Astronomy

Albedo

The fraction of the total light or other radiation falling on a non-luminous body, such as a planet, or on a planetary surface feature, that is reflected from it. In general, the albedo is equal to the amount of light reflected divided by the amount of light received. Albedo values range from 0.0 (0%) for a totally black surface that absorbs all incident light, to 1.0 (100%) for a perfect reflector. Planets or planetary satellites with dense atmospheres have much higher albedos than those with transparent or no atmospheres. The albedo may vary over the surface, so for practical purposes an average albedo is specified. Natural surfaces reflect different amounts of light in different directions, and albedo can be expressed in several ways according to whether the measurement is made in one direction or averaged over all directions. See Bond Albedo; Geometrical Albedo; Hemispherical Albedo.

Acronical

Referring to the rising or setting of a celestial object at or shortly after sunset. A planet's rising is acronical when it is at opposition.

Achromatism

Freedom from false colour (chromatic aberration) in an optical system. In reality, no optical system containing lenses can ever be completely free from false colour, but the aim is to reduce the amount of false colour to acceptable proportions. A mirror is completely achromatic.

Anaemic Spiral Galaxy

anaemic spiral galaxy definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy anaemic spiral galaxy A type of spiral galaxy with characteristics intermediate between normal gas-rich spirals and gas-poor lenticular galaxies. They are seen most frequently in rich clusters of galaxies. Examples are NGC 4941 and 4866 in the Virgo Cluster, and NGC 4921 in the Coma Cluster.

Analemma

analemma definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy analemma A curve that depicts the changing altitude of the Sun during the year at noon, and also the difference between apparent solar time and mean solar time. The analemma is shaped like a thin figure of eight, its height corresponding to the Sun's altitude and its width to the time difference. It was originally a refinement to sundials that allowed corrections to be made for the *equation of time.

Anthelion

anthelion definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy anthelion The point in the sky directly opposite the Sun.

Anthelion Radiant

anthelion radiant definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy anthelion radiant A broad, weak source of meteors (usually fewer than five per hour) located on the ecliptic some 15° east of the *anthelion point. Meteors from this radiant are caused by particles of interplanetary dust moving faster than the Earth which enter the atmosphere as they overtake us. Activity from the anthelion radiant was formerly attributed to a sequence of individual minor showers, but is now recognized to be part of a general background that is active throughout the year and that has no specific parent body. The only exception is the *Taurid meteor shower of October and November, which is associated with Comet *Encke.

Anthropic Principle

anthropic principle definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy anthropic principle The proposition that the presence of human life is connected with the properties of the Universe. There are various forms of the anthropic principle. The least controversial is the weak anthropic principle, according to which human life occupies a special place in the Universe because it can evolve only where and when the conditions are suitable for it. A more speculative version, the strong anthropic principle, asserts that the laws of physics must have those properties that allow life to evolve. In recent years, the weak anthropic principle has been used to explain many properties of the Universe, such as the values of the fundamental constants and the number of dimensions of space, on the grounds that if they did not have these values human life would never have evolved. However, the validity of arguments of this kind is very uncertain unless, as some speculative theories suggest, there are actually many universes, each having different values for the fundamental constants and possibly different numbers of dimensions (see Multiverse). If that is true, some of these universes would contain no life at all, some would contain very different life from ours, and our Universe would obviously contain our kind of life.

Airglow

A weak background emission of light over the whole sky, resulting mainly from the excitation of atmospheric oxygen at an altitude of around 100 km by solar radiation; also known as nightglow. Airglow emissions vary with the time of day. Green oxygen emission at a wavelength of 557.7 nm predominates at night. Sodium and red oxygen emissions are prominent in the twilight airglow. Airglow is also produced during daylight, and is 1000 times as intense as at night.

Alpha Scorpiid Meteors

A meteor shower, active from April 20 to May 19, producing rather low observed rates (maximum ZHR 10). Its parent meteor stream lies close to the ecliptic, and has been split into at least two branches by planetary perturbations. Maxima occur on April 28 from a radiant at RA 16h 32m, dec. −24°, and on May 13 from RA 16h 04m, dec. −24°. It is not recognized as a genuine shower by some authorities.

Altazimuth Mounting

A method of mounting a telescope so that it can pivot up and down (in altitude) around one axis and horizontally (in azimuth) around the other axis. Following an object across the sky thus usually requires simultaneous movements around each axis. Moreover, as objects cross the sky their orientation in the field of view changes. The equatorial mounting was therefore long preferred for large telescopes, but with the advent of readily available computer control, which can easily compensate for the varying movements and field rotation, the altazimuth mounting has been adopted for large telescopes as it simplifies construction.

A Star

A star definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy A star A star of spectral type A, whose spectrum is dominated by absorption lines of hydrogen (the *Balmer series); in fact, their hydrogen absorption lines are the strongest of any normal stars. They appear blue-white in colour. A-type stars on the main sequence have temperatures in the range 7200-9500 K, are 7-50 times more luminous than the Sun, and are of 1.5-3 solar masses. Sirius, the brightest star in the sky, is of spectral type A1, and Vega is type A0. A-type supergiants, such as Deneb, are more massive stars (up to 16 solar masses) evolving off the main sequence, with temperatures up to 9700 K, and luminosities over 35 000 times the Sun's. There are a number of peculiar groups among stars of this spectral type, particularly the *Ae stars, *Am stars, and *Ap stars. Also, two of the principal types of pulsating variable (*RR Lyrae stars and *Delta Scuti stars) are evolved stars whose surface temperatures are in the A-star range.

Alpha Canum Venaticorum Star

A type of main-sequence, extrinsic variable star; abbr. ACV. Stellar rotation produces brightness variations of 0.01-0.1 mag., accompanied by changes in the strengths of the spectral lines and the magnetic fields. The rotational periods range from 0.5 days to more than 160 days. The spectra (B8p-A7p) exhibit abnormally strong lines of silicon, strontium, chromium, and rare-earth elements. Stars in the ACVO subtype undergo non-radial pulsation (periods 0.004-0.01 days), with small (0.01-mag.) variations superimposed on the fluctuations caused by the rotation.

AM Canum Venaticorum Star

AM Canum Venaticorum star definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy AM Canum Venaticorum star A rare form of cataclysmic binary that appears to consist of a pair of white dwarfs. The few known are extremely hydrogen-deficient and have practically pure helium spectra. There are no outbursts; the variations are like those of a *W Ursae Majoris star or *ellipsoidal variable, accompanied by rapid *flickering. There may be mass transfer to an *accretion disk around the primary, and the system may represent an extinct nova. The type star has the shortest period (18 min) of any known eclipsing binary.

AM Herculis Star

AM Herculis star definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy AM Herculis star A type of cataclysmic binary whose light exhibits linear and, more significantly, extremely strong circular *polarization (hence the alternative name polar); abbr. AM. The polarization varies smoothly over the orbital period. Such systems consist of a K-M dwarf and a strongly magnetic compact object, accretion taking place down an accretion column over the latter's magnetic poles. The compact star's rotation is synchronized with the orbital period. Optical variations are in the form of *flickering, with amplitudes up to 4-5 mag. The type star also exhibits X-ray variability.

AR Lacertae Star

AR Lacertae star definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy AR Lacertae star A close binary containing two evolved stars neither of which has yet overflowed its *Roche lobe so that the pair are still a *detached system; abbr. AR. They belong to the *RS Canum Venaticorum class of binary. The rotational period of the stars is locked to the orbital period, resulting in enhanced activity from both components. Enhanced versions of solar-like flares excite the coronae of the stars, producing light at extreme ultraviolet and X-ray wavelengths, while the interaction of electrons and magnetic fields gives rise to radio emission. AR Lacertae itself is also an eclipsing binary.

ASTROSAT

ASTROSAT definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy ASTROSAT An Indian satellite for X-ray and ultraviolet astronomy, scheduled for launch in 2012. It will carry an Ultraviolet Imaging Telescope (UVIT) with two 0.38-m mirrors for imaging at far ultraviolet (130-180 nm), near ultraviolet (180-300 nm), and optical wavelengths; three Large-Area Xenon Proportional Counters (LAXPC) for timing and low-resolution spectrometry at X-ray energies 3-80 keV; a coded-mask camera with Cadmium-Zinc-Telluride detector array (CZT) for higher spectral resolution in the range 10-150 keV; a Soft X-ray Imaging Telescope (SXT) for energies 0.3-8 keV; and a Scanning X-ray Sky Monitor (SSM) for monitoring X-ray transients and other variable sources at 2-10 keV. Web Link: http://astrosat.iucaa.in/ Official mission website.

Am Star

Am star definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Am star A star of spectral type A whose spectrum additionally contains very strong metallic lines (hence the suffix 'm') more typical of type F. Am stars are usually members of close binary systems. They rotate more slowly than normal A stars, thereby allowing some elements to sink and some to rise in their atmospheres, and this produces the observed abundance anomalies. Sirius is an Am star.

Amalthea

Amalthea definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Amalthea The third-closest satellite of Jupiter, distance 181 200 km, orbital period 0.498 days; also known as Jupiter V. Its axial rotation period is the same as its orbital period. Amalthea was discovered in 1892 by E. E. *Barnard. It is noticeably elongated, measuring 250 × 146 × 128 km. Its surface is heavily scarred by impact craters, the largest 90 km wide. The surface has a very low albedo, 0.09, and is distinctly reddish, possibly because of sulphur from Io's volcanoes. Spacecraft measurements yield a mean density of 0.85 g/cm3, indicating a porous structure rich in water ice.

