1922-91), Uccle Observatory, Belgium, on photographic plates obtained on 1956 November 8. Comet Arend-Roland reached perihelion, 0.32 AU from the Sun, on 1957 April 8. After perihelion, the comet came closest to Earth (0.57 AU) on April 21, becoming a prominent naked-eye object in northern hemisphere skies in the latter parts of the month. Peak brightness approached mag. — 1.0, and the tail reached a length of 25-30°. A prominent, spiked antitail 15° long developed in late April to early May. The comet's orbit is hyperbolic.

areo- Prefix pertaining to the planet mars, such as in areography, the mapping of Martian surface features.

Ares Vallis Valley on mars (9°.7N 23°.4W). mars pathfinder landed here in 1997, not far from the viking 1 lander site chryse planitia. Ares Vallis exhibits the characteristics of an ancient flood-plain.

Argelander, Friedrich Wilhelm August (1799-1875) German astronomer, born in Memel (in modern Lithuania), noted for his compilation of fundamental star catalogues and atlases. From 1817 to 1822 Argelander worked at the observatory of the University of Konigsberg, under its director Friedrich Wilhelm bessel. There he revised the positions of 2848 bright stars catalogued by John flamsteed, amongst the first compiled after the invention of the telescope. In 1823 Argelander became director of the Finnish observatory at Turku and, later, Helsinki, where he remained until his appointment as director of the Bonn Observatory in 1836. In 1843 he published Uranometria nova, a catalogue of positions and magnitudes for stars visible to the naked eye; for this project, Argelander invented his 'stepwise method' of estimating a star's visual magnitude by comparing it to the relative brightness of neighbouring stars.

Argelander's greatest cartographic achievement was the bonner durchmusterung (BD). Employing Bonn Observatory's 75-mm (3-in.) Fraunhofer 'comet seeker' refractor, which they equipped with a micrometer eyepiece, from 1852 to 1863 he and his assistants Eduard Schonfeld (1828-91) and Adalbert Kriiger (1832-96) mapped 324,198 stars to the 9th magnitude between the north celestial pole and —2° declination. In 1886 Schonfeld extended the survey to include another 133,659 stars to the declination zone of —23°, resulting in the Southern Bonner Durchmusterung. In 1863 Argelander and Wilhelm Forster (1832-1921) co-founded the Astronomische Gesellschaft (Astronomical Society). This organization published the first 'AGK' catalogue of 200,000 fundamental stars in 1887.

Argelander step method Visual method of estimating the magnitude of a VARIABLE STAR, based upon assessing the ease with which the variable may be distinguished from individual comparison stars. The arbitrary steps thus derived (known as 'grades') may subsequently be used to obtain the variable's magnitude. The method has the advantage of being relatively objective, in that the magnitudes of any comparisons are unknown at the time of making the estimate.

Argo Navis Former southern constellation, one of Ptolemy's original 48, representing the ship of the Argonauts. Argo Navis was huge, covering a quarter as much sky again as the largest present-day constellation, Hydra. In the 18th century, Lacaille divided it into the new constellations CARINA, the ship's keel, PUPPIS, the deck, and VELA, the sails.

Angle measured along the orbital plane of a planet from the ASCENDING NODE to the PERIHELION. See also ORBITAL ELEMENTS

Argyre Planitia Classical circular feature on MARS (50°.0S 44°.0W). The Mariner spacecraft identified it as a basin 900 km (560 mi) in diameter, with a featureless floor covered by dust deposits. The floor is a light ochre colour when it is not covered by ice-fogs or frosts. The basin was formed by a meteoroid impact which created its 3-km-high (1.9-mi-high) mountainous rims. It is encircled by Nereidum Montes, on the north, and Charitum Montes, on the south.

Ariane Fleet of European satellite launchers developed by the EUROPEAN SPACE AGENCY and operated by Arianespace, a commercial satellite launching company. Arianespace became operational in 1981, two years after the maiden flight of Ariane 1. Further Ariane 2 and 3 models were developed and the present fleet consists of six types of Ariane 4 and an initial version of Ariane 5, four more models of which are under development. The Ariane 4 fleet will be retired in 2002-2003, leaving the Ariane 5 models in service, complemented by a fleet of smaller Vega boosters and Eurockot and Soyuz vehicles, operated by associated companies, for launches of lighter payloads into low Earth and other orbits. The major market for the Ariane 4 and 5 fleets is geostationary transfer orbit (GTO), the staging post for flights to equatorial geostationary orbit, using spacecraft on-board propulsion units. Launches take place from Kourou, French Guiana, which being located close the equator provides cost- and fuel-effective launches.

Communications satellites weighing up to 6 tonnes are being launched to GTO. On some flights, two smaller satellites can be launched together, reducing the launch cost for the customer. An Ariane launch costs in the region of US$100 million. The six Ariane 4 models provide a capability of placing 2.1-4.4 tonnes into GTO, with one model with no strap-on boosters and five with two to four solid or liquid boosters or combinations of these. Ari-ane 5 can place 6 tonnes into GTO. Four new Ariane 5 models, the 5E/S, 5E/SV, 5/ECA and 5/ECB, will improve GTO performance to between 7.1 and 12 tonnes.

Ariel Icy SATELLITE of URANUS, discovered in 1851 by William LASSELL. Ariel has a highly fractured surface, in marked contrast to its similar-size neighbour, UMBRIEL. Most fractures are paired to produce valleys with down-dropped floors, known as GRABENS. These fractures may have been caused by stresses induced by TIDES. Alternatively, if the interior had frozen after originally been molten, they could have been produced by the stretching of Ariel's LITHOSPHERE. Many areas, particularly valley floors, can be seen to have been flooded by some kind of viscous fluid during an episode of CRYOVOLCANISM. This flooding may have been a result of TIDAL HEATING, although Ariel no longer shares orbital RESONANCE with any other satellite of Uranus. Past heating events may have been sufficient to allow a small rocky CORE to settle out at Ariel's centre by DIFFERENTIATION.

The ice of which most of Ariel's volume is composed is believed to be rich in ammonia, although this has not been detected by spectroscopy. Ammonia-water ices have the remarkable property of being able to liberate small percentages of melt at temperatures as low as 176 K. As this is only about 100 K warmer than Ariel's present surface temperature, it goes some way to explaining the former molten nature of the interior and the apparent ease with which cryovolcanic melts have been able to reach and spread over the surface. However, even the youngest areas are scarred by abundant impact craters, indicating that Ariel was last geologically active a long time, maybe 2 billion years, ago. See data at URANUS

Ariel Voyager 2 captured this image of Uranus’ satellite Ariel in 1986. Ancient faults and valleys overlain by abundant impact craters indicate that the moon has not been geologically active for billions of years.

Ariel Series of six UK scientific satellites launched between 1962 and 1979. The first five were a collaboration with the USA. Ariel 1-4 studied the ionosphere and galactic radio sources; Ariel 5 and 6 carried X-ray and cosmic ray detectors.

Aries See feature article

A ring Outermost of the rings of SATURN visible from Earth, spanning a width of 14,600 km (9100 mi) to a maximum radius of 136,800 km (85,000 mi) from the planet's centre.

A ring This view of Saturn's A ring was obtained on August 23, when Voyager 2 was about 2.8 million km (1.7 million mi) from the planet. The Cassini Division is at bottom right, the Encke Division at top left and one of the F ring’s shepherding satellites near the top.

Aristarchus Relatively young lunar crater (23°.6N 47°.4W), 40 km (25 mi) in diameter; it is the brightest object on the Moon's surface. Aristarchus reflects 20% of the sunlight falling upon it, which gives it an albedo about twice that of typical lunar features. Its outer ramparts are highlighted by a white blanket of ejecta, and its brilliant rays radiate south and south-east. It is possible to see Aristarchus by earthshine. The crater walls are terraced through a drop of 3000 m (10,000 ft) to the lava-flooded floor, which is only half the diameter of the crater, supporting a small central peak. The dark vertical bands visible on the inner walls of Aristarchus are likely an effect of offsetting from the landslips that formed its terraces. With neighbouring herodotus and Vallis schroteri, this impact crater has long been the focus of intensive study. Many lunar transient phenomena have been observed here. There is evidence of volcanic activity, including plentiful volcanic domes, rilles and areas of fire fountaining.

Aristarchus This falsecolour image of the Aristarchus plateau on the Moon shows the composition of the surface. Red indicates pyroclastic flows from the region’s many volcanic craters, while the bright blue area is the 42-km-wide (26-mi) Aristarchus crater, surrounded by ejecta from the impact that caused it.

Aristarchus of Samos (c.310-c.230 bc) Greek astronomer and mathematician who measured the distances of the Sun and Moon, and who is also cited as the first astronomer to propound a heliocentric theory. Aristarchus reckoned the Sun's and Moon's distances by measuring the apparent angle between the Earth and Sun during the first quarter Moon, when the Moon-Sun angle is 90°. Though his method was mathematically valid, the exact moment at which the Moon was 50% illuminated proved very difficult to measure in practice, and any slight error in its value would introduce gross errors in the final result. Aristarchus observed the Sun-Earth-Moon angle as 87°, determining that the Sun was twenty times as far from the Earth as the Moon, and hence was twenty times the actual size of our natural satellite. Because the actual angle at first quarter is 89° 50', the Sun is really about 400 times as distant, but Aristarchus' result, announced in his work On the Sizes and Distances of the Sun and Moon, nevertheless demonstrated that the Sun is a much more distant celestial object than the nearby Moon.

The idea of a moving Earth seems to have originated with the followers of Pythagoras (c.580-500 bc), though they had it orbiting a central fire, not the Sun. But according to Archimedes (c.287-212 bc) and Plutarch (ad 46-120), it was Aristarchus who put the Sun at the centre of a very large Universe where the stars were 'fixed' and only appeared to move because of the Earth's rotation on its axis. Aristarchus' hypothesis called for a circular Earth orbit, but that could not account for the unequal lengths of the seasons and other irregularities, while a moving Earth was contrary to ancient Greek cosmology; his heliocentric model was therefore not accepted in his own day.

Aristotle (384-322 bc) Greek philosopher and polymath who developed a cosmology based upon a 'perfect' Earth-centred Universe. In 367 bc he enrolled at Plato's Academy, where he studied under eudoxus; in 335 bc he founded his own school at Athens, called the Lyceum. It was there that Aristotle developed a cosmology incorporating the 'four elements' of earth, air, fire and water, and the four 'fundamental qualities' -hot, cold, dry and wet. These ideas led him to reject the idea of a vacuum and, therefore, any atomic theory, because this demanded the presence of particles existing in empty space. Aristotle also considered and rejected any idea of a moving Earth. Aristotle taught that the Universe was spherical, with the Sun, Moon and planets carried round on concentric spheres nesting inside one another. The Earth itself was a sphere, Aristotle reasoned, because of the shape of the shadow cast on the Moon in a lunar eclipse. All these celestial bodies were eternal and unchanging, and he thought of them being composed of a fifth element or essence. The outermost sphere carried the 'fixed stars'; it controlled the other spheres, and was itself controlled by a supernatural 'prime mover'.

In Aristotle's Universe everything had its natural place. 'Earthy' materials fell downwards because they sought their natural place at the centre of the spherical Earth, which was itself at the centre of the Universe. Water lay in a sphere covering the Earth, hence its tendency to 'find its own level'. Outside the sphere of water lay that of the air, and beyond this was the sphere of fire; flames always burned upwards because they were seeking their natural place above the air. Change was confined to the terrestrial world, the 'sublunary' region lying inside the Moon's sphere. As a result meteors, comets and other transitory events were thought to occur in the upper air; they were classified as meteorological phenomena.

The Aristotelian world-view, as modified by Ptolemy, would hold sway for nearly two millennia, thanks in part to its incorporation into the doctrines of the Catholic Church. See also geocentric theory

Arizona Crater See meteor crater

Armagh Observatory Astronomical research institution close to the centre of Armagh, Northern Ireland. The observatory was founded in 1790 and equipped with a 2i-inch (60-mm) telescope, which is still in existence. A 10-inch (250-mm) refractor was added in 1885. During the 19th century, the observatory excelled in positional astronomy, its most important contribution being the new general catalogue compiled by J.L.E. Dreyer. Today's research topics include stellar physics and Solar System dynamics.

armillary sphere Oldest known type of astronomical instrument, used for both observing the heavens and teaching. It is a skeletal celestial sphere, the centre of which represents the Earth-bound observer. A series of rings represent great circles such as the equator and the ecliptic, while other rings represent the observer's horizon and meridian. The armillary sphere can be rotated to show the heavens at a particular time of day. Instruments of this kind were used from the time of the early Greek astronomers to that of Tycho Brahe.

Armstrong, Neil Alden (1930- ) American astronaut who, on 1969 July 20, became the first human to walk on the Moon. After serving as a fighter pilot, he became a test pilot for NASA's experimental X-15 and other rocket planes. He commanded the first docking mission (Gemini 8, 1966) and was back-up commander for Apollo 8 Ho 11 lunar landing mission, with 'Buzz' Aldrin and Michael Collins (1930- ) completing the crew. Armstrong served on the 1986 panel that investigated the space shuttle Challenger tragedy.

Armstrong, Neil Alden -bearer) Neil Armstrong is pictured here with an X-15 rocket-powered aircraft in 1960.

Arneb The star a Leporis, visual mag. 2.58, distance about 1300 l.y., spectral type F0 Ib. Its name comes from the Arabic meaning 'hare'.