Ambartsumian, Viktor Amazaspovich

Ambartsumian, Viktor Amazaspovich definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy (1908-96) Armenian astrophysicist. His major work was on the origin and evolution of stars, for which he was the first to take proper account of their physical properties. He discovered and named stellar *associations, and did important early work on radio galaxies and *active galactic nuclei. He also studied mass ejection from novae and planetary nebulae. Ambartsumian was instrumental in the founding of *Byurakan Astrophysical Observatory.

American Association of Variable Star Observers

American Association of Variable Star Observers definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy American Association of Variable Star Observers (AAVSO) An amateur society founded in 1911, with headquarters in Cambridge, Massachusetts. Observers around the world submit over 400 000 variable star estimates to the AAVSO each year. There are over 18 million archival observations available online. AAVSO activities also include solar observing. Web Link: http://www.aavso.org/ Official website.

American Astronomical Society

American Astronomical Society definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy American Astronomical Society (AAS) An organization founded in 1899 for the promotion of astronomy and related branches of science, with its headquarters in Washington, DC. It publishes the *Astronomical Journal, *Astrophysical Journal, and a quarterly Bulletin (BAAS). Web Link: http://www.aas.org/ Official website.

Ames Research Center

Ames Research Center definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Ames Research Center A NASA scientific and engineering establishment in Mountain View, California. It was founded in 1939 for aeronautical research and became part of NASA in 1958. As well as aeronautics, Ames specializes in space life sciences and various branches of astronomy including Solar System exploration and infrared astronomy. Web Link: http://www.nasa.gov/centers/ames/home/index.html Official website.

Amor Group

Amor group definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Amor group A group of asteroids that cross the orbit of Mars, but not that of Earth; also known as Earth-approaching (or Earth-grazing) asteroids. Their orbits have perihelia between 1.017 AU (Earth's aphelion distance) and 1.3 AU (the perihelion of Mars). Close approaches to Mars and Earth can turn Amors into Earth-crossers (see Apollo Group) temporarily, and vice versa (see also Near-Earth Asteroid). The group is named after (1221) Amor, diameter 1 km, discovered in 1932 by the Belgian astronomer Eugène Joseph Delporte (1882-1955). Amor's orbit has a semimajor axis of 1.921 AU, period 2.66 years, perihelion 1.09 AU, aphelion 2.76 AU, and inclination 11°.9. Amor group members show a broad variety of compositional types, evidently having originated from several sources. Asteroids may be perturbed into Amor-type orbits either by Jupiter, from near the 3 : 1 and 5 : 2 Kirkwood gaps in the main *asteroid belt, or by Mars, from near the inner edge of the main belt. The two largest members of the Amor group are the S-class (1036) Ganymed, diameter 32 km, and *Eros.

Ankaa

Ankaa definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Ankaa The star Alpha Phoenicis, magnitude 2.4. It is a K0 giant 85 l.y. away.

Ananke

Ananke definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Ananke A retrograde satellite of Jupiter, 21 048 000 km from the planet; also known as Jupiter XII. Ananke orbits Jupiter in 624.1 days at an inclination of almost 149° to Jupiter's equator. It is about 20 km in diameter, and was discovered in 1951 by the American astronomer Seth Barnes Nicholson (1891-1963). Ananke is the largest member of the so-called Ananke retrograde irregular group of Jovian satellites which all have similar orbital eccentricity, inclination, and semimajor axis: Euporie, Orthosie, Euanthe, Thyone, Mneme, Harpalyke, Hermippe, Praxidike, Thelxinoe, and Iocaste.

Anaxagoras

Anaxagoras definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Anaxagoras (c.500-c.428 bc) Greek philosopher, born in modern Turkey. In his cosmogony a vortex developed in the primordial Universe, causing dense, wet, dark, and cold matter to fall inwards and form the Earth, while rarefied, dry, light, and hot matter was forced outwards. The Sun, Moon, and stars were torn from the Earth by friction. His claim that the Sun is a red-hot stone allegedly led to his prosecution for impiety and banishment from Athens. Anaxagoras seems to have known the true cause of eclipses—that they are caused by the blocking of light from the Sun.

Anaximander

Anaximander definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Anaximander (c.610-c.540 bc) Greek philosopher, born in modern Turkey. He developed a cosmogony in which universes come into existence from an ageless and eternal reservoir, into which they are eventually reabsorbed. In our universe rotation has put heavy material at the centre (the Earth) and fire at the periphery (the stars). The significance of this world-view is its all-embracing principle governing everything, which can be regarded as the first exposition of a universal law. Anaximander has wrongly been credited with discovering the equinoxes and the obliquity of the ecliptic.

Andromeda

Andromeda definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Andromeda (And) (gen. Andromedae) A constellation of the northern sky, representing Princess Andromeda of Greek mythology. Its brightest stars are Alpha Andromedae (*Alpheratz) and Beta Andromedae (*Mirach). It contains the notable double star Gamma Andromedae (*Almach) and the 9th-magnitude planetary nebula NGC 7662. The constellation's best-known feature is the *Andromeda Galaxy.

Andromeda Galaxy

Andromeda Galaxy definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Andromeda Galaxy The spiral galaxy nearest to our own, and the largest member of the *Local Group; also called M31 and NGC 224. It lies 2.5 million l.y. away and is visible to the naked eye as an elongated patch of light of total magnitude 3.4 in the constellation Andromeda. The Andromeda Galaxy has two arms and is classified as an Sb spiral. Its total mass is over 400 billion solar masses, slightly more massive than our own Galaxy. On long-exposure photographs it can be traced across over 4° of sky, corresponding to a diameter of about 150 000 l.y. It has around 600 globular clusters, over three times as many as our own Galaxy. There are two close dwarf elliptical companion galaxies, both of 8th magnitude: M32 (NGC 221), and NGC 205 (sometimes known as M110), plus more than ten dwarf spheroidal systems. Web Link: http://www.esa.int/esaCP/SEMY1K0SDIG_index_1.html

Andromedid Meteors

Andromedid meteors definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Andromedid meteors A meteor shower seen in 1872, 1885, 1899, and 1904; also known as the Bielids. Activity resulted from the Earth's passage through debris from Comet *Biela, which is presumed to have disintegrated in the mid-19th century. The Earth's approach to the node of the comet's orbit on 1872 November 27 was marked by a meteor storm of around 6000 meteors/h. The 1885 return was even more spectacular, with rates of perhaps 75 000 meteors/h. Gravitational perturbations have pulled the meteor stream orbit away from that of the Earth, so that few, if any, Andromedids are now seen.

Anglo-Australian Telescope

Anglo-Australian Telescope definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Anglo-Australian Telescope (AAT) A 3.9-m reflector opened in 1974 at the Anglo-Australian Observatory (now the *Australian Astronomical Observatory), New South Wales. In 1995 it was fitted with a corrector lens at the prime focus which gives it a two-degree field of view (2dF); an array of fibre optics at the prime focus allows simultaneous spectroscopy of up to 400 stars or galaxies over this wide field of view. Web Link: http://www.aao.gov.au/AAO/about/aat.html Official telescope website.

Antares

Antares definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Antares The star Alpha Scorpii, an M1.5 supergiant over 10 000 times as luminous as the Sun and with a diameter about 400 times the Sun's. It is a semiregular variable, ranging between magnitudes 0.9 and 1.2 with a period of about 5 years. Antares lies 554 l.y. away. It has a B2 dwarf companion of magnitude 5.4 with an orbital period of about 1200 years. The name Antares is Greek, and is usually translated as 'rival of Mars', because of its strong red colour, but sometimes as 'like Mars'.

Antennae

Antennae definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Antennae A pair of interacting galaxies, NGC 4038 and 4039, lying 60 million l.y. away in the constellation Corvus. They have two long, faint, curved, tails extending for 360 000 l.y. (over 20′ on the sky), which give them their name. The tails are probably composed of stars pulled out by gravitational tidal interactions during the collision of a spiral and a lenticular galaxy. Web Link: http://hubblesite.org/newscenter/archive/releases/2006/46/image/c/

Antila

Antlia definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Antlia (Ant) (gen. Antliae) An unremarkable constellation of the southern sky, representing an air pump. Its brightest star, Alpha Antliae, is magnitude 4.3.

Antoniadi Scale

Antoniadi scale definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Antoniadi scale A scale of *seeing conditions, used by amateur astronomers, that describes the effect of atmospheric motions on an image, usually that of the Moon or a planet. Its categories are: I perfect, without a quiver II slight undulations, but with calm periods lasting several seconds III moderate, with some greater air movements IV poor, with the image in constant movement V very bad, making observations very difficult It was devised by E. M. *Antoniadi.

Antoniadi, Eugène Michael

Antoniadi, Eugène Michael definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Antoniadi, Eugène Michael (1870-1944) French astronomer, born in Turkey. He specialized in the planets, of which he was one of the most outstanding observers of his time. His highly detailed drawings show, for example, spots on Saturn's disk and 'spokes' in its rings, the South Tropical Disturbance on Jupiter, and surface details on Mercury. Antoniadi's chief interest, though, was Mars. He was sceptical about the Martian canals, and was able to demonstrate that they were illusory. He devised the *Antoniadi scale for the quality of seeing.