Arp, Halton Christian (1927- ) American astronomer known for his controversial theories of redshifts, which he developed from his studies of peculiar galaxies. In 1956 he determined the relation between the absolute magnitude of a nova at maximum brightness and the rate at which this brightness decreases after the outburst. From his studies of galaxies, galaxy clusters and active galactic nuclei (AGNs), Arp proposed that AGNs, which often display very high redshifts, are ejected from the cores of nearby active galaxies with lower redshifts, the higher apparent redshifts of the AGNs being the result of the violent ejection process.

array Arrangement of radio telescopes or antennae working together, most commonly in radio astronomy, to improve the resolution of the image created. The common shapes for an array are linear, Y-shaped or circular. An array is used in aperture synthesis and in radio interferometry. See also very large array

Array for Microwave Background Anisotropy (AMiBA) Compact array telescope being built on Mauna Loa, Hawaii, by the Academia Sinica Institute of Astronomy and Astrophysics (Taiwan) to investigate structure in the cosmic microwave background radiation. The instrument consists of 19 small dishes 1.2 and 0.3 m (48 and 12 in.) in diameter on a single mounting of unusual form; it should begin operation in 2004.

Arrhenius, Svante August (1859-1927) Swedish chemist known for his explanation of why liquid solutions of ions conduct electricity. In astronomy he proposed the panspermia theory, according to which life was brought to Earth by way of spore-containing meteorites.

Arsia Mons Shield volcano (8°.4S 121°.1W), 800 km (500 mi) in diameter, situated in the tharsis region of mars. It rises to 9 km (6 mi) in height, and has a large summit caldera and radiating volcanic flows.

artificial satellite Man-made object that is placed into orbit around the Earth, Sun or other astronomical body. The first artificial satellite, sputnik 1, was launched by the Soviet Union on 1957 October 4. By 2002 there had been almost 5000 successful launches, with an annual launch rate of around 90. Most of these carried single satellites, but it is increasingly common for two or more to be carried by one launch vehicle. At the same time, other objects such as the upper rocket stage and protective nose cone may also be released, creating a cloud of artificial debris - an undesirable class of satellites - around Earth.

artificial satellite If a high enough tower could be built, satellites could be fired from it directly into orbit. The type of orbit would depend on the initial velocity. If the starting velocity were low (A or B), the satellite would soon hit the ground. But at 8 km/s (5 mi/s) it would stay in its path (C), falling all the time, but never coming any closer to the ground.

All satellites move in elliptical orbits governed by newton'slawsofmotion. However, the orbit is modified continuously by external forces, such as friction with the upper atmosphere and variations in the gravitational pull of the Earth. The rates of change depend on the height of the satellite and the inclination of the orbital plane to the equator.

Changes in the density of the atmosphere, due principally to solar activity, cause variable drag on a satellite. The overall effect is to change the eccentricity, with the orbit becoming more and more circular. Eventually, the satellite will spiral inwards and experience increasing drag until it finally plunges into the denser regions of the atmosphere and burns up.

Other factors affecting a satellite's orbit include atmospheric tides and winds, whether the perigee of the orbit occurs in the northern or southern hemisphere and the local times at which this occurs. Solar radiation pressure has a major effect on satellites with a large area/mass ratio, such as those with large solar arrays. The overall result is that no satellite can have a stable orbit below a height of about 160 km (100 mi). At this height the orbital period is about 88 minutes.

Satellites are generally classified as military or civil, although there is considerable overlap. About 25% of current launches are for military purposes, such as photo-reconnaissance, communications, electronic listening and navigation. Categorizing US military satellites is fairly straightforward, but Russian or Chinese sources rarely divulge the purposes of their military satellites. For example, the 2386 Russian Cosmos satellites launched by the end of 2001 included all types of experimental, scientific and military spacecraft.

Common names are used extensively instead of technical designations. Such names as Spot, Landsat, Meteosat and Mir are well known, but confusion may arise when two organizations use the same or similar names. For example, the US GEOS was used for geodetic studies, while the european space agency used the same name for a satellite that investigated charged particles in the upper atmosphere. Acronyms have been used widely, adding to the confusion.

Fortunately, COSPAR (International Committee for Space Research) has provided a standard system for identifying all satellites and related fragments, based on the year of launch and the chronological order of successful launches in that year. For example, the mir space station, launched on 1986 February 19, was known as 1986-17A, since it was the 17th launch in 1986. The A referred to the satellite. The rocket that put it into orbit was designated 1986-17B. If several satellites are launched by a single rocket, they are usually given the letters A, B, C, and so on. If more than 24 fragments are created by an explosion, the letter Z is followed by AA, AB, BA, BB, and so on. Before 1963 a system involving Greek letters was used. For example, Sputnik 3 was designated 1958 8 2 whilst the associated rocket was known as 1958 8 1.

Photographic, visual and laser techniques are all used to track satellites. However, most satellites are tracked through their telemetry, by measuring the phase difference between a signal transmitted from the ground and a return signal from the satellite. This method can determine an Earth orbiting satellite's distance to within 10 m (33 ft).

If the sky is clear, satellites can be seen after dusk or just before dawn, when the sky is dark but the satellite is still illuminated by the Sun. The length of these visibility periods depends on the time of the year, the latitude of the observer and the orbit of the satellite. The satellite's brightness depends on the nature and curvature of the reflecting surface, the phase angle (the angle between the Sun and the observer as seen from the satellite) and its distance and altitude from the observer. Some objects, such as the international space station, rival the brightest planets, but others can only be seen through medium-sized telescopes.

The rate at which satellites cross the sky depends on their height above the Earth. Low satellites may cross the sky in about 2 minutes, but those at heights of about 2000 km (1200 mi) may take half an hour. Satellites at heights of about 36,000 km (22,400 mi) take 24 hours to complete one revolution. If the orbital inclination is 0°, the satellite appears motionless in the sky. Many meteorological and communication satellites have been launched into these geostationary orbits.

Roughly two-thirds of all the satellites launched have been from the former Soviet Union and most of these have been from the northern launch complex at Plesetsk, near Archangel. The other main launch site in the former Soviet Union is at baikonur (Tyuratam), north-east of the Aral Sea. A relatively small number of satellites have been launched from Kapustin Yar, near Volgograd, and a new Russian launch site is being developed at Svobodny in the Russian Far East.

The Americans have three main launch sites: Cape Canaveral in Florida, Vandenberg Air Force Base in California and Wallops Island in Virginia. A number of commercial launch sites are also being developed for small rockets.

In recent years, as other nations have begun to develop their space industries, launch sites have sprung up in South America and Asia. European ariane rockets lift off from Kourou in French Guiana. Japan uses two southern sites, Kagoshima and Tanegashima. India's main launch centre is on Sriharikota Island, northeast of Madras. China has three main launch centres, at Jiuquan, Taiyuan and Xichang, in the less populated areas of north, west and southwest China respectively. The most recent innovation is the development of an ocean-going launch platform for the US-Ukraine-Norway sea launch programme.

Aryabhata (ad 476-C.550) Indian mathematician and astronomer who wrote three astronomical treatises, only one of which, the AryabhatTya (ad 499), survives. In it he attributed the apparent movements of the planets in the sky to both their periods of revolution about the Sun and their orbital radii, presaging Copernican theory. Aryabhata also believed that the Earth rotated while the stars remain stationary.

Arzachel Latinized name of al-zarqalI

Arzachel Lunar walled plain (18°S 2°W), 97 km (60 mi) in diameter. Arzachel is the youngest of the three great walled plains that border Mare Nubium's east side. Its lofty (3000-4100 m/9800-13,500 ft), sharply defined walls show many terraces, especially on the west side. Its rugged, 1500-m-high (4900-ft-high) central peak confirms the crater's youth. A broad and lengthy canyon divides the south-east wall. A prominent rille runs from north-south across the east side of Arzachel's floor.

ASCA Abbreviation of Advanced Satellite for Cosmology and Astrophysics. See asuka

ascending node (ft) Point at which an orbit crosses from south to north of the reference plane used for the orbit. For the planets the reference plane is the ecliptic. For satellites it is usually the equator of the planet. See also inclination; orbital elements

Asclepius apollo asteroid, number 4581. On its discovery as 1989 FC it caused a media furore because of its close approach to the Earth, to within 685,000 km (426,000 mi), which is less than twice the lunar distance. Its discovery stimulated political action in the United States aimed at taking steps to determine whether any possible asteroid impact on the Earth might be foreseen and perhaps obviated. See table at near-earth asteroid

Ascraeus Mons Shield volcano situated in the tharsis region of mars (11°.9N 104°.5W). It is 18 km (11 mi) high, with complex caldera.

Aselli Latin name meaning 'the asses' applied to the stars 7 and 8 Cancri, which lie north and south, respectively, of the star cluster praesepe, 'the manger'. 7 Cancri (known as Asellus Borealis, the northern ass) is of visual mag. 4.66, 158 l.y. away, and spectral type A1 V. 8 Cancri (Asellus Australis, the southern ass) is visual mag. 3.94, 136 l.y. away and spectral type K0 III.

ashen light (Fr. lumiere cendre, ash-coloured light) Expression sometimes given to earthshine on the Moon, but more precisely given to the faint coppery glow infrequently seen on the dark side of venus, where the planet is visible as a thin crescent near inferior conjunction. Sometimes the entire night hemisphere is affected, at other times the glow is patchy and localized, but it has no preferred position. In appearance the phenomenon is analogous to earthshine, although it is produced in a very different way. The ashen light was apparently seen by Giovanni Battista riccioli in 1643, but it was first accurately described in 1715 by the English cleric William Derham, Canon of Windsor. It is possibly the oldest unsolved mystery in the observational history of the Solar System.

The ashen light phenomenon is rare, fugitive and suspect. It is only seen, and then with extreme difficulty, if Venus is observed on a dark sky and has its bright part hidden by an occulting device located inside the eyepiece of the telescope. Even this precaution is insufficient to dispel thoughts of illusion, and in the absence of photographic confirmation there is a natural tendency towards scepticism. Still, too many experienced observers have claimed sightings of ashen light to completely discount the phenomenon.

Volcanic activity, phosphorescence of the surface, self-luminosity, accidental combustion and other illuminating processes, including lightning, have all been proposed by way of explanation. The only conceivable physical mechanism that would be dependent on the phase or position of Venus as viewed from Earth would in fact be earthlight, but calculation clearly demonstrates that theoretical earth-light falls well below the threshold of visibility. Possibly the cause is to be found in an electrical phenomenon in Venus' upper atmosphere. Another possibility emerged in 1983 when David A. Allen (1946-94) and John W. Crawford imaged the planet in infrared and found cloud patterns on its night side. These clouds, with a retrograde rotation period of 5.4 ± 0.1 days, are thought to be at a lower level than the ultraviolet features and may be sufficiently lit by radiation scattered from the day side to become visible occasionally in integrated light.

ASP Abbreviation of astronomical society of the pacific

aspect Position of the Moon or a planet, relative to the Sun, as viewed from Earth. The angle on the celestial sphere between the Sun and another Solar System body is known as the elongation of that body. When the elongation is 0°, that body is said to be at conjunction. When it is 180°, and the two are opposite one another in the sky, it is at opposition. When the elongation is 90°, the body is at quadrature.

association, stellar Group of stars that have formed together but that are more loosely linked than in a star cluster. Stellar associations lie in the spiral arms of the Galaxy and help to define the shape of the arms, being highly luminous. ob associations are groups of massive O and B stars. R associations illuminate reflection nebulae and are similar stars but of slightly lower mass (3 to 10 solar masses). t associations are groups of t tauri stars.

Association of Lunar and Planetary Observers (ALPO) Organization of amateur observers that collects reports of observations of all Solar System objects (Sun, Moon, planets, comets, meteors and meteorites), founded in 1947. These observations, especially of Mars, Jupiter and Saturn, are summarized annually in the organization's Journal, which provides synopses of notable happenings on these dynamic bodies.

Association of Universities for Research in Astronomy (AURA) Consortium of US universities and other educational and non-profit organizations; it was founded in 1957. It operates world-class astronomical observatories. Current members include 29 US institutions and six international affiliates. The facilities operated by AURA include the gemini observatory, the national optical astronomy observatory and the space telescope science institute. In addition, AURA has a New Initiatives Office formally established in January 2001 to work towards the goal of a 30-m (98-ft) giant segmented-mirror telescope.

A star Member of a class of white stars, the spectra of which are characterized by strong hydrogen absorption lines. Ionized metals are also present. main-sequence dwarf A stars (five times more common than B stars) range from 7200 K at A9 to 9500 K at A0. Their zero-age masses range from 1.6 to 2.2 times that of the Sun, and their zero-age luminosities from 6 to 20 times that of the Sun. Their lifetimes range from 2.5 billion to 900 million years. Evolved A dwarf stars can have masses up to three times that of the Sun. Class A giant stars and supergiants are distinguished by the narrowing of the hydrogen lines with increasing luminosity.

Though A dwarfs can rotate rapidly, averaging from 100 km/s (60 mi/s) at A9 to 180 km/s (110 mi/s) at A0, their envelopes are not in a state of convection, and they lack solar-type chromospheres. A dwarfs exhibit a variety of spectral anomalies, especially among the slower rotators. Metallic-line am stars are depleted in calcium and scandium, while enriched in copper, zinc and the rare earths. The ap stars (A-peculiar) have strong magnetic fields that range to 30,000 gauss. They exhibit enhancements of silicon, chromium, strontium and the rare earths (europium is enriched by over 1000 times), which are concentrated into magnetic starspots. These odd compositions result principally from diffusion, in which some chemical elements gravitationally settle, while others are lofted upwards by radiation. Rare lambda bootis stars have weak metal lines.

delta scuti stars (Population IA dwarfs and sub-giants) pulsate in under a day with multiple periods and amplitudes of a few hundredths of a magnitude (the large-amplitude variety are known as dwarf Cepheids). Population II rr lyrae stars (A and F horizontal branch) have similar pulsation periods, but amplitudes of a few tenths of a magnitude.

Bright A stars include Sirius Aim V, Vega AO V, Altair A7 V and Deneb A2 Ia.

asterism Distinctive pattern formed by a group of stars belonging to one or more constellations. Perhaps the most famous asterism is the PLOUGH (Big Dipper), a shape formed by seven stars in Ursa Major. Other famous asterisms within individual constellations include the SICKLE OF LEO and the TEAPOT in Sagittarius. The SQUARE OF PEGASUS is an example of an asterism that is composed of stars from two constellations (in this case, Andromeda and Pegasus); another is the SUMMER TRIANGLE. The term is also used for smaller groupings of stars visible in binoculars or telescopes, some of which have been given descriptive names such as the COATHANGER (in Vulpecula) and Kemble's Cascade (in CAMELOPARDALIS).

asteroid (minor planet) Rocky, metallic body, smaller in size than the major planets, found throughout the Solar System. The majority of the known asteroids orbit the Sun in a band between Mars and Jupiter known as the MAIN BELT, but asteroids are also found elsewhere. There is a population known as NEAR-EARTH ASTEROIDS, which approach the orbit of our planet, making impacts possible. Other distinct asteroid classes include the TROJAN ASTEROIDS, which have the same orbital period as Jupiter but avoid close approaches to that planet. Farther out in the Solar System are two further categories of body that are at present classed as asteroids, although in physical nature they may have more in common with comets in that they seem to have largely icy compositions. These are the CENTAURS and the TRANS-NEPTUNIAN OBJECTS, members of the EDGEWORTH-KUIPER BELT.

asteroid Plots of the larger known asteroids: most of them lie in the main belt between the orbits of Mars and Jupiter, while the Trojans move in the same orbit as Jupiter, 60 in front of or behind it.

Although it is likely that there might be larger bodies in the Edgeworth-Kuiper belt awaiting discovery (see VARUNA), the largest asteroid in the inner Solar System is (1) CERES, which is c.933 km (c.580 mi) in diameter. It was discovered in 1801. Through a telescope, asteroids generally appear as pin-pricks of light, which led to the coining of the term 'asteroid', meaning 'star-like', by William HERSCHEL. Broadly spherical shapes are attained by asteroids if their self-gravity is sufficient to overcome the tensile strength of the materials of which they are composed, setting a lower limit for sphericity of about 200 km (120 mi); there are about 25 main-belt asteroids larger than this. All other asteroids are expected to be irregular in shape (see CASTALIA, DEIMOS, EROS, GASPRA, GEOGRAPHOS, GOLEVKA, IDA, KLEOPATRA, MATHILDE, PHOBOS, TOUTATIS, VESTA). Use of the HUBBLE SPACE TELESCOPE, sophisticated techniques employing ground-based telescopes and the application of radar imaging have recently led to the ability to resolve the shapes of some asteroids. OCCULTATIONS also allow asteroid sizes and shapes to be determined observationally: on rare occasions when asteroids pass in front of brighter stars, observers on the ground may see a fading or disappearance of the star, the duration of which (usually a few seconds) defines a cord across the asteroid profile. For many asteroids, sizes have been calculated on the basis of a comparison of their brightness both in scattered visible light from the Sun and also in the thermal infrared radiation emitted, which balances the solar flux they absorb. Such measurements allow the ALBEDO to be derived. In most cases, however, sizes are estimated simply on the basis of the observed absolute magnitude and an assumed value for the albedo.

The lower limit on size at which a solid body might be considered to be an asteroid is a matter of contention. Before the application of photography, starting in the 1890s, asteroids were discovered visually. Long photographic exposures on wide-field instruments, such as astrographic cameras and Schmidt telescopes, led to many thousands of asteroids being detected thereafter. However, the limited sensitivity of photographic emulsions makes it impossible to detect asteroids smaller than a few hundred metres in size, even if they are in the vicinity of the Earth. In the late 1980s the introduction by the SPACEWATCH project of charge-coupled devices for asteroid searching made it feasible to detect objects only a handful of metres in size during passages through cis-lunar space. A convenient size at which to draw a line is 10m (30ft): larger solid objects may be regarded as being asteroids, while smaller ones may be classed as METEOROIDS.

The total mass of all the asteroids in the inner Solar System (interior to Jupiter's orbit) is about 4 X 1021 kg, which is about 5% of the mass of the Moon. A large fraction of that total is held in the three largest asteroids, Ceres, PALLAS and Vesta. A decreasing fraction of the overall mass is represented by the smaller asteroids: although there may be over a million main-belt asteroids each about 1 km (0.6 mi) in diameter, their total mass is less than that of a single asteroid of diameter 200 km (120 mi). As a rule of thumb the number of asteroids increases by about a factor of one hundred for each tenfold decrease in size. Because the mass depends upon the cube of the linear dimension, the smaller asteroids thus represent a decreasing proportion of the total mass of the population.

When an asteroid is found its position is reported to the International Astronomical Union's Minor Planet Center, and it is allotted a preliminary designation. This is of the form nnnn pq, where nnnn is the year and pq represents two upper-case letters. The first letter (p) denotes the half-month in which the discovery was made, January 1-15 being labelled A, January 16-31 as B, and so on. The letter I is not used, so that December 16-31 is labelled Y in this convention, and the letter Z is also not used. The second letter (q) provides an identifier for the particular asteroid; in this case Z is used but not I, making 25 letters/asteroids in all. Thus the third asteroid reported in the second half of March 1989 was labelled as 1989 FC. Until the last couple of decades this designation system was adequate because no more than 25 asteroids were being discovered in any half-month, but now thousands are reported and so an additional identifier is required. This takes the form of a subscripted numeral. Thus 1999 LD31 was the 779th asteroid reported in the first half of June 1999. Many asteroids are reported After an asteroid has been observed for a sufficient length of time, such that our determination of its orbit is secure (this generally requires three oppositions), it is allocated a number in the master list, which begins with (1) Ceres and continues with (2) Pallas, (3) Juno, (4) Vesta and so on. This list exceeded 25,000 during the year 2001. Upon ^^^^^^^^^^wiiscoverers of each asteroid have the right to suggest a name for it to the naming committee of the International Astronomical Union, and in most cases that suggestion is adopted. Certain conventions apply to distinct categories of asteroid, such as the Trojans and the Centaurs. In the past, before strict rules were adopted for numbering and naming, a few exceptions to these procedures occurred, such as with albert and hermes.

Although some asteroids may be cometary nuclei that have become dormant or extinct, most of these bodies are thought to have originated in the region of the present main belt. They seem to be the remnant debris from a large complex of planetesimals that failed to form a major planet due to the gravitational stirring effect of Jupiter. Soon after their formation, some of the larger proto-asteroids underwent sufficient internal heating, through radioactive decay, to melt to some degree, acquiring metallic cores and layered mantles. This chemical differentiation is represented in the variety of known meteorite classes - these being in effect asteroid fragments.

A consequence of this differentiation is that asteroids present a variety of spectroscopic classes, representing different surface (and, assumedly, bulk) compositions. The most abundant group is called C-type (for 'carbonaceous') asteroids, these being especially prevalent as one moves outwards through the main belt. C-type asteroids reflect more light at the red end of the spectrum, but their low albedos (below 0.05) make them very dark, so they may be thought of as being black-brown. They are thought to contain the same materials as carbonaceous chondrite meteorites. The inner part of the main belt is dominated by S-type (stony or 'silicaceous') asteroids. These have moderate albedos (0.15-0.25) and are thought to be analogous to the metal-bearing stony meteorites known as ordinary chondrites. The third most populous class are the M-type ('metallic') asteroids. These also have moderate albedos and are thought to be derived, like nickel-iron meteorites, from the metal-rich cores of large differentiated parent bodies that have become exposed by collisional break-up. Members of a less common class, the E-types ('enstatite'), have elevated albedos of 0.40 or more, as has the V-class Vesta. There are many other subdivisions of each of these classes, and other rare categories that have been recognized as being distinct from the common groupings. The distributions of the various asteroid types in the main belt - the preponderance of S-types at the inner edge but with more C-types at greater distances - suggest that the volatile-rich asteroids formed farther from the Sun, as a result of the presumed temperature gradient in the solar nebula. Similarly, the homogeneous composition of the hirayama families suggests that most asteroids remain in orbits similar to those where they formed in that nebula.

Another important clue to the nature of asteroids comes from their spin rates. The rotation period of an asteroid may be measured from observations of its repeating trends in brightness variation, generally displaying two maxima and two minima in each revolution. This pattern indicates that the changes in brightness are dominated by the effect of the asteroid's non-spherical shape, rather than any albedo distribution across its surface. Typical amplitudes are only up to 0.2 mag., or 20% in the brightness, although some near-Earth asteroids, such as Geographos, vary by a much greater amount, as a result of their elongated shapes. Typically, asteroids have rotation periods in the order of ten hours, although a few are much longer (some weeks), possibly due to the damping effect of a companion satellite (as with Ida, eugenia and pulcova). At the other end of the scale, short rotation periods are of interest because self-gravitation cannot hold together an asteroid having a rotation period of less than about two hours. Until recently all asteroids for which light curves were available displayed periods in excess of this amount, meaning that they could be 'rubble piles' held together by gravity. (Another piece of evidence pointing to such a structure is the presence of voids within asteroids, as indicated by the low average densities determined for some of them, for example Ceres and Mathilde.) The first asteroid to be measured to have a shorter period (about 97 minutes) was 1995 HM, which thus appears to be a monolith. Since then several other asteroids have been shown to have even higher spin rates, with periods as short as ten minutes. These must similarly be single rocks rather than rubble piles, being held together by their tensile strength.

The meteoroids that produce meteorites, and indeed some complete asteroids, leave the main belt through a variety of mechanisms. Inter-asteroid collisions may grossly change their orbits (evidence of past collisions derives from the Hirayama families), and such collisions also change the spin rates discussed above. A more significant avenue through which asteroids can escape the main belt is the rapid dynamical evolution that occurs if an object acquires an orbital period that is a simple fraction of that of Jupiter. This leads to depleted regions of orbital space known as the kirkwood gaps. Asteroids leaving those regions tend to acquire high-eccentricity orbits, which can lead to them either being ejected from the Solar System by Jupiter, or else joining the population of mars-crossing asteroids or near-Earth asteroids.

Viewed in three dimensions, the main belt is a somewhat wedge-shaped torus, increasing in thickness from its inner to its outer edge. The median inclination increases from about 5° to about 9°, whereas the median orbital eccentricity is around 0.15 throughout the main belt. Most asteroids, therefore, follow heliocentric orbits that are only a little more inclined than those of the planets, and with modestly greater eccentricities. It follows that the majority of main-belt asteroids move among the stars, as seen from the Earth, in a similar way to the planets: they stay close to the ecliptic, and retrograde through opposition. There are, however, exceptions among various dynamical classes such as the near-Earth asteroids, the Trojans, and various specific main-belt classes that are grouped together in terms of common dynamical behaviour rather than composition or origin.

asteroid belt General term for the region between the orbits of Mars and Jupiter in which the orbits of most asteroids lie. See main-belt asteroid

asteroseismology Study of the internal structure of stars using observations of the frequencies and strengths of global oscillations detected at the surface by their Doppler shifts. The very low amplitude of stellar oscillations has limited its current application to pulsating stars. See also helioseismology.

asthenosphere Weak, uppermost layer of a planet's mantle, in which solid-state creep first plays a predominant role; it lies immediately below the relatively rigid lithosphere. At the asthenosphere the rise in temperature with depth reaches the threshold at which plastic flow may occur in response to small stress differences. In the case of the Earth, seismological evidence - the attenuation of S waves and a decrease of P wave velocities between depths of about 100-250 km (62-155 mi) - is taken to define the asthenosphere. The viscosity of the asthenosphere is largely similar to that of the bulk of the underlying mantle, but it is far less than that of the lithosphere, enabling plate-tectonic motions to take place. At subduction zones the asthenosphere appears to penetrate down to approximately 700 km (430 mi), which is comparable with the transition zone between the upper and lower mantle. Other planets such as Venus and Mars are assumed to have asthenospheres, as are some of the larger satellites. The Moon is known to have an asthenosphere at a depth of approximately 1000 km (620 mi), and recent seismological evidence suggests that it may be even more fluid at a greater depth. See also seismology; tectonics astigmatism aberration in lenses and mirrors that prevents star images away from the centre of the field of view from being focused into sharp points. When astigmatism is present point objects such as stars usually appear elongated, either into a line or an ellipse. Attempts to reduce the elongated image to a point by refocusing will cause the orientation of the elongation to change through 90°. The best or least distorted result occurs as the orientation changes and the image becomes round; this is known as the circle of least confusion. See also anastigmat

Astraea main-belt asteroid; number 5. It is about 117 km (73 mi) across, and was found in 1845, almost four decades after the discovery of (4) vesta.