Antu

Antu definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Antu The first 8.2-m Unit Telescope (UT1) of the European Southern Observatory's *Very Large Telescope in Chile, opened in 1998. Its name means 'the Sun' in the local Mapudungun language.

Ap Star

Ap star definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Ap star A peculiar star (hence the suffix 'p') of spectral type A that has much stronger absorption lines than normal of elements such as manganese, mercury, silicon, chromium, strontium, and europium; also known as a peculiar A star. Such stars have unusually strong magnetic fields, thousands of times stronger than the Sun's typical surface field. This leads to certain elements being localized in spots, similar to sunspots. As the star rotates the spectral features therefore vary. Hotter and cooler versions of Ap stars are, respectively, Bp and Fp stars.

Apache Point Observatory

Apache Point Observatory definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Apache Point Observatory An observatory at an altitude of 2788 m in the Sacramento Mountains near Sunspot, New Mexico. It houses a 3.5-m reflector opened in 1994; a 2.5-m reflector built for the *Sloan Digital Sky Survey which began operation in 1998; and a 1-m reflector owned by New Mexico State University. The observatory is owned by the Astrophysical Research Consortium (ARC), whose members are the University of Chicago, the University of Colorado, Johns Hopkins University, Princeton University, the University of Virginia, the University of Washington, and New Mexico State University; the last-named also operates the observatory for the Consortium. Web Link: http://www.apo.nmsu.edu/ Official observatory website.

Aphrodite Terra

Aphrodite Terra definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Aphrodite Terra A large upland area on Venus, near the equator. It is about 17 500 km long, and has about the same area as Africa. It includes four smaller highland areas: Ovda Regio and Thetis Regio in its western half, and Atla Regio and Ulfrun Regio to the east. There are many troughs in Aphrodite, including *Diana Chasma near the centre, and volcanoes such as Maat Mons, 8.4 km high.

Apollo Group

Apollo group definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Apollo group A group of asteroids that cross the Earth's orbit, but whose average distances from the Sun are greater than that of Earth; also known as Earth-crossing asteroids. Their perihelion distances are 1.017 AU (Earth's aphelion) or less. Most Apollo asteroids are small (up to 5 km diameter) and highly irregular in shape. They are named after (1862) Apollo, the first of the group to be discovered, by the German astronomer Karl Reinmuth (1892-1979) in 1932. Apollo is a Q-class asteroid of diameter 1.5 km. It came within 0.07 AU (10.5 million km) of Earth in 1932, but was then lost until 1973. Apollo's orbit has a semimajor axis of 1.470 AU, period 1.78 years, perihelion 0.65 AU, aphelion 2.30 AU, and inclination 6°.4. Apollo can approach to within 0.028 AU (4.2 million km) of Earth's orbit and can also make close approaches to Venus and Mars. Some Apollo asteroids such as *Phaethon and *Icarus approach closer to the Sun than Mercury. The largest Apollo asteroid is (1866) Sisyphus, diameter 8 km. Other notable named Apollos include *Hephaistos, *Toro, and *Toutatis. The Apollo asteroids were once thought to be extinct cometary nuclei, but in fact Jupiter may perturb asteroids from near the 3:1 Kirkwood gap in the main asteroid belt into Apollo-type orbits. See also Near-Earth Asteroid.

Apollo Project

Apollo project definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Apollo project The American space programme to land humans on the Moon. The three-seater Apollo spacecraft were launched to the Moon by Saturn V rockets. In orbit around the Moon, two astronauts entered the Lunar Module and descended to the surface. After four manned test flights, the first manned landing was made on 1969 July 20 by Neil Alden Armstrong (1930-2012) and Edwin Eugene 'Buzz' Aldrin (1930- ) during the Apollo 11 mission. On subsequent missions, astronauts spent longer on the lunar surface and were able to explore greater distances by means of an electrically powered lunar rover. A total of 381 kg of rock samples was brought back by the Apollo astronauts. (See Table 1, Appendix.)

Apollonius of Pergus

Apollonius of Perga definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Apollonius of Perga (c.262-c.190bc) Greek mathematician, born in modern Turkey. He showed that the ellipse, parabola, and hyperbola are all curves formed by a plane intersecting a cone in different ways, i.e. that they are conic sections. The orbital path of an unperturbed body moving in a gravitational field follows one of these three curves, as would come to be appreciated by later astronomers such as E. *Halley, who translated Apollonius' book Conics. Apollonius also originated the mathematical concept of motion based on epicycles and deferents, later taken up by *Hipparchus and *Ptolemy to explain planetary motion.

Apus

Apus definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Apus (Aps) (gen. Apodis) An insignificant constellation near the south celestial pole, representing a bird of paradise. Its brightest star is Alpha Apodis, magnitude 3.8.

Aquarid Meteors

Aquarid meteors definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Aquarid meteors Any of several meteor showers that are active from Aquarius during July and August, principally the *Delta Aquarids and *Iota Aquarids. Meteors emanating from this part of the sky that cannot be identified with a specific radiant are simply termed 'Aquarids' to distinguish them from sporadic meteors.

Aquarius

Aquarius definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Aquarius (Aqr) (gen. Aquarii) A constellation of the zodiac, popularly known as the water-carrier, through which the Sun passes from the third week of February to the second week of March. Its brightest stars are Alpha Aquarii (Sadalmelik) and Beta Aquarii (Sadalsuud), both of magnitude 2.9. It contains the 7th-magnitude globular cluster M2, the *Helix Nebula, and the *Saturn Nebula. Three meteor showers radiate from Aquarius each year, the *Delta Aquarids, the *Eta Aquarids, and the *Iota Aquarids.

Aquila

Aquila definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Aquila (Aql) (gen. Aquilae) A constellation on the celestial equator, representing an eagle. It is marked by the bright star *Altair (Alpha Aquilae), flanked by Beta Aquilae (*Alshain) and Gamma Aquilae (*Tarazed). Eta Aquilae is one of the brightest Cepheid variables, ranging from magnitude 3.5 to 4.4 with a period of 7.2 days.

Ara

Ara definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Ara (Ara) (gen. Arae) A small constellation of the southern sky, representing an altar. Its brightest stars are Alpha and Beta Arae, both magnitude 2.8. Ara contains the open cluster NGC 6193 and the globular cluster NGC 6397.

Arago Ring

Arago ring definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Arago ring A ring of Neptune, fourth in order from the planet, named after F. *Arago. It lies 57 600 km from Neptune's centre and is less than 100 km wide.

Arago, (Dominique) François (Jean)

Arago, (Dominique) François (Jean) definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Arago, (Dominique) François (Jean) (1786-1853) French scientist and statesman. With the French physicist Augustin Jean Fresnel (1788-1827), he established the wave theory of light and studied its *polarization. In 1811 he invented the polariscope for measuring the degree of polarization of light. This instrument allows light from solid and fluid surfaces to be distinguished, and Arago used it in several astronomical studies. One was an extensive study of the total solar eclipse of 1842, in which he determined that the Sun's limb is gaseous, and examined polarized light from the chromosphere and corona.

Arcturus

Arcturus definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Arcturus The star Alpha Boötis, magnitude −0.05, the brightest star north of the celestial equator and the fourth-brightest star of all. It is a K1.5 giant, 37 l.y. away.

Arecibo Observatory

Arecibo Observatory definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Arecibo Observatory The site of the world's largest radio astronomy dish, 305 m in diameter, opened in 1963 and upgraded in 1974 when the original wire-mesh surface was replaced by solid panels. The observatory is situated 12 km south of Arecibo, Puerto Rico, and is operated by the National Astronomy and Ionosphere Center (NAIC), which has its headquarters at Cornell University, New York State. The dish is suspended in a natural hollow in the ground, and scans a strip of sky overhead as the Earth rotates. Radio sources can be tracked within 20° of the zenith by using movable feeds suspended above the stationary reflector, giving coverage in declination from about 38° north to 1° south. The Arecibo dish is used for atmospheric studies as well as astronomy. Web Link: http://www.naic.edu/ Official observatory website.

Arend-Roland, Comet

Arend-Roland, Comet (C/1956 R1) definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Arend-Roland, Comet (C/1956 R1) A long-period comet discovered in 1956 November by the Belgian astronomers Silvain Arend (1902-92) and Georges Roland (1922-91); formerly designated 1957 III. It reached perihelion, 0.32 AU, on 1957 April 8. Closest approach to Earth, 0.57 AU, was on April 21. At peak brightness, the comet reached magnitude −1, with a tail of 25-30°. It also had a prominent *antitail. Comet Arend-Roland has a hyperbolic orbit (eccentricity 1.0002) of inclination 119°.9. Web Link: http://cometography.com/lcomets/1956r1.html Information page at Cometography website.

Argelander Step Method

Argelander step method definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Argelander step method A visual method of estimating the magnitude of a variable star, described by F. W. A. *Argelander. It involves comparing the variable with a comparison star, assigning a step value that reflects the ease with which the brightness of the variable can be distinguished from that of the comparison. Estimates of the form 'A(3)V, V(1)B' result, and the magnitude of the variable (V) can be calculated from the known magnitudes of the comparisons (A and B), as with the *fractional method. See also Pogson Step Method.