Astro Fleet of Japanese artificial satellites. When launched, they are given individual names.

astrochemistry Chemistry in stars. The chemical composition of most stars is dominated by hydrogen, with helium in second place and the remaining elements a long way behind. The relative proportions (or abundances) of the elements are quantified either by the number of atoms or the mass involved. In terms of mass, average material from outer layers of the Sun and the rest of the Solar System contain 70.7% hydrogen, 27.4% helium and 1.6% of all the other elements. These three quantities are called mass fractions and are conventionally labelled X, Y and Z. In terms of the number of atoms, hydrogen dominates even more, with 92.0% hydrogen, 7.8% helium, and all the rest just 0.12%. Elements other than hydrogen and helium are often termed the heavy elements, and their relative abundances are shown in the accompanying table.

These figures, which are representative compositions throughout the Universe, are changed by nucleosynthesis reactions inside stars. Thus at the centre of the Sun, the abundance of hydrogen has currently fallen to 38% by mass, and that of helium has increased to about 60%. Stars that have completed their main-sequence lives, will have cores of almost pure helium. Solar mass stars will go on to synthesize carbon and oxygen, and higher mass stars will create the elements all the way to iron. The remaining elements are produced during supernova explosions.

Nucleosynthesis implies that hydrogen is continually being converted into heavier elements, and so abundances change slowly. But only a few per cent of helium by mass can have been produced this way during the lifetime of the Universe. Most helium, therefore, was produced during the early stages of the Universe (see cosmochemistry). The remaining elements have been produced inside stars, and then ejected to mix with the interstellar medium through stellar winds, supernova explosions and so on. The mass fraction, Z, of the heavy elements is therefore increasing with time. The oldest stellar populations, known as Population II stars, found in the galactic nucleus and halo and in globular clusters, have values of Z around 0.5%. Stars in the disk of the Galaxy are younger, have a higher proportion of heavy elements and are called Population I stars. Thus the Sun, which formed about 4.5 billion years ago, has Z = 1.6%, while stars forming today have Z ~ 4%.

Some stars are exceptions to the norms, the most important being the white dwarfs. These stars, which are at the ends of their lives, have lost their outer layers and so have their cores exposed. The cores contain the products of nucleosynthesis, and thus the composition of white dwarfs ranges from helium-rich through relatively pure carbon to calcium. wolf-rayet stars subdivide into the WN stars, formed of helium and nitrogen, and the WC stars, within which helium, carbon and oxygen predominate. ttauri stars have an over-abundance of lithium. ap stars are over-abundant in elements such as mercury, strontium, silicon, europium, holmium and chromium. It is thought that these latter peculiarities occur just in a thin surface layer and are perhaps brought about by diffusion. Beneath that thin layer the remaining parts of the star are of normal composition. carbon stars have over-abundances of lithium and carbon, while S-type stars additionally have over-abundances of zirconium and yttrium. Technetium has been detected in both carbon and S stars, and since all isotopes of technetium are radioactive with short half-lives, it must have been produced inside the stars. Nucleosynthesis products are thus being brought to the surface in these stars. There are other anomalies to be found, and such stars are termed chemically peculiar stars (CP).

In the outer layers of cooler stars, simple molecules such as CH, TiO, CN and C2 may be observed, but stars are too hot for most molecules to form. Molecules are therefore to be found in cooler regions such as planets (see geochemistry) and giant molecular clouds (GMCs). Some 120 molecule species have been identified so far in GMCs, including water (H2O), ammonia (NH3), salt (NaCl) and ethanol (CH3CH2OH). Much of the material between the stars is molecular hydrogen (H2).

Astro E Japanese X-ray observation satellite that has not yet been scheduled for launch. The satellite will also carry a US X-ray instrument, which was to have flown originally on the chandra x-ray observatory. Astro E will observe in the soft X-ray region, with its main observational subjects being hot plasmas, spectroscopy of black hole candidates and pulsars.

astrograph Telescope specially designed for taking wide-angle photographs of the sky to measure star positions. Traditionally, astrographs are refracting telescopes characterized by their objectives, which have relatively fast focal ratios (sometimes as fast as f4) and fields of view up to 6° wide - large by astronomical standards. The objectives may have three or four component lenses to achieve this performance, and the biggest astrographs have lenses 50 cm (20 in.) or so in diameter. Astrographs are always placed on equatorial mountings, which are motor-driven to allow the telescope to follow the apparent motion of the sky.

The first recognizable astrograph was built in 1886 at the Paris Observatory. It had an aperture of 330 mm (13 in.) and its objective was designed to give the best images in blue light, early photographic emulsions being insensitive to other colours. The success of this telescope led, in 1887, to a major international conference of astronomers agreeing to embark on a photographic survey of the whole sky - the carte du ciel. The Paris astro-graph was adopted as a standard model for this ambitious project, and several similar telescopes were built with focal lengths of 3.4 m (134 in.) and objectives corrected for colour and coma, including one at Greenwich in 1890.

Astrographic photographs were always taken on glass plates rather than film, which lacks the stability of glass. The instruments were principally used for astrometry, the accurate measurement of star positions, but have now been superseded by the schmidt camera.

Astrographic Catalogue See carte du ciel

astrolabe Early disk-shaped astronomical instrument, for measuring positions on the celestial sphere, equippped with sights for observing celestial objects. The classical form, known as the planispheric astrolabe, originated in ancient Greece, attained its greatest refinement in the hands of medieval Arab astronomers (see islamic astronomy) and reached the Christian West in about the 10th century ad. The basic form consists of two flat disks, one of which (the mater) is fixed and represents the observer on the Earth. The other (the rete) is movable and represents the celestial sphere. The altitudes and azimuths of celestial bodies can be read. A pointer or alidade can be used for measuring altitudes when the instrument is suspended by a string.

Given the latitude, the date and the time, the observer can read off the altitude and azimuth of the Sun, the bright stars and the planets, and measure the altitude of a body and find the time. Its function of representing the night sky for a certain latitude at various times is reproduced in the modern planisphere. The astrolabe can be used as an analogue computer for many problems in spherical trigonometry, and it was used even for terrestrial surveying work such as determining the altitude of a tower.

The mariner's astrolabe is a simplified instrument designed for observing the altitudes of the Sun and stars while at sea, as an aid to navigation. It was developed by the Portuguese and was used mainly in the 17th century, being supplanted by the sextant.

The precision (or prismatic) astrolabe is a 19th-century instrument for determining local time and latitude by accurately measuring when a star reaches a certain altitude. It was further developed as the impersonal astrolabe (so called because it eliminates the observer's personal error) by Andre danjon in 1938.

astrology In its modern form, a pseudoscience that claims to be able to assess personality traits and predict future events from the positions of celestial objects, particularly the visible planets. Today, more newspaper space is devoted to horoscopes than to astronomy, and if anything the influence of astrology is probably increasing. Yet there is no scientific justification for the claims made by astrologers.

Astrology has a long history and was of great importance to primitive and superstitious peoples. The positions of the planets and the constellations at the time of a person's birth were believed to have a great influence on their subsequent life and career.

Unusual celestial events such as eclipses of the Sun and Moon, and the appearance of comets and 'guest stars', were significant in times when most people believed that the heavens could influence their fates, and there emerged a class of astrologers who made their living from the gullibility of their clients. Kings often employed astrologers whose job was to keep up the morale of their subjects by assuring them that the dates of particular events such as royal births were auspicious ones. The efforts of astrologers and priests to predict such apparently ominous events as eclipses helped to establish astronomy as a legitimate subject of study.

astrometric binary Binary star system in which one component is too faint to be observed directly. Its binary nature is detected by perturbations in the visible component's proper motion, caused by the orbital motion of the unseen companion. Some astrometric binaries, just beyond the limit resolution, appear as a single elongated image, with the orientation of the long axis changing as a result of orbital rotation.

astrometry Branch of astronomy concerned with the precise measurement of the apparent positions of celestial objects through the creation of a fundamental non-rotating reference frame on the celestial sphere. This frame is used not only for precisely recording the relative positions of celestial objects, but also for measurements of stellar parallax and proper motion, and as a reference for relating optical observations to those in other regions of the electromagnetic spectrum. The frame has the Sun at its centre and is based on the plane of the Earth's equator (from which declination is measured) and the first point of aries (the zero point for measuring right ascension).

The position of the equator and the First Point of Aries for a given date (called the epoch) has traditionally been defined by making observations of the Sun and planets using a meridian or transit circle. This instrument is mounted on a fixed east-west horizontal axis and can only rotate in a north-south vertical plane. The declination of a celestial object is found from the inclination of the telescope when the object is observed crossing the meridian; right ascension is reckoned by timing the exact moment this occurs.

Once the positions of the celestial equator and the First Point of Aries are established, the positions of stars can be measured relative to these by further observations. Measuring stellar positions in this way provides fundamental observations of the celestial bodies since they are measured with respect to an inertial reference frame rather than to other 'fixed' stars. The initial, raw observations from the transit circle first need to be corrected for the Earth's movement through space, that is, referred to the centre of the Sun (heliocentric coordinates), and also corrected for the individual motions of the stars themselves. The components of these motions across the line of sight are called proper motions and were first discovered in 1718 by Edmond halley.

The observations are first referred to the centre of the Earth by allowing for the effect of refraction, diurnal parallax and diurnal aberration. By making further corrections for precession, nutation and polar drift, the coordinates are then referred to the fundamental plane of the equator. Heliocentric coordinates are obtained by correcting for the orbital motion of the Earth by allowing for the effects of annual parallax and aberration. It is then possible to go one step further and make a correction for the motion of the Sun itself, which appears 'reflected' in the proper motions of nearby stars. The ultimate accuracy of our measurement of stellar positions therefore depends on our knowledge of the astronomical constants of precession, aberration, nutation and the stellar proper motions.

In the past, much of our knowledge of positions, parallaxes and proper motions was achieved through photographic astrometry, undertaken over many years using long-focus telescopes to obtain photographic plates of individual areas of the sky. The stellar images on these plates were then measured very accurately on dedicated measuring machines to determine their relative positions.

Nowadays our knowledge comes from instruments such as the Carlsberg Meridian Telescope (CMT) at the Roque de los Muchachos Observatory on the Canary Island of La Palma and the astrometric satellite hipparcos. The Carlsberg Meridian Telescope is dedicated to carrying out high-precision optical astrometry and is currently being used to map the northern sky using a ccd detector. This will provide accurate positions of stars, allowing a reliable link to be made between the bright stars measured by the Hipparcos satellite and fainter stars seen on photographic plates. So far, the CMT has made measurements of some 180,000 positions, proper motions and magnitudes for stars down to magnitude 15, and over 25,000 positions and magnitudes of 180 Solar System objects, ranging from the outer planets including Pluto to some of the many asteroids that orbit the Sun between Earth and Mars.

Hipparcos was the first space experiment designed specifically for astrometry. The satellite, which operated between 1989 and 1993, extended the range of accurate parallax distances by roughly l0 times, at the same time increasing the number of stars with good parallaxes by a much greater factor. Of the 118,218 stars in the Hipparcos catalogue, the distances of 22,396 are now known to better than 10% accuracy. Prior to Hipparcos, this number was less than 1000. The companion Tycho database, from another instrument on the satellite, provides lower accuracy for 1,058,332 stars. This includes nearly all stars to magnitude 10.0 and many to 11.0.

Information on star positions, parallaxes and proper motion is published in fundamental catalogues and between 1750 and 1762 James bradley compiled a catalogue that was to form the basis of the first fundamental one ever made, by Friedrich bessel, in 1830. By comparing Bradley's work with that of Giuseppe piazzi in Palermo in about 1800, Bessel was able to calculate values of proper motions for certain bright stars and also to derive the constants of precession for the epochs involved. Bessel's name is also inextricably linked with two other aspects of astrometry. He was the first, in 1838, to measure the trigonometric parallax of a star and derived a distance of 9.3 l.y. (the modern value is 11.3 l.y.) for the nearby binary star sixty-one cygni. From his work on the fundamental catalogue he also found that the proper motion of sirius was not constant and he correctly attributed this 'wobble' to the presence of an unseen companion pulling Sirius from its path. The companion was discovered in 1862, making Sirius the first example of an astrometric binary.

Further fundamental catalogues followed with those of Simon newcomb (1872, 1898) and Arthur auwers (1879) being especially important. The latter was the first in the Fundamental Katalog series compiled in Germany, the latest of which is the FK5 Part II. This catalogue provides mean positions and proper motions at equinox and epoch J2000.0 for 3117 new fundamental stars.

The European Space Agency also has plans for a next-generation astrometric satellite, called GAIA. Instead of orbiting the Earth, GAIA will be operated at the L1 Sun-Earth Lagrangian point, located 1.5 million km (0.9 million mi) from Earth in the direction away from the Sun. Here it will have a stable thermal environment and will be free from eclipses and occultations by the Earth. The purpose of the satellite will be repeatedly to measure positions of more than a billion stars to an accuracy of a few microarcseconds, the goal being to achieve 10-microarcsecond precision for stars as faint as 15th magnitude, with potentially four or five microarcseconds achievable for stars brighter than 10th magnitude. This will be more than a hundred times more accurate than the observations from Hipparcos.

astronautics Science of space flight. See also apollo programme; rocket astronomy

Astronomer Royal Honorific title conferred on a leading British astronomer. Until 1971 the Astronomer Royal was also the director of the royal greenwich observatory. Those who have held the position are listed in the accompanying table.

The post of Astronomer Royal for Scotland was created in 1834 initially to provide a director for the Royal Observatory, Edinburgh; the present holder, appointed in 1995, is John Campbell Brown (1947- ). The last Astronomer at the Cape, working originally at the Royal Observatory at the Cape of Good Hope (now in South Africa), was Richard Hugh Stoy (1910-94); the post was abolished in 1968. The post of Royal Astronomer for Ireland was created to provide a director for the dunsink observatory. It lapsed in 1921; the last incumbent was Henry Crozier Plummer (1875-1946).

Astronomical Almanac The Publication containing fundamental astronomical reference data for each calendar year. It includes positions of the Sun, Moon and planets, data for physical observations, positions of planetary satellites, sunrise and sunset times, phases of the Moon, eclipses, locations of observatories and astronomical constants. It is published jointly by the US Naval Observatory and Her Majesty's Nautical Almanac Office.

Astronomical Data Center (ADC) Part of the Space Sciences Directorate at NASA's goddard space flight center, located in Greenbelt, Maryland. The ADC specializes in archiving and distributing astronomical datasets, most of which are in the form of machine-readable catalogues rather than images. The Center's data collection is accessible via the World Wide Web.

Astronomical Journal Major USjournal for the publication of astronomical results. Founded in 1849 by Benjamin gould, the journal is published monthly by the University of Chicago Press, with a bias towards observational rather than theoretical papers. Historically, the 'AJ emphasized traditional fields of astronomy, such as galactic structure and dynamics, astrometry, variable and binary stars, and Solar System studies. It has now broadened its coverage to all aspects of modern astronomy.

Astronomical League (AL) Blanket organization for amateur astronomers in the United States. It is the largest astronomical organization in the world, with nearly 20,000 members from some 250 local societies. Its goal is to promote amateur astronomy through educational and observational activities. Founded in 1946, the AL holds a nationwide meeting every year and publishes a quarterly newsletter, The Reflector, which describes amateur activities throughout the country.

Astronomical Society of Australia Australian organization for professional astronomers, founded in 1966. The Society publishes a peer-reviewedjournal, PASA (Publications of the Astronomical Society of Australia). In 1997 it introduced an electronic counterpart (called el-PASA) to improve international access to Australian astronomical research.

Astronomical Society of the Pacific (ASP) Claimed to be the largest general astronomy society in the world, founded in 1889 by Californian amateur and professional astronomers. It retains its function as a bridge between the amateur and professional worlds, and it has developed into a leading force in astronomy education. It publishes a monthly magazine, Mercury, and original research is reported in the Publications of the ASP. The Society's headquarters are in San Francisco.

Astronomical Technology Centre (ATC) UK's national centre for the design and production of state-of-the-art astronomical technology. It is located on the site of the royal observatory edinburgh (ROE). It was created in 1998 to replace the instrument-building functions of both ROE and the royal greenwich observatory. The ATC provides expertise from within its own staff and encourages collaborations involving universities and other institutions. Customers include the gemini observatory (for which ATC was a prime contractor for the GMOS multi-object spectrometers), the Herschel Space Observatory, the UK infrared telescope and the james clerk maxwell telescope.

astronomical twilight See twilight

astronomical unit (AU) Mean Sun-Earth distance, or half the semimajor axis of the Earth's orbit, used as a unit of length for distances, particularly on the scale of the Solar System. Its best current value is 149,597,870.66 km.

The determination of the astronomical unit was one of the most important problems of astronomy until comparatively recent times. From the ancient Greeks up to the early 18th century, the Earth-Sun distance was known very imperfectly because it depended on very fine angular measurements that were beyond the capabilities of the available instruments. Johannes Kepler's best estimate, for example, was only one-tenth of the actual value. In the 18th and 19th centuries attempts to measure the AU were made by making accurate observations of Mars or Venus from two widely separated places on the Earth's surface, and calculating the planet's distance by triangulation. Kepler's third law was then used to compute the Sun-Earth distance. The first such attempt was made by Giovanni Domenico cassini, who got within 7% of the true value. Some improvement was achieved by measuring the distances of asteroids, especially close-approaching ones, such as 433 Eros. Such refinements were achieved in the late 19th century by David gill and by Harold Spencer jones in the 20th. The current technique makes use of radar measurements, and the probable error has been reduced by several orders of magnitude compared to the best visible-light values.

astronomy history of Astronomy is the oldest of the sciences and predates the written word. archaeoastronomy has shown that early peoples made use of astronomical observations in the alignment of their megalithic monuments.

Written records of astronomical observations in Egypt, Babylon and China (see egyptian astronomy; babylonian astronomy; chinese astronomy) and Greece date from pre-Christian times and it is to the Greeks that we owe the beginnings of scientific theorizing and systematic efforts to understand the Universe. The main legacy of Greek astronomy was a system of thought dominated by the work of aristotle, though many other Greek scientists of great originality contributed to early astronomical knowledge.

Greek ideas survived the medieval period, largely through the almagest of Ptolemy, a Graeco-Roman compendium of astronomical knowledge preserved by scholars in the Islamic world and the Byzantine Empire (see medieval european astronomy; islamic astronomy).

'Modern' astronomy can be said to have begun with the rejection by Nicholas copernicus of Aristotle's complex picture of planetary orbits, which assumed that the Sun was the centre of the Universe. Although Copernicus put forward a simpler and more logical view, which placed the planets at distances from the Sun that increased with their periods, its details were very complex and unconvincing to many of his contemporaries (see copernican system). Only after the meticulous observations of Tycho brahe and their interpretation by Johannes kepler did the new ideas begin to gain general acceptance (see renaissance astronomy).

The invention of the telescope and its exploitation by galileo began an era of astronomical discovery that completely destroyed the picture formulated by the ancients. Building on the mathematical and physical discoveries of the 17th century, Isaac newton produced a synthesis of ideas which explained earthly mechanics and planetary motions, based on his three laws of motion and his inverse-square law of gravitational attraction.

The last few centuries show an ever-increasing number of astronomical discoveries. As early as the 17th century, Ole romer showed that the speed of light is finite. Investigations of the heavens beyond the Milky Way began in the 1700s and are principally associated with William her-schel. He also began the trend towards building telescopes of greater and greater power, with which the Universe could be explored to ever-increasing distances.

The 19th century saw the development of astrophysics as the centre of astronomical interest: astronomers became interested in the physics of celestial phenomena and the nature of celestial bodies, rather than just describing their positions and motions. The development of stellar spectroscopy and its use by William huggins and others led the way to understanding the nature of the many nebulous objects in the sky. astrophotography emerged as a sensitive and reliable technique that allowed sky maps to be prepared in an objective and repeatable way. Sky surveys began to be made that showed up spiral nebulae and variable phenomena that had been beyond the reach of purely visual techniques.

At the start of the 20th century were two astounding discoveries that revolutionized physics and astronomy:

Max planck's realization that energy is quantized (1901), and Albert einstein's theory of special relativity (1905). They were the heralds of what was to come. Observational astronomy has grown exponentially ever since. The giant telescopes in excellent climates promoted by George Ellery hale soon led to the discovery by Edwin hubble that the spiral nebulae are external galaxies and that the Universe is expanding. The morphology of our own Galaxy began to be understood.

The great outburst of technological development associated with World War II led to the development of sensitive radio and infrared detectors that allowed the Universe to be investigated at wavelengths where new phenomena such as pulsars and quasars were discovered. In the late 1950s space probes had been developed that could take X-ray and gamma-ray detectors above the Earth's atmosphere and open yet further vistas to astrophysics. Even visible-light observations have been improved by the development of CCD detectors many times more sensitive than photographic plates.

The early 21st century is characterized by a new generation of giant telescopes and a vigorous investigation of conditions near the time when the Universe was formed. Massive data-handling techniques are allowing surveys of unprecedented detail to be made, searching for new phenomena hitherto unobserved or unobservable because of their faintness or rarity. See also amateur astronomy, history of; infinite universe, idea of

astrophotography Recording images of celestial objects on traditional photographic emulsion (film), as opposed to electronic means. From the late 19th century until the last two decades of the 20th, almost all professional astronomical observations were made using photography. From its introduction in 1839, it was 50 years before photography became accepted as a legitimate research tool by professional astronomers, and another quarter of a century before improvements to equipment and plates had made it vital to the progress of astronomical research.

What the human eye cannot see in a fraction of a second it will never see, whereas with both photography and electronic imaging light is accumulated over a period of time, revealing objects that are invisible even through the largest telescope, and creating a permanent record that is not affected by the observer's imagination or accuracy. However, early astrophotography suffered from insufficiently accurate telescope drives, lack of sensitivity ('speed') in film, and film's inability to record light equally well across the whole visible spectrum - for many years it was 'blind' to red.

Despite these drawbacks, the new technology was put to immediate use, mainly by enthusiastic amateurs. In this first form of practicable photography, a silvered copper plate sensitized with iodine vapour was exposed, developed with mercury vapour and fixed with sodium hyposulphite to produce a Daguerrotype, named after Louis-Jacques-Mande Daguerre (1789-1851), who perfected the process. The Moon was an obvious (because bright) target, and the first successful Daguerreotype of it is attributed to John draper (1840 March); the exposure time was 20 minutes, and the image showed only the most basic detail. Hippolyte fizeau and Leon fou-cault obtained a successful Daguerreotype of the Sun in 1845 April. In 1850 W.C. bond obtained excellent images of the Moon and the first star images, of Vega and Castor.

With the replacement of Daguerrotypes by the faster and more practical wet plate collodion technique in the early 1850s came a more systematic approach to astrophotography. John herschel proposed using photography on a daily basis to record sunspot activity, and this 'solar patrol' was taken up and directed by Warren de la rue. This trail-blazing work, which continued until 1872, was continued and developed by Walter maunder and Jules janssen. De La Rue subsequently obtained excellent images of the 1860 July 18 total solar eclipse, which proved that the prominences were solar and not lunar phenomena. In the United States, G.P. bond pursued stellar photography with improved equipment and considerable success.

An improvement came in the 1870s with the introduction of the more convenient dry photographic plates, and talented individuals such as Lewis rutherford and Henry draper in the United States, and subsequently Andrew common and Isaac Roberts (1829-1904) in Britain, pushed forward the frontiers of astrophotogra-phy. In 1880 Draper succeeded in taking a photograph of the Orion Nebula (M42), and a few years later Common and Roberts obtained excellent photographs of M42. There were also significant advances in photographic spectroscopy, by Draper, William huggins, E.C. pickering and others.

The 1880s marked photography's breakthrough in astronomy. In 1882 David gill obtained the first completely successful image of a comet. He was impressed by the huge numbers of stars on the images, accumulated in exposures lasting up to 100 minutes. With Jacobus kapteyn, he set out to construct a photographic star map of the southern sky; the resulting catalogue contained the magnitudes and positions of over 450,000 stars. Equally enthusiastic were the henry brothers at Paris Observatory, who helped to instigate the international project to prepare a major photographic sky chart that became known as the carte du ciel.

With the new century came a desire to discern the true nature of astronomical bodies by photographing their spectra. Increasing telescope apertures enabled greater detail to be resolved in the spiral nebulae by, for example, James keeler and George ritchey. Edwin hubble and others, using the 100-inch (2.5-m) Hooker Telescope at Mount Wilson Observatory, showed these objects to be other galaxies far beyond the Milky Way, and spectroscopy showed them to be receding from Earth, in an expanding Universe.

As the 20th century progressed, photographic materials became increasingly sensitive and pushed the boundaries of the known Universe ever outward. But by the end of the century the technique that had contributed so much to astronomy had been dropped far more rapidly than it was ever accepted, in favour of electronic imaging using ccds. Although astrophotography was well established and initially cheaper, their greater light sensitivity, linear response to light and digital output saw CCDs rapidly sweep the board. As CCDs have become larger and cheaper, electronic imaging has almost completely replaced astrophotography

Astrophotography at major observatories may be a thing of the past, but many amateur astronomers still use conventional film, despite the availability of CCDs and digital cameras, partly because of its simplicity but also because it continues to yield results of superlative quality at low cost. Some advanced amateurs use film for the exposures themselves, but then scan the images for manipulation on a computer, borrowing one of the main advantages of digital photography.

For astrophotography and CCD imaging alike, the telescope or camera must be driven so as to follow objects as they are carried across the sky by the Earth's rotation. The long exposure times required by photographic film, often over an hour, necessitate a means of monitoring the accuracy of tracking and making corrections to the drive rate: either a separate guide telescope or an off-axis guiding system. In simple systems the astrophotographer looks through an eyepiece equipped with cross-wires and adjusts the drive rate manually, while more advanced systems use an autoguider, which makes automatic corrections if the chosen guide star starts to drift out of the field of view. Autoguiders on amateur instruments incorporate a CCD whose purpose is to monitor the guide star - the imaging task may be left to the superior data acquisition of photography.

Virtually every type of film has its use in amateur astrophotography. An important characteristic of film is its graininess: the surface is covered with minute light-sensitive grains, which are visible under high magnification after exposure. Fine-grained films provide excellent sharpness and contrast, but are relatively insensitive to light. More sensitive films can record images with shorter exposure times, or fainter objects in a given exposure time, but have the disadvantage of coarse grain. Negative films generally record a wider range of brightness than transparency films, but require more effort afterwards to get a satisfactory print. This drawback is now largely overcome with the ready availability of film scanners, computer manipulation and high-quality colour printers. hypersensitization is an advanced technique to increase the sensitivity and speed of film for long exposures.