Argelander, Friedrich Wilhelm August

Argelander, Friedrich Wilhelm August definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Argelander, Friedrich Wilhelm August (1799-1875) German astronomer, born in modern Lithuania. From a study of the proper motions of several hundred stars he confirmed F. W. *Herschel's calculated position of the Sun's *apex. His main achievement, begun in 1852, was a survey of all stars down to 9th magnitude in the northern hemisphere. The resulting star atlas and catalogue were published in 1859-62 as the *Bonner Durchmusterung. He subsequently initiated the first *AGK catalogue. Argelander also introduced the subdivision of magnitudes into tenths, the step method for estimating stellar magnitudes, and the practice of assigning capital roman letters to variable stars.

Argo Navis

Argo Navis definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Argo Navis One of the 48 constellations known to the Greeks, representing the ship of the Argonauts, but broken up in the 18th century by N. L. de *Lacaille into *Carina, *Puppis, and *Vela.

Argyre Planitia

Argyre Planitia definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Argyre Planitia A lowland plain within the Argyre impact basin in the southern hemisphere of Mars, visible telescopically from Earth as a pale circular patch, centred at −50° lat., 44° W long. Argyre is the second-largest impact basin on Mars, about 800 km wide and over 4 km deeper than its surroundings.

Ariel

Ariel definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Ariel The fourth-largest satellite of Uranus, 1158 km in average diameter; also known as Uranus I. Ariel orbits Uranus in 2.52 days at a distance of 190 950 km, keeping the same face towards the planet. It was discovered in 1851 by W. *Lassell. Ariel is an icy body, with a surface scored by giant steep-sided troughs, termed chasmata, between lightly cratered regions. The longest trough, Kachina Chasmata, is over 600 km long, and the largest crater, Yangoor, is 78 km wide. The erosion of some craters and the presence of smooth-floored material in the chasmata is evidence of volcanic or tectonic resurfacing by viscous ice flows.

Ariel Satellites

Ariel satellites definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Ariel satellites A series of UK scientific satellites launched by NASA between 1962 and 1979. Ariel 1, the first UK satellite, studied solar X-rays and the Earth's outer atmosphere. The next three studied the atmosphere and radio astronomy. The longest-lived mission was the X-ray astronomy satellite Ariel 5, which operated from 1974 to 1980. Ariel 6, the last UK satellite, was primarily concerned with cosmic rays but also carried X-ray instruments. Web Link: http://heasarc.gsfc.nasa.gov/docs/ariel5/ariel5.html Information page at Goddard Space Flight Center

Aries

Aries definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Aries (Ari) (Arietis) A constellation of the zodiac, representing a ram. The Sun lies in Aries for the last 10 days of April until mid-May. Its brightest star is Alpha Arietis (*Hamal). Gamma Arietis is a pair of white stars of magnitudes 4.6 and 4.7.

Aristarchos Telescope

Aristarchos Telescope definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Aristarchos Telescope A 2.28-m reflector at an altitude of 2340 m at Helmos Observatory on Mt Helmos, Greece, 130 km west of Athens, opened in 2004. It is owned and operated by the National Observatory of Athens. Web Link: http://www.astro.noa.gr/helmos/ Official telescope website.

Aristarchus of Samos

Aristarchus of Samos definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Aristarchus of Samos (c.320-c.250 bc) Greek mathematician and astronomer. He attempted to calculate the sizes and distances of the Sun and Moon, establishing that the Sun is much larger than the Earth and much farther off than the Moon. To Aristarchus is due the first heliocentric theory. He placed the Sun at the centre of the Earth's orbit, and the fixed stars on a sphere a great distance from the Sun. The theory attracted little support mainly because *Aristotle's geocentric theory was held in high esteem, but also because the idea of a moving Earth was disliked.

Aristotle

Aristotle definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Aristotle (384-322 bc) Greek philosopher. He based his geocentric model of the Universe on the system of concentric spheres proposed by *Eudoxus (as modified by *Callippus), increasing the number of spheres to 49 to account for the movement of all celestial bodies. (This was later modified by *Ptolemy, who replaced spheres by epicycles.) The outermost sphere, which carried the fixed stars, controlled the motion of the others and was itself controlled by a supernatural agency. Aristotle's world-view of a Universe in which everything had its natural place, with a changing Earth surrounded by the eternal, perfect, and incorruptible heavens, was not seriously challenged for almost two thousand years. Aristotle demonstrated that the Earth was spherical, from the shadow it cast during a lunar eclipse, and calculated its size, obtaining a result that was 50% greater than the true value.

Astronomer Royal for Scotland

Astronomer Royal for Scotland definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Astronomer Royal for Scotland A title created in 1834, originally given to the director for the Royal Observatory at Edinburgh but since 1995 a separate honorary position.

Arizona Radio Observatory

Arizona Radio Observatory definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Arizona Radio Observatory (ARO) A division of *Steward Observatory founded in 2002 which owns and operates a 12-m millimetre-wave dish on Kitt Peak, Arizona, and the 10-m Submillimeter Telescope (SMT) on Mount Graham, Arizona. The 12-m dish, opened in 1967 and upgraded in 1983, was originally owned by the National Radio Astronomy Observatory but was transferred to ARO in 2002, while the SMT, formerly known as the Heinrich Hertz Telescope, was opened in 1994 and transferred to ARO in 2002. Web Link: http://aro.as.arizona.edu/ Official observatory website.

Arneb

Arneb definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Arneb The star Alpha Leporis. It is an F0 supergiant about 2200 l.y. away, of magnitude 2.6.

Asclepius

Asclepius definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Asclepius Asteroid 4581, a member of the *Apollo group, discovered in 1989 by the American astronomers Henry Edward Holt (1929- ) and Norman Gene Thomas (1930- ). It can approach to within 0.004 AU (600 000 km) of Earth's orbit. Asclepius is about 400 m in diameter. Its orbit has a semimajor axis of 1.023 AU, period 1.03 years, perihelion 0.66 AU, aphelion 1.39 AU, and inclination 4°.9.

Association of Universities for Research in Astronomy

Association of Universities for Research in Astronomy definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Association of Universities for Research in Astronomy (AURA) A consortium of US universities founded in 1957 to provide observing facilities for astronomers. AURA operates the *National Optical Astronomy Observatory, the *National Solar Observatory, the *Space Telescope Science Institute, and the *Gemini Observatory. Its headquarters are in Washington, DC. It has 37 US member institutions plus international affiliates in Australia, Canada, the Canary Islands, Chile, Germany, and Japan. Web Link: http://www.aura-astronomy.org/ Official website.

Astraea

Astraea definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Astraea Asteroid 5, the fifth asteroid to be discovered, by the German amateur astronomer Karl Ludwig Hencke (1793-1866) in 1845. Astraea is an S-class asteroid of diameter 120 km. Its orbit has a semimajor axis of 2.577 AU, period 4.14 years, perihelion 2.09 AU, aphelion 3.07 AU, and inclination 5°.4.

Astrographic Catalogue

Astrographic Catalogue definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Astrographic Catalogue A series of star catalogues covering the whole sky, obtained from measurements of photographs taken with *normal astrographs at many observatories between about 1890 and 1950. It was published in 21 sections, each covering a zone of declination measured from a particular observatory, between 1902 and 1963. The general limiting magnitude is about 11 (photographic), but some observatories measured fainter stars. In 1997 the US Naval Observatory published the AC 2000 catalogue, containing 4 621 836 stars measured as part of the original Astrographic Catalogue project but with their positions converted to the same reference frame as the modern *Hipparcos Catalogue. A revised version, AC 2000.2, was released in 2001, incorporating photometry for 2.4 million stars from the *Tycho-2 catalogue. Web Link: http://cdsarc.u-strasbg.fr/viz-bin/Cat?I/275 Detailed description and full catalogue downloadable from the CDS.

Astronomer Royal

Astronomer Royal definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Astronomer Royal An honorary title held by a prominent British astronomer, created in 1675 by King Charles II when the Royal Observatory was founded. Until 1971 the Astronomer Royal was also Director of the Royal Greenwich Observatory, but the post was subsequently made separate.

The Astronomical Almanac

Astronomical Almanac, The definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Astronomical Almanac, The A yearly publication containing tables of the Sun, Moon, and planets and their satellites, and other astronomical data. It is compiled and published jointly by the US Naval Observatory and HM Nautical Almanac Office. It began in Britain in 1766 under the title The Nautical Almanac and Astronomical Ephemeris, but from 1914 two separate versions were issued, one for astronomers and one for navigators (see Nautical Almanac, The). In 1960 the name of the astronomical version was abridged to The Astronomical Ephemeris. The current title was adopted in 1980 when it merged with The American Ephemeris and Nautical Almanac, published in the US since 1852 (for the year 1855). Web Link: http://asa.hmnao.com UK official site. Some free online content. Web Link: http://asa.usno.navy.mil">http://asa.usno.navy.mil US official site. Some free online content.

Astronomical Journal

Astronomical Journal definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Astronomical Journal (AJ) A publication containing original research and observations, founded in 1849 by B. A. *Gould and now issued monthly by the *American Astronomical Society. Web Link: http://iopscience.iop.org/1538-3881 Official website with contents and abstracts of recent issues and free online access to older issues.

Astronomical Netherlands Satellite

Astronomical Netherlands Satellite definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Astronomical Netherlands Satellite (ANS) A Dutch satellite for ultraviolet and X-ray astronomy, launched in 1974 August. It carried a 0.22-m telescope for broad-band photometry at ultraviolet wavelengths. Two X-ray devices, consisting of reflecting collectors with proportional counter detectors, covered the energy range 2-40 keV (0.03-0.62 nm). ANS operated until 1976. Web Link: http://heasarc.gsfc.nasa.gov/docs/ans/ans.html Information page at Goddard Space Flight Center.

Astronomical Society of the Pacific

Astronomical Society of the Pacific definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Astronomical Society of the Pacific (ASP) An international scientific and educational organization based in San Francisco, USA, founded in 1889. Its monthly Publications contain technical papers, and it also publishes a bimonthly popular magazine, Mercury. Web Link: http://www.astrosociety.org/ Official website.

Astronomy and Astrophysics

Astronomy and Astrophysics definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Astronomy and Astrophysics (A&A) A publication founded in 1969 by the merger of several existing European journals. It is a joint enterprise of scientific organizations in various European and South American countries. It is published four times monthly and contains papers on all aspects of astronomy and astrophysics. Web Link: http://www.aanda.org/ Official website with contents and abstracts of recent issues and free online access to older issues.

Astrophysical Journal

Astrophysical Journal definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Astrophysical Journal (ApJ) A publication founded in 1895 by G. E. *Hale and James Edward Keeler (1857-1900), now issued by the *American Astronomical Society three times a month. It publishes research papers in astronomy and, especially, astrophysics. Published with it is the Astrophysical Journal Letters, founded in 1958, which contains shorter and more topical papers. The Astrophysical Journal Supplement series, founded in 1954, appears monthly and carries longer and more technical papers. Web Link: http://iopscience.iop.org/0004-637X/ Official website with contents and abstracts of recent issues and free online access to older issues.

Astrophysics Data System

Astrophysics Data System definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Astrophysics Data System (ADS) An online database of journals and books hosted at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, and funded by NASA. Publications in the ADS cover the fields of astronomy, astrophysics, physics, and geophysics. The ADS provides open access not only to published papers but also to preprints of forthcoming papers. The ADS first went online in 1994. Web Link: http://adsabs.harvard.edu/index.html Official ADS website, allowing search of their digital library.

Atacama Cosmology Telescope

Atacama Cosmology Telescope definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Atacama Cosmology Telescope (ACT) A 6-m millimetre-wave telescope at an altitude of 5190 m on Cerro Toco in the Atacama desert of northern Chile designed to map the cosmic microwave background at wavelengths of 2, 1.4, and 1.1 mm. It was opened in 2007 and is operated by an international consortium of observatories and institutions led by Princeton University. The ACT is similar to the *South Pole Telescope. Web Link: http://www.physics.princeton.edu/act/ Official telescope website.

Australian Square Kilometre Array Pathfinder

Australian Square Kilometre Array Pathfinder definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Australian Square Kilometre Array Pathfinder (ASKAP) An aperture synthesis radio telescope consisting of 36 dishes, each of 12 m diameter with baselines up to 6 km, being built at the *Murchison Radio-astronomy Observatory in Western Australia. It is due for completion in 2013 and will become part of the *Australia Telescope National Facility. As well as being a major instrument in its own right, ASKAP will serve as a testbed for the *Square Kilometre Array. Web Link: http://www.atnf.csiro.au/projects/askap/ Official telescope website.

Atacama Large Millimeter Array

Atacama Large Millimeter Array definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Atacama Large Millimeter Array (ALMA) An aperture-synthesis array for millimetre- and submillimetre-wavelength astronomy being jointly built by the European Southern Observatory, the US National Radio Astronomy Observatory, and the National Astronomical Observatory of Japan, at an altitude of 5000 m on the Chajnantor plateau of the Atacama desert in northern Chile. When it comes into full operation in 2013, ALMA will consist of 66 dishes operating at wavelengths of 0.3 to 3.6 mm. The ALMA main array consists of fifty 12-m dishes that can be arranged with baselines up to 16 km, while the Atacama Compact Array (ACA) consists of four 12-m dishes and twelve 7-m ones in an area 250 m wide for imaging extended sources. Web Link: http://www.eso.org/public/images/alma-chajnantor-scene1/ Web Link: http://www.almaobservatory.org/ Official telescope website. Web Link: https://science.nrao.edu/facilities/alma Information page at NRAO.

Atacama Pathfinder Experiment

Atacama Pathfinder Experiment definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Atacama Pathfinder Experiment (APEX) A 12-m antenna for submillimetre astronomy, jointly owned by the Max Planck Institut für Radioastronomie, *Onsala Space Observatory, and the *European Southern Observatory (ESO), sited at an altitude of 5100 m on the Chajnantor plateau, Chile. It works at wavelengths between 0.2 and 1.5 mm and began operation in 2005. APEX is a forerunner of the *Atacama Large Millimeter Array at the same site. Web Link: http://www.apex-telescope.org/ Official telescope website.

Atacama Submillimeter Telescope Experiment

Atacama Submillimeter Telescope Experiment definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Atacama Submillimeter Telescope Experiment (ASTE) A Japanese 10-m dish for submillimetre astronomy at an altitude of 4860 m at Pampa La Bola in the Atacama desert, northern Chile. It observes at wavelengths from 0.4 to 1.1 mm and began operation in 2004. Web Link: http://www.das.uchile.cl/astechile/ASTEinicio.html Official telescope website.

Aten Group

Aten group definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Aten group A group of asteroids that cross the Earth's orbit, and whose average distances from the Sun are less than that of Earth; along with the more abundant *Apollo group, they are termed Earth-crossing asteroids. Their aphelia are greater than 0.983 AU (Earth's perihelion distance) and their orbital periods are shorter than 1 year. The group is named after (2062) Aten, the first of the group to be discovered, by the American astronomer Eleanor Kay Helin, née Francis (1932-2009), in 1976. It is an S-class asteroid of diameter about 1 km. Aten's orbit has a semimajor axis of 0.967 AU, period 0.95 years, perihelion 0.79 AU, aphelion 1.14 AU, and inclination 18°.9. Other major members of the group include (3753) Cruithne, diameter 3 km; *Hathor; and *Ra-Shalom.

Atlas

Atlas definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Atlas The second-closest satellite of Saturn, distance 137 670 km, orbital period 0.602 days; also known as Saturn XV. It was discovered in 1980 on images from the Voyager 1 spacecraft. It is 41 × 35 × 18 km in size, and orbits at the outer edge of Saturn's A Ring, where its gravitational field affects the ring structure.

Atria

Atria definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Atria The star Alpha Trianguli Australis, a K2 giant of magnitude 1.9, lying 391 l.y. away.

Auriga

Auriga definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Auriga (Aur) (gen. Aurigae) A constellation of the northern sky, representing a charioteer. Its brightest star is *Capella (Alpha Aurigae). Auriga contains three 6th-magnitude open clusters, M36, M37, and M38. *Epsilon Aurigae is a long-period eclipsing binary. Zeta Aurigae is another eclipsing binary, varying between magnitudes 3.7 and 4.0 with a period of 972 days; it is the prototype of the *Zeta Aurigae stars.

Australia Telescope National Facility

Australia Telescope National Facility definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Australia Telescope National Facility (ATNF) The collective name for a group of eight radio antennas, located at three observatories in New South Wales, Australia, owned and operated by the Commonwealth Scientific and Industrial Research Organisation (CSIRO). The antennas can be used individually, or in various combinations as a long-baseline interferometer. The heart of the ATNF is the Compact Array at the *Paul Wild Observatory, Narrabri, consisting of six dishes, 22 m in diameter, in a line 6 km long. The largest individual instrument is the 64-m dish at *Parkes Observatory, 300 km to the south of the Compact Array. Between these two sites lies another 22-m dish at Mopra Observatory, near the town of Coonabarabran. The Australia Telescope began operation in 1988. Its headquarters are in Epping, NSW. Web Link: http://www.atnf.csiro.au/ Official telescope website.

Australian Astronomical Observatory

Australian Astronomical Observatory definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Australian Astronomical Observatory An observatory at an altitude of 1150 m on Siding Spring Mountain near Coonabarabran, New South Wales, founded in 1973 and owned and operated by the Australian Department of Innovation, Industry, Science and Research. Its headquarters are in Epping, NSW. Its instruments are the 3.9-m Anglo-Australian Telescope and the 1.2-m United Kingdom Schmidt Telescope. It was originally known as the Anglo-Australian Observatory and was jointly owned by the UK and Australia, but came under full Australian ownership in 2010. Web Link: http://www.aao.gov.au/ Official observatory website.

Avior

Avior definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Avior A name used by navigators for the star Epsilon Carinae, magnitude 1.9. It is a K3 giant, 605 1.y. away.

Altitude

The angular distance of a celestial object above or below the observer's horizon. Altitude is 0° at the horizon and 90° at the zenith. See also Zenith Distance.

Aluminizing

The process of depositing a reflective aluminium coating on a mirror, the successor to silvering. The mirror is placed in a vacuum chamber and the aluminium is vaporized by heating. With no air molecules to collide with, the aluminium atoms travel directly to the mirror, coating it with a thin, even layer (about 100 nm thick). Subsequent layers of other materials, such as silicon dioxide, may then be deposited in a similar way to protect the coating. Large telescopes have aluminizing chambers housed within the observatory building so that the mirror can be re-aluminized on site whenever necessary. The reflectivity of a fresh aluminium coating with overcoating is 89%, declining by a few per cent a year as a result of oxidation of the surface.

Alpheratz

The star Alpha Andromedae, magnitude 2.1. It is a B9 subgiant 97 l.y. away. An alternative name is Sirrah.