As CCDs become cheaper and capable of recording images of ever higher resolution, even amateur astropho-tography is declining in importance. It is rare to find advanced amateurs using film for planetary imaging or for supernova patrolling, for example. It remains ideal for aurora and total eclipse photography, and for general constellation shots. Many deep-sky photographs, particularly of wide fields, remain firmly in the province of film. It is not hard to foresee the day when even ordinary film is hard to find, but until then, photography will still have a place in amateur astronomy.

Astrophysical Journal Foremost research journal for theoretical and observational developments in astronomy and astrophysics. Dating from 1895, when it was launched by George Ellery hale and James keeler, the 'ApJ reported many of the classic discoveries of the 20th century. In 1953 The Astrophysical Journal Supplement Series was introduced to allow the publication of extensive astronomical data papers in support of the main journal.

astrophysics Branch of astronomy that uses the laws of physics to understand astronomical objects and the processes occurring within them. The terms 'astronomy' and 'astrophysics' are however often used as synonyms. Astrophysics effectively dates from the first applications of the spectroscope to study astronomical objects. Sir William huggins' identification of some of the chemical elements present in the Sun and stars in the 1860s was the first major result of astrophysics. Today, the use of computers for data processing, analysis and for modelling objects and processes is an integral part of almost all aspects of astrophysics spectroscopy is the most powerful technique available in astrophysics. Individual atoms, ions or molecules emit or absorb light at characteristic frequencies (or 'lines', from their appearance in a spectroscope) and can thus be identified and their quantity assessed. Information on the physical and chemical state of an object can be obtained from an analysis of the way its spectrum is modified by the collective environmental conditions within the object. Line-of-sight velocities of the whole object or regions within it are indicated by the doppler shift. The most intensively studied spectral region is the ground-accessible 'optical' region (the near-ultraviolet to the near-infrared), but much information now comes from observations in the radio, microwave, far-ultraviolet, x-ray and gamma-ray regions.

Stellar spectra characteristically show a continuous bright background that by its colour distribution approximately indicates the 'surface' temperature (but is made up of radiation contributed in some measure from throughout the whole of the stellar atmosphere). Several different processes produce these continua. In cooler stars (like the Sun) the interaction chiefly involves electrons and hydrogen atoms; in hotter stars, electrons and protons; and in the hottest stars, electrons alone. The dark spectral lines superimposed on the continuum, which are due to atoms, ions and molecules, strictly do not arise from absorption by a cooler outer layer as a simple visualization suggests, but in a complex depth-dependent manner in which radiation dominantly escapes from outer and therefore cooler (darker) regions of the atmosphere near the centre of an absorption line, or deeper, hotter (brighter) regions for the continuum. The fact that the lines appear dark indicates that the outer layers are cooler, and indeed the temperature generally is expected to decrease outwards from the centre of any star. For the Sun and certain other stars some bright lines are also seen, particularly in the far-ultraviolet region, indicating that in these the temperature is rising again in extended, extremely tenuous layers of the outer atmosphere. The strengths, widths and polarization of spectral lines give information on the chemical composition, ionization state, temperature structure, density structure, surface gravity, magnetic field strength, amount of turbulent motion within the atmosphere of the star and stellar rotation.

A striking result of astronomical spectroscopy is the uniformity of chemical composition throughout the Universe (see astrochemistry), extending even to the most distant galaxies. Although the atomic constituents of the Universe are everywhere the same, the proportions of these elements are not identical in all astronomical objects. Measurements of these differences give information on the nuclear processes responsible for the formation of the elements and the evolutionary processes at work. A star is kept in a steady state for a long time by the balance of the inward pull of gravity and the opposing force arising from the increase of pressure with depth that is maintained by a very high central temperature. For a star like the Sun (mass of 2 x 1030 kg, radius 700,000 km/435,000 mi) the central temperature is about 15,600,000 K. The energy from such hot matter works its way tortuously outwards through the star until it is radiated away from the atmospheric region near its surface. This process yields a relationship between the masses and luminosities of stars, with the luminosity increasing very dramatically with mass. Thus the luminosities of stars within the relatively small range of masses between 0.1 and 10 times that of the Sun differ by a factor of over a million. The high rate of loss of energy requires a powerful energy-producing process within the star, identified as thermonuclear fusion of light atomic nuclei (mainly hydrogen nuclei) into heavier ones (mainly helium nuclei). The transformation of hydrogen into helium in the case of the Sun - a small star - makes enough energy available to enable it to shine as it does today for a total of about 1010 years. In contrast, the lifetime of massive, very luminous stars, which burn up their hydrogen very rapidly, can be as short as a million years. During this burning process the core contracts, and it eventually becomes hot and dense enough for helium itself to ignite and to form carbon and oxygen. Although the contraction of the core is thus halted, the star then has a rather complicated structure, with two nuclear burning zones, and may become unstable. Later stages in the evolutionary cycle present many possibilities depending principally on the initial mass of the star. It may become a white dwarf, such as the companion of Sirius, which is a very dense, hot star with about the mass of the Sun but a radius nearer that of the Earth. The immense pressures built up in such stars result in the matter forming them becoming degenerate.

More massive stars than the Sun progress to advanced stages of nucleosynthesis, experiencing ignition of successively heavier nuclei at their cores. In some rare cases much of the outer layers of the star may be violently ejected in a supernova outburst in which heavy elements (some synthesized in the explosion) are scattered widely into the general interstellar medium. Out of this enriched material later generations of stars are formed. The residues of these events may be neutron stars, which can be observed as pulsars, and represent a still higher state of compression than white dwarfs, with electrons and protons crushed together to form a degenerate neutron gas. On the other hand, a star may go on contracting until it becomes a black hole, whose violent accretion of nearby material makes its influence observable by strong emission in the X-ray region.

A great deal of theoretical work has gone into explaining how the distribution of radiation over the spectrum of a star depends on the structure of its outermost layers, and, more broadly, the processes occurring during the life cycles of stars. The latter includes the stars' formation from collapsed regions of the interstellar medium, their active burning phases, and their deaths, which can range from being an event in which a star mildly fades away to being a violent explosion that at its maximum brightness can appear as brilliant as a whole galaxy.

Such applications of physics to astronomical problems in general are progressing strongly on most fronts and much success has been achieved. Currently a great deal of attention is being directed towards the processes that occurred during the earliest phases of the formation of the Universe, very close to the time of the Big Bang. But here astronomers and physicists are venturing out hand in hand, for not only is our knowledge of the behaviour of the Universe incomplete, but so too is our knowledge of physics itself. New physical theories unifying all forces of nature are needed before we can begin to understand the most fundamental properties of the evolving Universe and, indeed, the origin of matter itself.

Asuka Japan's fourth cosmic X-ray astronomy satellite. It is called the Advanced Satellite for Cosmology and Astrophysics (ASCA), also known as Astro D. It was launched in 1993 February, equipped with four large-area telescopes. ASCA was the first X-ray astronomy mission to combine imaging capability with a broad-wavelength bandwidth, good spectral resolution and large-area coverage, as well as the first to carry charge-coupled devices (CCDs) for x-ray astronomy.

Atacama Large Millimetre Array (ALMA) Millimetre-wavelength telescope planned by a major international consortium that includes the national radio astronomy observatory (USA), the particle physics and astronomy research council (UK) and the european southern observatory, planned for completion in 2010. The instrument, at an elevation of 5000 m (16,400 ft) at Llano de Chajnantor in the Chilean Andes, will consist of at least 64 antennae 12 m (39 ft) in diameter, allowing resolution to a scale of 0".01. The high altitude will give ALMA access to wavebands between 350 |jum and 10 mm, otherwise accessible only from space.

ataxite See iron meteorite

ATC Abbreviation of astronomical technology centre

Aten asteroid Any member of a class of asteroids that, like the apollo asteroids, cross the orbit of the Earth, but that are distinguished by having orbital periods of less than one year. In consequence they spend most of the time on the sunward side of our planet. The archetype is (2062) Aten, discovered in 1976, although an asteroid of this type was also discovered in 1954 and subsequently lost. By late 2001 over 120 Aten-type asteroids were catalogued. Characteristics of the following are listed in in the near-earth asteroid table: 1954 XA, (2062) Aten, (2100) Ra-Shalom and (2340) Hathor. See also intraterrestrial asteroid

Athena Small US satellite launch vehicle powered by solid propellants. It was originally called the Lockheed Martin Launch Vehicle (LMLV). There are two Athena models, with two and three stages, respectively, capable of placing satellites weighing a maximum of 2 tonnes into low Earth orbit. The booster has flown six times since 1995, with four successful launches. The launcher has now been discontinued.

Atlantis One of NASA's space shuttle orbiters. Atlantis first flew in 1985 October.

Atlas Lunar crater (47°N 44°E), 88 km (55 mi) in diameter, located to the east of Mare Frigoris. It forms a notable pair with the smaller crater Hercules to the west. Atlas' walls vary in altitude between 2750 and 3500 m (9000-11,500 ft). The floor of Atlas is uneven and has been observed to 'glitter'. It contains a group of central hills, which resemble a ruined crater ring, many craterlets and a very noticeable cleft on the east side. To the north is O'Kell, a large, heavily eroded, ancient ring.

Atlas One of the small inner satellites of saturn, discovered in 1980 by Richard Terrile during the voyager missions. It is spheroidal in shape, measuring about 38 X 34 X 28 km (24 X 21 X 17 mi). Atlas has a near-circular equatorial orbit just outside Saturn's a ring, at a distance of 137,700 km (85,600 mi) from the planet's centre, where its orbital period is 0.602 days. It appears to act as a shepherd moon to the outer rim of the A ring.

Atlas One of the USA's primary satellite launchers. It began its career as the country's first intercontinental ballistic missile in 1957. The Atlas was equipped with a Centaur upper stage in 1962 and, since that failed maiden flight, has operated in a number of Atlas Centaur configurations, making over 100 flights. This configuration later became known as the Atlas 2, 2A and 2AS, which by 2001 had together made almost 50 flights. The new Atlas 3, which first flew in 1999, and the Atlas 5 fleets (there is no Atlas 4) will use a Russian first-stage engine. The Atlas 2 fleets will be retired in 2002, leaving the Atlas 3 and 5 offering launches to geostationary transfer orbit within the 4- to 8.6-tonne range. The Atlas 5 has been built primarily as a US Air Force launcher, under the Evolved Expendable Launch Vehicle programme.

Atlas An Atlas 2 is launched in 1966. The rocket is carrying an Agena target vehicle towards a rendezvous with Gemini 11.

atlas, star Collection of charts on which are plotted the positions of stars and, usually, numbers of deep-sky objects, over the whole of the sky or between certain limits of declination, down to a certain limiting magnitude. The classic star atlases of the 17th and 18th centuries in particular are now appreciated for their beauty. The 19th century saw a division appear between atlases produced for professional and for amateur astronomers, and the emergence of the photographic atlases. Today's professionals have little use for printed atlases, and amateurs are beginning to rely more on virtual atlases than on printed ones.

Until the 17th century, accuracy of position in atlases was often the victim of artistic licence, stars being placed where they looked best on intricately drawn constellation figures. The first truly modern atlas was Johann BAYER's Uranometria of 1603, which plotted accurate positions from Tycho BRAHE's star catalogue, distinguished between different magnitudes, and introduced the practice of labelling stars with Greek letters (see BAYER LETTERS); the allegorical constellation figures were depicted faintly to the stars. Other notable atlases, based on observations by their originators, were those of Johannes HEVELIUS and John FLAMSTEED. The last great general all-sky 'artistic' star atlas was Johann BODE's Uranographia (1801), which showed over 17,000 stars and nebulae on huge pages with beautifully engraved constellation figures.

The 19th and 20th centuries saw the introduction of photographic atlases for the professional, such as the carte du ciel and the PALOMAR OBSERVATORY SKY SURVEY, and the appearance of atlases for amateur observers. The most venerable is perhaps Norton's Star Atlas, by Arthur Philip Norton (1876-1955), first published in 1919 and with its 20th edition planned for 2002. The lavish two-volume Millennium Star Atlas (R.W. Sinnott and M. Perryman, 1997), showing stars to magnitude 11, may be the last such large printed star atlas for the amateur observer, now that virtual atlases are readily available (see SOFTWARE).

atmosphere Gaseous envelope that surrounds a planet, satellite or star. The characteristics of a body that determine its ability to maintain an atmosphere are the temperature of the outer layers and the ESCAPE VELOCITY, which is dependent on the body's mass. Small bodies, such as the Moon, Mercury and the satellites of the planets in our Solar System (apart from TITAN and TRITON), do not have any appreciable permanent atmosphere. The escape velocity for these bodies is sufficiently low for it to be easily exceeded by gas molecules travelling with the appropriate thermal speeds for their masses and temperature. The speed of a gas molecule increases with temperature and decreases with the molecular weight. Consequently, the lighter molecules, such as hydrogen, helium, methane and ammonia, escape more readily to space than the heavier species, such as nitrogen, oxygen and carbon dioxide. MERCURY has a transient, tenuous atmosphere, consisting of material that it captures from the SOLAR WIND, which it retains for a short period. In the cold outer reaches of the Solar System, PLUTO and Triton have potential atmospheric gases frozen out on their surfaces. When Pluto is close to perihelion (as in the late 20th century), the gases sublime to produce a tenuous atmosphere of methane and ammonia. It is estimated that these gases will refreeze on to the surface by about the year 2020.