Altair

The star Alpha Aquilae, magnitude 0.76, the twelfth-brightest star in the sky. It is an A7 dwarf 16.7 l.y. away. Altair forms one corner of the so-called Summer Triangle of stars, completed by Deneb and Vega.

Alphecca

The star Alpha Coronae Borealis, also spelt Alphekka and also known as Gemma. It is an eclipsing binary of the Algol type, varying between magnitudes 2.2 and 2.3 with a period of 17.4 days. The two stars are dwarfs of spectral types A0 and G5. Alphecca lies 75 l.y. away.

Alrescha

The star Alpha Piscium, also spelt Alrischa. It is a close binary consisting of two A-type stars of peculiar spectra and unknown luminosity class, magnitudes 4.2 and 5.2, with an orbital period of about 930 years. Together they appear as a star of magnitude 3.8. They lie 151 l.y. away.

Alshain

The star Beta Aquilae. It is a G8 subgiant of magnitude 3.7, lying 45 l.y. away.

Ambipolar Diffusion

ambipolar diffusion definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy ambipolar diffusion The process by which ions and neutral atoms or molecules in interstellar gas travel at different speeds in the presence of the interstellar magnetic field. A gas cloud that is collapsing to form stars can detach itself from the interstellar magnetic field through this process, so avoiding the build-up of magnetic pressure that would otherwise halt the collapse. Provided that the collapse is slow, the neutral atoms and molecules can slip past the magnetic field lines in the gas. In this way the bulk of the cloud collapses but some of the magnetic flux leaks out.

Amplitude

amplitude definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy amplitude 1 The overall range of brightness of a variable star: the difference between its maximum and minimum magnitudes.

Anastigmatic

anastigmatic definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy anastigmatic Describing a lens or optical system that has zero *spherical aberration, coma (see Coma, Optical), and *astigmatism. Certain designs of *Maksutov and *Schmidt-Cassegrain telescope are anastigmats, as are all photographic lenses. Anastigmats usually have at least three optical elements, and often many more.

Angstrom

angstrom definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy angstrom (Å) A unit of length equal to 10−10 m. It was used mainly for specifying the wavelength of radiation, but has been replaced by the *nanometre (nm). One angstrom is 0.1 nm. It is named after A. J. Ångström.

Angular Acceleration

angular acceleration definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy angular acceleration The rate of change in the *angular velocity of a spinning body, or in the angular velocity of a celestial object in its orbit about another body.

Angular Diameter

angular diameter definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy angular diameter The apparent size of an object, such as the diameter of a planet, expressed in degrees, minutes, or seconds of arc.

Angular Momentum

angular momentum definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy angular momentum The momentum a body has by virtue of its rotation. A body such as a planet has two types of angular momentum: one type results from its motion in orbit around the Sun, and the other from its spin on its own axis. Angular momentum in orbit is given by the body's mass multiplied by its orbital *angular velocity and by its distance from the Sun. Angular momentum of spin depends on the mass of the body's individual parts and their distances from its centre (the moment of inertia) multiplied by its angular velocity of spin. As a consequence of conservation of angular momentum, a body spins faster as it gets smaller, such as when a gas cloud shrinks to become a star.

Angular Resolution

angular resolution definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy angular resolution The distance, in angular measure, between two close objects that can just be separated by an optical system, such as a telescope. See also Resolving Power.

Angular Separation

angular separation definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy angular separation The apparent distance between two objects, such as two stars, expressed in degrees, minutes, or seconds of arc.

Angular Velocity

angular velocity definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy angular velocity (ω) The rate of rotation of a body, either about its own axis or in its orbit about another body. For example, the Earth rotates with a certain angular velocity about its polar axis, but it also has angular velocity in its revolution about the Sun.

Anisotropy

anisotropy definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy anisotropy A characteristic of a substance or body in which physical properties are different in different directions. In astronomy, the temperature of the cosmic microwave background is observed to be anisotropic on a large angular scale as a result of the Sun's motion through space (dipole anisotropy), and on a small scale as a result of fluctuations in the density of the early Universe. See also Isotropy.

Annealing

annealing definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy annealing In optical manufacture, the process of allowing glass or ceramic to cool slowly at a controlled rate, to avoid stress patterns within the material. A large mirror blank, for example, is cast from molten material. If cooling is too rapid, temperature variations will arise and some areas will solidify more rapidly than others, creating internal stresses that can cause the mirror blank to shatter when it is being ground to shape. Annealing of a large telescope mirror can take several months.

Annual Aberration

annual aberration definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy annual aberration The small displacement in position of a star's image during the year due to the motion of the Earth around the Sun. Annual aberration was discovered by J. *Bradley in 1728 from observations of the changes in distance from the zenith of the star Gamma Draconis. The ratio of the Earth's mean velocity to the speed of light gives the constant of aberration, 20″.5. This is the maximum amount by which a star can appear to be displaced from its mean position. During the course of a year, the star appears to move around its mean position in a shape that ranges from a circle for a star at the ecliptic pole, via a progressively flattened ellipse, to a straight line for a star on the ecliptic. See also E-Terms.

Annual Equation

annual equation definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy annual equation A periodic disturbance in the celestial longitude of the Moon, resulting from the changing gravitational pull of the Sun around the Earth's elliptical orbit; also known as annual inequality. It has an amplitude of 11 minutes of arc and a period of one *anomalistic year.

Annual Parallax

annual parallax definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy annual parallax The maximum apparent difference in position of a star during the course of a year due to the changing position of the Earth in its orbit around the Sun; also known as heliocentric parallax. The amount of displacement is equal to the angular separation between the Earth and Sun as seen from the star. See also Trigonometric Parallax.

Annual Variation

annual variation definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy annual variation The yearly changes in the right ascension and declination of a star due to annual precession and proper motion. These quantities are tabulated in star catalogues.

Annular Eclipse

annular eclipse definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy annular eclipse A solar eclipse occurring when the Moon is near apogee and its apparent diameter is smaller than that of the Sun, so that a ring, or annulus, of the Sun's disk remains visible at mid-eclipse along the narrow central ground track on the Earth. Annular eclipses can last up to 12 m 30 s.

Anomalistic Month

anomalistic month definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy anomalistic month The mean interval of time between successive passages of the Moon through its closest point to Earth (its perigee), equal to 27.55464 days. The Moon's orbit is not fixed in space but can be regarded as a slowly rotating ellipse, the rotation due principally to perturbation by the Sun. Hence the perigee point advances slowly around the orbit, completing one circuit in 8.85 years. As a consequence, the anomalistic month is about 5.5 hours longer than the *sidereal month.

Anomalistic Year

anomalistic year definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy anomalistic year The interval of time between successive passages of the Earth through the perihelion of its orbit, equal to 365.2596 days. Due to perturbations by the other planets, the Earth's orbit is not a fixed ellipse but one that is slowly advancing. As a result the anomalistic year is about 5 min longer than the Earth's orbital period (the *sidereal year).

Anomalous Cepheid

anomalous Cepheid definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy anomalous Cepheid A star which has a period characteristic of comparatively long-period RR Lyrae variables of type RRAB, but which is considerably more luminous; abbr. BLBOO from the prototype BL Boötis.

Anomalous Iron

anomalous iron definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy anomalous iron An *iron meteorite with a chemistry and structure unlike those of any of the primary groups into which other iron meteorites are divided.

Anomaly

anomaly definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy anomaly An angle that describes a body's position at a given time in an elliptical orbit. There are three types of anomaly: *eccentric anomaly, *mean anomaly, and *true anomaly. All are measured in the direction of the body's motion and take as their starting-point the body's periapsis (i.e. its closest point to the object it orbits).

Anorthosite

anorthosite definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy anorthosite A medium- to coarse-grained igneous rock, grey to white in colour. It is composed almost entirely of calcium-rich plagioclase feldspar (95%), one of the most common rock-forming minerals, together with minor amounts of pyroxene (4%), olivine, and iron oxides. Anorthosite is thought to comprise a significant fraction of the lunar crust, and is present at all the Apollo and Luna landing sites. Samples of anorthosites 4.4-4.5 billion years old were among the oldest lunar rocks returned by spacecraft.

Ansae

ansae definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy ansae The extensions of Saturn's rings either side of the planet; sing .ansa. The word is Latin for 'handles', since the rings project from the planet like handles.

Antenna

antenna definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy antenna A device used in radio astronomy to detect radio waves; also known as an aerial. In an antenna, radio waves induce an oscillating electric current in a conductor. The simplest antenna is a *dipole antenna, which is a simple metal rod. More complex antennas, such as the *Yagi antenna, may consist of a dipole or folded dipole (the driven element) with other conducting rods (parasitic elements) which direct or reflect the waves on to the dipole and so increase the antenna's *directivity and *gain. The most powerful radio telescopes, especially at high frequencies, use parabolic (dish) antennas to collect waves and bring them to a focus. Large numbers of antennas may be combined in an array to achieve high resolution. Antennas may also be used to transmit radio waves.

Antenna Pattern

antenna pattern definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy antenna pattern A graph of the sensitivity or gain of a radio telescope in various directions. A typical antenna pattern shows several sensitivity maxima (lobes) of which the strongest (the main lobe or main beam) defines the direction in which the telescope is most sensitive. An antenna pattern with a narrow main lobe of circular cross-section and negligible *side lobes is known as a pencil beam, while a pattern with a wide, flat main lobe is known as fan beam. See also Polar Diagram.