The primary atmospheres of the bodies in the Solar System originated from the gaseous material in the SOLAR NEBULA. The lighter gases were lost from many of these objects, particularly the terrestrial planets VENUS, EARTH and MARS, which now have secondary atmospheres formed from internal processes such as volcanic eruptions. The main constituent of the atmospheres of Venus and Mars is carbon dioxide (CO2); the main constituents of the Earth's atmosphere are nitrogen and oxygen (approximately 78% and 21% by volume, respectively). Free oxygen in the Earth's atmosphere is uniquely abundant and is believed to have accumulated as a result of photosynthesis in algae and plants. Earth's is thus regarded by some authorities as a tertiary atmosphere. The only other significant nitrogen atmosphere found in the Solar System is on Titan, the largest satellite of Saturn, although nitrogen gas produces the active geyser-like plumes observed on Triton. Only the giant outer planets, JUPITER, SATURN, URANUS and NEPTUNE, still retain their primordial atmospheres of mainly hydrogen and helium. However, these major planets are also surrounded by envelopes of escaped hydrogen. Titan, too, is associated with a torus of neutral hydrogen atoms, which have escaped from the satellite's upper atmosphere and populate the satellite's orbit. It is also possible for a body to gain a temporary atmosphere through a collision with an object containing frozen gases, such as a cometary nucleus.

The gaseous layers of an atmosphere are divided into a series of regions organized on the basis of the variation of temperature with altitude. For worlds with definite solid surfaces, such as the Earth, Venus, Mars and Titan, the levels start from the ground and relate to existing observations. The outer planets are huge gaseous bodies with no observable solid surface, and with atmospheres that have been investigated to just beneath the cloud tops. Consequently, the characteristics of the lowest layers are based upon theoretical models and the levels usually start from a reference level at which the pressure is 1 bar. The names given to the various structured layers of the terrestrial atmosphere are also applied to all other planetary atmospheres. The precise variation of the atmospheric temperature with altitude depends on the composition of the atmosphere and the subsequent solar (and internal) heating and long-wave cooling at the various levels.

The lowest layer is the TROPOSPHERE, which on the Earth extends from sea level, where the average pressure is 1 bar, to the TROPOPAUSE,at an altitude of between 5-8 km (3-5 mi) at the poles and 14-18 km (9-11 mi) at the equator. This region contains three-quarters of the mass of the atmosphere; it is the meteorological layer, containing the cloud and weather systems. The troposphere is heated from the ground and the temperature decreases with height to the tropopause, where the temperature reaches a minimum of approximately 218 K at the poles and 193 K over the equator. Above the tropopause, the temperature increases with height through the STRATOSPHERE because the heating from ozone (03) absorption dominates the long-wave cooling by CO2. The temperature increases to about 273 K at an altitude of 50 km (31 mi), which marks the stratopause. In the Earth's atmosphere, the region between 15 and 50 km (9 and 31 mi), where the ozone is situated, is often called the ozonosphere. Beyond the stratopause the temperature again decreases with altitude through the MESOSPHERE to the atmospheric minimum of 110-173 K at the mesopause, which lies at an altitude of 86 or 100 km (53 or 62 mi) at different seasons. The atmosphere at these levels is very tenuous and is therefore sensitive to the heating from the Sun. Consequently, this region and the further layers of the atmosphere have a significant diurnal temperature cycle. Beyond the mesopause, we enter the THERMOSPHERE, where the temperature increases with height until it meets interplanetary space. The heating that produces this layered structure is primarily created by the absorption of far-ultraviolet solar radiation by oxygen and nitrogen in the atmosphere. Solar X-rays also penetrate through to the mesosphere and the upper atmospheric layers, which are therefore sensitive to changes in the solar radiation and atmospheric chemistry. The solar ultraviolet radiation photo-ionizes the atmospheric constituents in these outer layers to produce ionized atoms and molecules; this results in the formation of the IONOSPHERE at an altitude of about 60-500 km (40-310 mi). This region is also the domain of METEORS and AURORAE, which regularly occur in the upper atmosphere. Above 200-700 km (120-430 mi) is the EXOSPHERE, from which the atmospheric molecules escape into space.

The individual planetary atmospheres each possess some or all of these basic structures but with specific variations as a consequence of their size, distance from the Sun, and atmospheric chemistry. The massive CO2 atmosphere of Venus has a deep troposphere, which extends from the surface to an altitude of about 100 km (62 mi). About 90% of the volume of the atmosphere is contained in the region between the surface and 28 km (17 mi). Venus has no stratosphere or mesosphere but does have a thermosphere. Its planetary atmosphere is, therefore, quite different from that of the Earth.

By terrestrial standards, the Martian atmosphere is very thin, since the surface pressure is only 6.2 millibars. The atmosphere does have some similarity in the vertical layering, however, with the presence of a troposphere, stratosphere and thermosphere. During the global dust storms, the structure of the Martian troposphere may change dramatically to an isothermal layer (that is, one with a constant temperature throughout) or may even display an inversion. However, there is much less ozone in the Martian atmosphere than in that of Earth, so there is no reversal in the temperature gradient as there is in the terrestrial stratosphere. Lower temperatures are found in the Martian thermosphere as a consequence of the CO2 cooling effect.

The basic structure of the atmospheres of the outer planets resembles that of the Earth. There is a troposphere, which extends to unknown depths beneath the clouds, and a well-defined stratosphere, created by the heating due to absorption of solar radiation by methane (CH4). Knowledge of the higher levels of the atmospheres is still incomplete. Titan has a well-defined atmospheric structure, comprising a troposphere, stratosphere and thermosphere, created more by the complex aerosol chemistry than by gaseous composition alone.

These atmospheric structures vary on a local scale as a consequence of the planetary weather systems. The atmosphere of the Earth receives its energy from the Sun. The atmosphere (including clouds) reflects 37% of the incident radiation back to space and absorbs the remainder, which becomes redistributed so that 48% of the incident radiation actually reaches the Earth's surface, which then reflects 5% of the total directly back to space. The surface reradiates most of its absorbed energy back into the atmosphere as long-wave radiation, with a small amount (8%) passing directly to space. The heated atmosphere reradiates some of the long-wave radiation to the surface while a portion (50%) is ultimately lost to space. The atmosphere behaves like a giant heat engine, trying to balance the absorbed solar energy with the long-wave radiation emitted to space: the small imbalances that are always present give rise to the weather, and any longer-term variations produce changes in climate. In the Earth's atmosphere, clouds, which cover approximately 50% of the surface, are a key factor in weather and climate because a small change in their geographical and vertical distribution will have a profound effect on the environment.

Exploration of the Solar System has now provided a unique opportunity to examine the weather systems of all the planetary atmospheres. They have very different properties: Venus' atmosphere is slowly rotating and totally covered with cloud; Mars' thin atmosphere is strongly affected by the local topography and annual global dust storms; and the outer planets Jupiter, Saturn and Neptune have huge, rapidly rotating gaseous envelopes and meteorologies that are largely driven by the planets' internal sources of energy. These planetary atmospheres of the Solar System are natural laboratories for investigating geophysical fluid dynamics.

atmospheric extinction Reduction in the brightness of light from astronomical objects when it passes through the Earth's atmosphere. The atmosphere attenuates the light from these objects so that they appear fainter when seen from the surface than they would outside the atmosphere. This is caused by absorption and scattering by gas molecules, dust and water vapour in the atmosphere. The amount of extinction is variable and can be determined by measuring the brightness of standard stars photometrically. Atmospheric extinction is most pronounced close to the horizon, where astronomical bodies are observed through a greater volume of air.

atmospheric pressure Force exerted by the gas forming an atmosphere on a unit area. The units are pascals (Pa), with 1 Pa=1 N/m2. Commonly encountered non-SI units are the bar or millibar (1 bar = 105 Pa) and the atmosphere (1 Atm = 101,325 Pa). Sea-level pressure on the Earth is around 105 Pa, surface pressures on Venus and Mars are 9.2 X 106 Pa and 620 Pa respectively. Atmospheric pressure represents the weight of a vertical column of the atmosphere whose cross-section is 1 m2.

atmospheric refraction Small increase in the apparent altitude of a celestial object, as viewed by an observer on the surface of the Earth, caused by light from the object changing direction as it passes through the Earth's atmosphere. When light passes from one medium to another it is bent, or refracted, and the same is true when light from a star passes from the vacuum of space into the Earth's atmosphere. The result is to make the object appear at a higher altitude than is really the case, and the nearer it is to the horizon, the more pronounced the effect, even causing objects below the horizon to appear visible. The degree by which the light is refracted is also dependent on atmospheric pressure and temperature. All astronomical observations have to be corrected for atmospheric refraction to obtain true, as opposed to apparent, positions.

ATNF Abbreviation of australia telescope national facility

atom Smallest part of an element. Atoms have a nucleus containing protons and neutrons, together with a surrounding cloud of electrons (see atomic structure). The nucleus is positively charged, and contains almost all the mass of the atom. The mass of the atom is called the atomic mass, symbol A, and is given approximately in atomic mass units by the total number of protons and neutrons in the nucleus. The number of protons in the nucleus is the atomic number, symbol Z, and it determines the element to which the atom belongs. In the normal atom there are equal numbers of protons and electrons, so that the atom as a whole is electrically neutral. If an atom loses or gains one or more electrons it becomes an ion. The number of neutrons in the nucleus may vary and results in the different isotopes of the element.

There are a number of variations in the nomenclature for atoms. Amongst the commonest are the chemical symbol plus atomic number and mass as superscripts and subscripts, for example: Fe2565.847, 26Fe55.847 or 26Fe55.847. Here, the atomic mass is the average for the element as it is found on Earth with the isotopes in their natural abundances. When an individual isotope is symbolized, the atomic mass will be much closer to a whole number. Thus the naturally occurring isotopes of iron on Earth are Fe53 9396 ( 5.8%), Fe55 9349 (91.7%) and Fe56.9354 (2.2%). An isotope may also be written down as the element name or symbol followed by the number of nuclear particles, for example iron-56 or Fe-56 or in the previous notation Fe56.

atomic mass unit (amu) Unit used for the masses of ATOMS. It is defined as one-twelfth of the mass of the carbon-12 isotope. It is equal to 1.66033 X 10—27 kg.

atomic structure The gross structure of an ATOM comprises a nucleus containing PROTONS and NEUTRONS, together with a surrounding cloud of ELECTRONS. The nucleus has a density of about 2.3 X 1017 kg/m3 and is a few times 10—15 m across. The atom as a whole has a size a few times 10—11 m (note that neither the size of the nucleus nor of the atom can be precisely defined). Although there is structure inside the nucleus, the term 'atomic structure' is usually taken to mean that of the electrons.

atomic structure The Bohr–Sommerfeld model of atomic structure visualizes electrons orbiting around the nucleus in discrete shells. The first three shells are: K (blue), which can contain a maximum of two electrons; L (red), which can contain a maximum of eight electrons; and M (yellow), which has a maximum of eighteen.

Quantum mechanics assigns a probability of existence to each electron at various points around the nucleus, resulting in electron orbitals. While the quantum mechanical description may be needed to describe some aspects of the behaviour of atoms, the conceptually simpler Bohr-Sommerfeld model is adequate for many purposes. This model was proposed in 1913 by the Danish physicist Niels Bohr (1885-1962). It treats the subatomic particles as though they are tiny billiard balls and the electrons move in circular orbits around the central nucleus. Different orbits correspond to different energies for the electrons, and only certain orbits and hence electron energies are permitted (that is, the orbits are quantized). Arnold Sommerfeld (1868-1951, born in East Prussia at Konigs-berg, now Kaliningrad) modified the theory in 1916 by adding elliptical orbits. When an electron changes from one orbit to another a PHOTON is emitted or absorbed, the wavelength of which corresponds to the energy difference between the two levels. Since the orbits are fixed, so too are the energy differences, and thus the photons are emitted or absorbed at only specific wavelengths, giving rise to the lines observed in a SPECTRUM.

The electron energies are determined by quantum numbers; the principal quantum number, n, the azimuthal quantum number, l, the magnetic quantum number, mv and the spin, ms. These numbers can take values: n = 1, 2, 3, 4, ... °o; l =0, 1, 2, ... n — 1; ml = 0, ±1, ±2, ... ±l; ms = ±i. The value of n determines the shell occupied by the electron, with n =1 corresponding to the K shell, n =2 to the L shell, n = 3 to the M shell, and so on. The value of l determines the ellipticity of the orbit, l = n - 1 gives a circular orbit; smaller values of l correspond to increasingly more elliptical orbits.

The PAULI EXCLUSION PRINCIPLE requires that no two electrons within an atom can have the same set of quantum numbers, and its operation determines the electron structure of the atom and so the properties of the chemical elements. For n = 1, the other quantum numbers must be l = 0, m = 0, and ms = ±\. The K shell can thus contain at most two electrons, with quantum numbers, n = 1, l =0, m = 0, ms = +i and n = 1, l = 0, ml = 0, and ms = —\. Thus we have helium with two electrons in the K shell and a nucleus with two protons and (normally) two neutrons. Hydrogen has just one electron in the K shell. But the third element, lithium, has to have one electron in the L shell as well as two in the K shell, and so on. The maximum number of electrons in each shell is 2n2, so the L shell is filled for the tenth element, neon, and the eleventh, sodium, has two electrons in the K shell, eight in the L shell, and one in the M shell. Atoms with completed shells are chemically very unreactive, giving the noble gases, helium, neon, argon, krypton, xenon and radon.

atomic time System of accurately measuring intervals of time based on the transitions between energy levels of the caesium-133 atom. When an atom of caesium changes from a lower-energy level to a higher one, it absorbs radiation of a very precise frequency, namely 9,192,631,770 Hz. This varies by less than one part in 10 billion and has therefore been used to define the basic unit of time, the SI second, which is used in international timekeeping. In 1967 the SI second was defined as being 'the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the caesium-133 atom'.

Prior to this, the second was defined in terms of the mean solar day and astronomical observations provided a definitive measure of time to which clocks were accordingly adjusted to keep them in step. The Earth, however, is not a good timekeeper; its rotation rate is irregular and slowing due to the effects of tidal braking. With the advent of more accurate quartz clocks, followed in 1955 by the caesium-beam atomic clock, it therefore became necessary to re-define the standard unit for measuring time. Today's atomic clocks are now accurate to one second within a few thousand years and are used to form INTERNATIONAL ATOMIC TIME (TAI). See also TIMEKEEPING

AU Abbreviation of ASTRONOMICAL UNIT

aubrite (enstatite achondrite) Subgroup of the ACHONDRITE meteorites. Aubrites are highly reduced meteorites with mineralogies and oxygen isotopic compositions similar to those of ENSTATITE CHONDRITES, leading to the suggestion that aubrites might have formed by partial melting of an enstatite chondrite precursor.

Aura Abbreviation of ASSOCIATION OF UNIVERSITIES FOR RESEARCH IN ASTRONOMY

Auriga See feature article

aurora Illumination of the night sky, known popularly as the northern and southern lights, produced when electrons accelerated in Earth's MAGNETOSPHERE collide with atomic oxygen and molecular and atomic nitrogen in the upper atmosphere, at altitudes in excess of 80 km (50 mi). These collisions produce excitation, and as the atmospheric particles return to their ground state, they re-emit the excess energy as light at discrete wavelengths; aurorae are often coloured green and red.

Geographically, the aurora is present more or less permanently in two oval regions, encircling either geomagnetic pole. The ovals show a marked offset, being on average 20° from the pole on the dayside, 30° on the nightside. Under normal conditions, the ovals remain comparatively narrow and are confined to high latitudes. Release of stress within the magnetosphere gives rise to occasional brighten-ings of the ovals, with a westward-travelling surge of increased activity during substorms, of which as many as five may occur each day even in quiescent conditions.

AURIGA (gen. aurigae, abbr. aur)

The rather more major disturbances brought about by geomagnetic storms, following the arrival of energetic, highly magnetized plasma thrown from the Sun during coronal mass ejection or flare events, can dramatically alter the configuration of the auroral ovals. During the biggest disturbances, each oval brightens and broadens, particularly on the nightside, pushing auroral activity towards the equator. During such expansions, the aurora can become visible from lower latitudes, such as those of the British Isles and southern USA. Aurorae at these latitudes are most commonly seen a year or so ahead of sunspot maximum, with a secondary, less intense peak 12-18 months after sunspot maximum.

During a major display at lower latitudes, the aurora may first appear as a structureless glow over the polewards horizon. The glow may rise higher, taking on the form of an arc, with folding giving rise to a ribbon-like structure described as a band. Arcs and bands may be homogeneous, lacking internal structure, or may show long, vertical striations known as rays. Individual rays can appear like searchlight beams, stretching over the horizon. Where a rayed band has a high vertical extent, its movement gives rise to the commonly portrayed 'curtain' effect. If a display is particularly intense, the rays and other features may pass overhead and on into the equa-torwards half of the sky. At this stage - which is comparatively rare in displays at lower latitudes - the aurora takes on the form of a corona, with rays appearing to converge on the observer's magnetic zenith as a result of perspective. Corona formation often marks the short-lived peak of an auroral display, following which activity again retreats polewards. In the most major low-latitude storms - perhaps three or four times in each roughly 11-year sunspot cycle - activity may go on all night, with several coronal episodes separated by quieter interludes.

As well as showing occasionally rapid movement, auroral features change in brightness. Some changes are slow (pulsing), others rapid (flaming - waves of brightening sweeping upwards from the horizon). No two displays are ever quite the same, and at lower latitudes the most an observer might see on many occasions are simply the upper parts of a display as a horizon glow towards the pole. It is from this appearance, the 'northern dawn' (a description first used in the 6th century by Gregory of Tours), that the aurora borealis takes its name.

Increased auroral activity is often found in the declining years of the sunspot cycle at times when Earth becomes immersed in persistent high-speed solar wind streams. These coronal hole-associated aurorae are much less dynamic and extensive than the events that follow coronal mass ejections, and they often recur at intervals of 27 days, roughly equivalent to the Sun's rotation period as seen from Earth. Activity from these displays usually comprises a quiet homogeneous arc, with only occasional rayed outbursts, and often lasts for several successive nights.

Auroral rayed bands may have a vertical extent of several hundred kilometres and a lateral extent covering tens of degrees in longitude. Their base extent in latitude, however, is only a few kilometres, reflecting the narrow field-aligned sheets along which electrons undergo their final acceleration in the near-Earth magnetosphere before impacting on the upper atmosphere.

The aurora shows an emission spectrum that is brightest at its red end. The human eye, however, is most sensitive to the green oxygen 557.7 nm emission, which predominates in the aurora at altitudes of about 100 km (62 mi). Higher up, from about 150-600 km (90-370 mi) altitude, red oxygen emissions are found; auroral rays often show a colour gradation from green at their base to red at the top. At extreme altitudes of 1000 km (600 mi), molecular nitrogen can produce purple emissions under further excitation by solar ultraviolet radiation. During intense displays, the lower border of bands may show red nitrogen emissions.

Ionization produced during auroral activity enhances the ionospheric E layer at 112 km (70 mi) altitude. Radio Northern constellation, representing a charioteer (possibly Erichtonius, legendary king of Athens and inventor of the four-horse chariot), between Perseus and Gemini. It is easily recognized by virtue of capella, at mag. 0.1 the sixth-brightest star in the sky. epsilon aurigae is a remarkable eclipsing binary, mag. range 3.7-4.0, period 27.1 years - the longest of any such known star; another eclipsing binary is zeta aurigae. The constellation's deep-sky objects include the open clusters M36, M37 and M38, which each contain several dozen stars fainter than mag. 8-9 operators can use the aurora to scatter short-wave signals over longer-than-normal distances to make 'DX' contacts. Doppler shifts allow radar measurements of the particle motions in auroral arcs.

References to the aurora are found in pre-Christian, Greek, Chinese, Japanese and Korean texts. European medieval church records often mention 'battles' in the sky, equating the auroral red emissions with blood; such imagery also appears in Viking chronicles. Treated with due caution, such records provide useful information on solar-terrestrial activity from pre-telescopic times.

Scientific investigations of the aurora began in earnest in the 18th century, with the work of Edmond halley, Jean-Jacques de Mairan (1678-1771) and others. The first modern account of the aurora australis was made by Captain Cook in 1773. The long-suspected connection between solar activity and the aurora was confirmed when a major solar flare was observed in 1859, followed a day or so later by a huge auroral storm. Theoretical work and experiments by Kristian Birkeland (1867-1917) and Carl Stormer (1874-1957) in the early 20th century began to clarify some of the mechanisms by which the aurora occurs. Stormer undertook an intensive programme of parallactic auroral photography from Norway beginning in 1911. Great progress in understanding the global distribution of auroral activity was made during the international geophysical year of 1957-58. Current models owe much to the work of the Japanese researcher Syun-Ichi Akasofu (1930- ).

Continuing investigations of the aurora, which is the most visible of several effects on near-Earth space resulting from the Sun's varying activity, are further aided by observations from spacecraft, which can directly sample particles in the solar wind and magnetosphere, and image the auroral ovals from above.

Australia Telescope Compact Array See Australia telescope national facility

Australia Telescope Long Baseline Array See australia telescope national facility

Australia Telescope National Facility (ATNF) Set of eight radio telescopes that can be used either individually or together. Six of the telescopes, at Narrabri in New South Wales, constitute the Australia Telescope Compact Array. Each dish has an aperture of 22 m (72 ft). A seventh 22-m antenna, the Mopra Telescope, is located near siding spring observatory, 100 km (60 mi) to the south-west. The final component is the parkes radio telescope, a further 220 km (140 mi) away. When all eight instruments are used together they constitute the Australia Telescope Long Baseline Array. The ATNF is funded by the Commonwealth Scientific and Industrial Research Organisation (CSIRO).

autoguider Electronic device that ensures a telescope accurately tracks the apparent movement of a celestial object across the sky. Although a telescope is driven about its axes to compensate for the rotation of the Earth, it is still necessary to make minor corrections, particularly during the course of a long observation. An autoguider achieves this by using a photoelectric device, such as a quadrant photodiode or a CCD, to detect any drifting of the image. The light-sensitive surface of the detector is divided into four quarters, with light from a bright guide star in the field of view focused at the exact centre. If the image wanders from the centre into one of the four quadrants, an electrical current is produced. By knowing from which quadrant this emanated, it is possible to correct the telescope drive to bring the image back to the exact centre again.

autumnal equinox Moment at which the Sun's centre appears to cross the celestial equator from south to north, on or near September 23 each year at the first point of libra. At the time of the equinox, the Sun is directly overhead at the equator and rises and sets due east and due west respectively on that day, the hours of daylight and darkness being equal in length. The term is also used as an alternative name for the First Point of Libra, one of the two points where the celestial equator intersects the ecliptic. The effects of precession cause these points to move westwards along the ecliptic at a rate of about one-seventh of an arcsecond per day. See also vernal equinox

Auwers, (Georg Friedrich Julius) Arthur (1838-1915) German astronomer, a specialist in astrometry, and a founder of Potsdam Astrophysical Observatory. He compiled many important star catalogues, and computed the orbits of the binaries Sirius and Procyon before astronomers confirmed the existence of those bright stars' faint companions by direct observation.

Auzout, Adrien (1622-91) French astronomer who, independently of William gascoigne, invented the micrometer. He developed the wire micrometer for measuring separations in the eyepiece, and was one of the first to fit graduated setting circles to telescopes to facilitate the measurement of coordinates. As a member of the 'Paris School' of scientists, which also included Giovanni Domenico Cassini, Jean Picard, Ole Romer and Christiaan Huygens, Auzout was instrumental in founding paris observatory.

averted vision Technique for viewing faint objects through a telescope. The retina of the eye is equipped with two different types of photoreceptors: cone cells and rod cells. Cone cells are responsible for colour vision and respond only to high levels of light; rod cells are more sensitive to light, but do not provide colour vision. This is why colours cannot be seen under low levels of light. The fovea, a small area of the retina located directly behind the lens, contains only cones, so when one looks directly at an object, no rods are used. A dim object that cannot be seen when viewed directly will often come into view if one looks slightly to the side. This allows light from the object to fall on a region of the retina populated by rods, which are about 40 times (equivalent to a difference of 4 stellar magnitudes) as sensitive as cones. The best averted vision is achieved when the object is about 8° to 16° away from the fovea, towards the nose. The regions above and below the fovea are almost as sensitive, but the area towards the ear should be avoided since the blind spot (where the optic nerve enters the eye) is 13° to 18° in that direction.

Avior The star e Carinae, visual mag. 1.86 (but slightly variable), distance 632 l.y., spectral type K3 III. The name Avior is of recent application and its origin is unknown.

AXAF Abbreviation for Advanced X-Ray Astrophysics Facility. It was renamed the chandra x-ray observatory.

axion Elementary particle proposed to explain the lack of CP (Charge and Parity) violation in strong interactions. Axions have been proposed as a type of cold dark matter, the mass of which contributes to the gravitational potential in galaxy haloes, thus helping to explain the radial velocity curves of galaxies. Recent background radiation measurements and Hubble Space Telescope observations of brown dwarfs in the haloes of galaxies have limited the need for axions in cosmology.

axis Imaginary line through a celestial body, which joins its north and south poles, and about which it rotates or has rotational symmetry. The angle between this spin axis and the perpendicular to its orbital plane is called the axial inclination, which in the case of the Earth is 23°.45. In the cases of Venus, Uranus and Pluto, the values exceed 90°, because their axial spins are retrograde. The gravitational pull of the Sun and Moon on the Earth's equatorial bulge causes its rotational axis to describe a circle over a period of 25,800 years, an effect known as precession. If a body possesses a magnetic field, its rotational and magnetic axes do not necessarily coincide. On the Earth the two are inclined at about 10°.8 to one another, while on Uranus the magnetic axis lies at 58°.6 to the rotational axis and is also offset from the centre of the planet.

azimuth Angular distance of an object measured westwards around the horizon from due north at 0°, through due east at 90°, due south at 180° and so on. An object's azimuth is determined by the vertical circle (meridian) running through it. Azimuth is one of the two coordinates in the horizontal (or horizon) coordinate system, the other being altitude. See also celestial coordinates

Azophi Latinized name of al-sufI