Antenna Temperature

antenna temperature definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy antenna temperature A measure of signal strength in radio astronomy. It is defined as the temperature of a black-body enclosure which, if completely surrounding a radio telescope, would produce the same signal power as the source under observation. Antenna temperature is a property of the source, not of the antenna itself. Where a uniform extended source fills the telescope beam, the antenna temperature is equal to the *brightness temperature of the source.

Antimatter

antimatter definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy antimatter Matter composed of antiparticles: subatomic particles that have identical *rest mass to corresponding particles of ordinary matter but opposite charge, and are opposites in other fundamental properties. For example, the antiparticle of the electron is the positron, which has a positive charge equal to the electron's negative charge; the antiproton has a negative charge equal to the proton's positive charge. When matter and antimatter meet, they annihilate each other, releasing energy. The Universe seems to be almost entirely in the form of matter rather than antimatter; why this should be so is presumably related to events shortly after the Big Bang.

Antitail

antitail definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy antitail A protuberance from a comet that appears to point towards the Sun, often looking like a spike. It is actually part of the comet's dust tail, and consists of larger (millimetre-sized) particles lagging behind the comet in its orbit. The antitail does not actually point sunwards, but results from a perspective effect when the comet is seen from a certain angle. Antitails are uncommon. Where present, they are usually most prominent when the Earth passes through, or close to, the plane of the comet's orbit. The cometary dust, which lies in a thin sheet, is then seen edge-on.

Apastron

apastron definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy apastron The point in an elliptical orbit around a star that is farthest from the centre of the star.

Aperture

aperture definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy aperture The diameter of the main lens or mirror of a telescope, or, in radio astronomy, of the collecting dish. For *Schmidt cameras, however, it is the diameter of the *corrector plate. See also Unfilled Aperture.

Aperture Efficiency

aperture efficiency definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy aperture efficiency A measure of the efficiency with which a telescope can collect radio waves. It is expressed as the *effective aperture divided by the geometrical area. For a simple parabolic antenna, aperture efficiencies rarely exceed 65%, due to the difficulties of designing a primary feed to be equally sensitive to waves reflected from all points of the dish.

Aperture Ratio

aperture ratio definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy aperture ratio The ratio of the aperture (diameter) of a lens or mirror to its focal length, usually expressed as a true ratio, e.g. 1 : 8. It is more common, however, to refer to the *focal ratio, which is the inverse of the aperture ratio and is expressed in the form f/8 (see f/Number).

Aperture Synthesis

aperture synthesis definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy aperture synthesis A technique used in radio astronomy to achieve high angular resolution. It uses an array of telescopes to simulate a single telescope of large aperture (see the diagram). In principle, an aperture of any size can be synthesized by moving the two elements of an *interferometer to occupy all possible positions in the desired aperture. In practice, all synthesis telescopes make use of the fact that over a period of 12 hours the Earth's rotation will move the elements to sweep out half a ring of the synthesized aperture (supersynthesis, or Earth-rotation synthesis); the other half of the ring can be derived from the observations of the first half. The elements need then be moved only to sweep out successive rings. In practice, some aperture-synthesis telescopes employ several movable dishes to reduce observing time, while in others the dishes remain fixed (an unfilled aperture). Aperture synthesis requires complex data-reduction techniques and powerful computers. Examples of aperture synthesis telescopes are the *Multi-Element Radio-Linked Interferometer Network (MERLIN), the *Ryle Telescope, the *Very Large Array, and the Westerbork Synthesis Radio Telescope (see Westerbork Radio Observatory).

Aphelion

aphelion definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy aphelion The point in an elliptical orbit around the Sun that is farthest from the Sun's centre.

Aplanatic

aplanatic definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy aplanatic Describing a lens or optical system that is free from both *spherical aberration and coma (see Coma, Optical). Examples of aplanatic systems include some types of achromatic lens and the *Ritchey-Chrétien telescope.

Apoapsis

apoapsis definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy apoapsis The point in an elliptical orbit that lies farthest from the centre of the object being orbited.

Apocentre

apocentre definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy apocentre The point in an elliptical orbit that lies farthest from the centre of mass of the orbiting system, such as a binary star or a planet and satellite.

Apochromatic

apochromatic definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy apochromatic Describing a lens or optical system with very little *chromatic aberration. In an apochromatic lens (also known as an apochromat) the chromatic aberration is completely cancelled out at three or more wavelengths, compared with two for a typical achromatic lens. Apochromats usually have at least three optical *elements, made from different types of glass. *Fluorite has become a popular material for apochromats, giving virtually perfect colour correction with only two elements, but at considerable cost.

Apodization

apodization definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy apodization A technique in which the normal performance of an instrument is deliberately degraded in such a way that the instrument's performance is actually improved for one specific application. It may be applied to optical and radio telescopes (where it is called tapering the antenna), and is particularly important in *Fourier transform spectrometers where it leads to the elimination of spurious spectral features at the expense of a slight reduction in spectral resolution. A common application is the use of an apodizing screen in the observation of a close visual binary where the image of a faint companion star falls on one of the diffraction rings surrounding the Airy disk. A circular screen which gradually becomes opaque towards the edge reduces the brightness of the rings; a screen with a square aperture produces a cross-shaped diffraction pattern which can be rotated clear of the companion star.

Apogee

apogee definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy apogee The point in an elliptical orbit around the Earth that is farthest from the Earth's centre. Web Link: http://solarsystem.nasa.gov/multimedia/display.cfm?IM_ID=442

Apohele

apohele definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy apohele An asteroid whose orbit lies entirely within that of the Earth (an aphelion of 0.983 astronomical units or less). The first proposed member of the group was 1998 DK36, but its orbit is poorly determined. The first confirmed apohele was 2003 CP20, now known as (163693) Atira, aphelion 0.98 AU, and the second was 2004 JG6, aphelion 0.97 AU.

Apparent Diameter

apparent diameter definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy apparent diameter The size of a celestial object or feature as seen by an observer, expressed in angular measure; the same thing as angular diameter.

Apparent Magnitude

apparent magnitude definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy apparent magnitude (m) The brightness of a celestial object as measured by the observer. Sirius, the brightest star, has an apparent magnitude of −1.44, and the faintest stars visible to the naked eye under the most favourable conditions have magnitudes of about +6.5. Stars of magnitude +23 are regularly measured at modern professional observatories, and as faint as +30 with the Hubble Space Telescope. If m carries no subscript it is assumed to be the *visual magnitude. The notation m bol denotes the apparent *bolometric magnitude.

Apparent Noon

apparent noon definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy apparent noon The time at which the Sun crosses the observer's meridian, and hence reaches its greatest altitude above the horizon. Due to the *equation of time, apparent noon can differ by some minutes from mean noon as recorded by a clock keeping local mean solar time.

Apparent Place

apparent place definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy apparent place The predicted coordinates of a star as it would be seen from the centre of the Earth, referred to the true equator and equinox at a specific date. It includes displacements from the heliocentric direction given in a star catalogue due to precession, nutation, aberration, proper motion, annual parallax, and gravitational deflection of light.

Apparent Retrogression

apparent retrogression definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy apparent retrogression The temporary reversal in the direction of movement of an outer planet on the celestial sphere as it is overtaken by the Earth; also known as retrograde motion.

Apparent Sidereal Time

apparent sidereal time definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy apparent sidereal time Time as given by direct observation of the stars; strictly, the hour angle of the true equinox. The vernal equinox (the First Point of Aries) is not a completely fixed sidereal direction but varies due to general precession and nutation. Nutation is a periodic variation and imposes irregularities of about a second on *sidereal time. The principal period of this variation is 18.6 years, which is the period of regression of nodes of the Moon's mean orbit around the ecliptic. Apparent sidereal time corrected for this effect gives mean sidereal time.

Apparent Solar Time

apparent solar time definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy apparent solar time Time as given by the daily movement of the Sun across the sky; strictly, the *hour angle of the Sun plus 12 hours, which is added so that the solar day begins at midnight. Apparent solar time is the time shown on a sundial. The Sun's hour angle increases due to the Earth's rotation, but slightly more slowly than that of the stars because the Sun moves against the star background as the Earth orbits it. However, this movement is not entirely uniform because the Earth's orbit is elliptical, and consequently apparent solar time can be anything up to a quarter of an hour ahead of or behind *mean solar time. The difference between apparent solar time and mean solar time is known as the *equation of time.

Apparition

apparition definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy apparition The period of time during which an object in the Solar System is visible from Earth, such as an evening apparition of Venus or the apparition of a periodic comet. The term is not usually applied to objects that are more regularly visible, such as the Sun or stars.

Appulse

appulse definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy appulse The apparent close approach of two celestial bodies when one moves into the same line of sight as the other, such as when a planet appears to pass another planet or a star.

Apsidial Motion

apsidal motion definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy apsidal motion A progressive change in the orientation of the major axis (i.e. the line of apsides) of an orbit; also known as *advance of perihelion. There are several possible causes, including perturbations by other bodies; mass transfer between stars in a binary system; the elliptical shape of stars in a close binary; and the curvature of space around a massive body.

Apsides

apsides definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy apsides The two points in an elliptical orbit that lie closest to or farthest from the centre of the body being orbited; sing. apsis or apse. The closest point is the periapsis (or periapse) and the farthest point the apoapsis (or apoapse). The straight line joining these two points is the line of apsides, and is the same as the major axis of the orbit. In the case of the Earth's orbit around the Sun, the apsides are perihelion and aphelion.

Aqueous Alteration

aqueous alteration definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy aqueous alteration A process in which changes to minerals occur through reactions with water. The effects of aqueous alteration are apparent in certain meteorites (particularly carbonaceous chondrites) and in the surface layers of some asteroids, notably Ceres.

Arachnoid

arachnoid definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy arachnoid A cobweb-shaped landform on Venus showing radiating lineaments centred on a volcanic dome and surrounded by concentric ridges or fractures. Arachnoids are up to 150 km in diameter and occur mostly in two lowland regions of Venus, Sedna Planitia and Ganiki Planitia. Other sparse groupings are known.

Arc

arc definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy arc A part of a circle. Angular measure is given in units of arc. See Degree of Arc; Minute of Arc; Second of Arc; Radian.

Arch Filament System

arch filament system definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy arch filament system Narrow arches or loops above the Sun's photosphere, visible in hydrogen light as darker absorption features against the solar disk, in a similar manner to normal *filaments; when seen at the limb, though, they appear bright. Arch filament systems are a characteristic feature of developing *active regions on the Sun and connect areas of opposite magnetic polarity (usually sunspots) in the innermost part of young *bipolar groups. Individual arches have typical lengths of 30 000 km and lifetimes of some tens of minutes, although the entire arch filament system can last for several days. Their absorption of hydrogen light implies the arches are cool and dense, and so they are believed to be photospheric plasma that has risen into the corona.

Archeoastronomy

archaeoastronomy definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy archaeoastronomy The study of the astronomical significance of archaeological remains and artefacts, particularly large stone structures (megaliths). Structures such as Stonehenge in England and the Great Pyramid in Egypt have components which are oriented to align with the positions of celestial bodies at significant times of the year. This suggests that they were constructed at least partly for astronomical purposes, although further interpretation remains largely speculative.

Arcus

arcus definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy arcus An arc-shaped feature on a planetary body; pl. arcus The term is not a geological one, but is used in the nomenclature of individual features on planets and satellites. An example is Hotei Arcus on Titan.

Area Photometer

area photometer definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy area photometer A specialized photometer used to measure simultaneously the light at each point of an extended object such as a galaxy or nebula; also known as an imaging photometer. Most area photometers rely on CCD detectors or infrared arrays.

Argument of Perihelion

argument of perihelion definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy argument of perihelion (ω) An angle that defines the direction of the major axis of an orbit around the Sun. It is the angle between the ascending node and the perihelion of an orbit, measured in the plane of the orbit and in the direction of orbital motion. See also Elements, Orbital.

Arm Population

arm population definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy arm population Those young stars concentrated in the spiral arms of galaxies; they are *extreme Population I stars. Their youth is indicated by their high content of heavy elements, by their proximity to the gas and dust from which they formed, and by the presence among them of massive stars which have only a short lifetime. They are found in loose open clusters or associations.

Armillary Sphere

armillary sphere definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy armillary sphere An instrument used since ancient times to demonstrate and observe the movements of the heavens. It consists of a number of rings (armillaries) representing celestial great circles such as the meridian, horizon, celestial equator, and ecliptic, arranged to form a skeletal celestial sphere. Armillary spheres can still be found in the form of equatorial sundials, in which the ring representing the celestial equator is marked with hours. The name comes from the Latin word armilla, meaning 'ring' or 'bracelet'.

Array

array definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy array 1 A group of telescopes or antennas (elements) arranged to operate as a single instrument. See Interferometer, Phased Array. 2 A regular arrangement of detectors such as CCDs or infrared detectors placed in the focal plane of a telescope, either in a line (a one-dimensional array) or in a plane (a two-dimensional array), and designed to record an image of a source.

Artificial Satellite

artificial satellite definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy artificial satellite A spacecraft sent into orbit around the Earth. The first artificial Earth satellite was Sputnik 1, launched by the former Soviet Union on 1957 October 4. A spacecraft can become an artificial satellite of the Moon or a planet if it goes into orbit around it.

Ascending Node

ascending node definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy ascending node (☊) The point in an orbit at which a body moves from south to north across a reference plane, such as the plane of the ecliptic or of the celestial equator. The *longitude of the ascending node is one of the elements of an object's orbit. See Elements, Orbital; Node.

Ashen Light

ashen light definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy ashen light A phenomenon in which the unlit portion of Venus's globe glows faintly when the phase is a thin crescent. It may be caused by refraction of sunlight in Venus's very dense atmosphere or by atmospheric electrical phenomena. The ashen light is fainter than *earthshine on the Moon, and can be seen only when Venus is very close to inferior conjunction, but even then it is rare.

Aspect

aspect definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy aspect The position of a body in the Solar System relative to the Sun, as seen from Earth. The main aspects are *conjunction, *greatest elongation, *opposition, and *quadrature.

Aspheric

association, stellar definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy association, stellar A very loose concentration of young stars (perhaps 100 in a region several hundred light years across), conspicuous only because it consists of distinctive stars. An *OB association consists predominantly of the most massive stars, of spectral types O and B; an *R association consists of medium-mass stars, surrounded by reflection nebulae; and *T associations are the birthplaces of the lowest-mass stars, in the form of T Tauri stars. All three types of association can be found together, and often have a star cluster within them. Like open clusters, associations are born from nebulae in the spiral arms of our Galaxy, but are so scattered that they disperse in 10 million years or so.

Association, Stellar

association, stellar definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy association, stellar A very loose concentration of young stars (perhaps 100 in a region several hundred light years across), conspicuous only because it consists of distinctive stars. An *OB association consists predominantly of the most massive stars, of spectral types O and B; an *R association consists of medium-mass stars, surrounded by reflection nebulae; and *T associations are the birthplaces of the lowest-mass stars, in the form of T Tauri stars. All three types of association can be found together, and often have a star cluster within them. Like open clusters, associations are born from nebulae in the spiral arms of our Galaxy, but are so scattered that they disperse in 10 million years or so.

Asterism

asterism definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy asterism A distinctive pattern of stars that forms part of one or more constellations. For example, the familiar shape of the Plough or Big Dipper is an asterism within the constellation Ursa Major, while the *Square of Pegasus and the *False Cross are asterisms formed from stars of more than one constellation. The term is also used to describe a group of stars that appear to form a cluster, even though they are not related.

Astronomical Twilight

astronomical twilight definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy astronomical twilight The period before sunrise and after sunset when the centre of the Sun's disk is between 12° and 18° below the horizon. It is regarded as the time during which 6th-magnitude stars are visible at the zenith in a clear sky. In summer, astronomical twilight can last all night at latitudes higher than 48½° because the Sun is never more than 18° below the horizon. See also Twilight.

Asteroid

asteroid definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy asteroid Any of the many small rocky or metallic objects in the Solar System, mostly lying in a zone (the *asteroid belt) between the orbits of Mars and Jupiter; also known as a minor planet. They range in diameter from almost 1000 km for *Ceres (the first asteroid discovered, in 1801) down to less than 10 m for the smallest so far detected. The total mass of all asteroids is 4 × 1021 kg, about one-twentieth the mass of the Moon. At the end of 2010 over half a million asteroids were known and the total was growing by about 10% per year.

Asteroid Belt

asteroid belt definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy asteroid belt The region of the Solar System between the orbits of Mars and Jupiter in which most asteroids are found; also known as the main belt. It extends from 2.1 to 3.3 AU from the Sun, corresponding to orbital periods of 3.0-6.0 years. Asteroids are not uniformly distributed throughout the belt. There are sparsely populated zones, known as *Kirkwood gaps, as well as concentrations of asteroids with similar orbital elements, forming groups and *Hirayama families. The proportion of various asteroid classes changes markedly through the belt (see Asteroid). Web Link: http://sse.jpl.nasa.gov/multimedia/display.cfm?IM_ID=850

Asteroseismology

asteroseismology definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy asteroseismology The measurement and interpretation of oscillations or pulsations of stellar photospheres. It is the extension of *helioseismology to other stars. Asteroseismology is often used to refer to the study of weak, solar-like oscillations in stars, but can also include the larger amplitude variations of, for example, rapidly oscillating *Ap stars and *Beta Cephei stars. Solar-like oscillations have been identified in several dwarf and subgiant stars, with over 30 oscillation modes detected in Alpha Centauri A. In most cases the disk of the star cannot be resolved so it is global oscillations that are measured, which probe the deeper layers of the star. Two techniques are used to measure stellar oscillations: photometry and Doppler spectroscopy. The former is difficult for detecting solar-like oscillations as the brightness variations are typically a few millionths of a magnitude, at the limit of what is measurable from ground-based observatories. The perfect seeing available in space has, however, allowed such tiny oscillations to be measured by satellites such as *Microvariability and Oscillations of Stars (MOST), *COROT, and *Kepler. Solar-like oscillations have been detected at ground-based observatories using *radial-velocity spectrometers that measure the Doppler motions on the stars' surfaces. Observations are made at several sites around the globe to provide continuous coverage for long periods.

Asthenosphere

asthenosphere definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy asthenosphere A weak layer within a planetary body, beneath the rigid outer layer known as the *lithosphere. It is slightly weaker than the lithosphere and deformable over long periods, allowing continental drift to take place on the Earth. On other planets and satellites the asthenosphere allows the topography to smooth out slowly, highlands becoming lower and deep basins shallower. The depth of the asthenosphere varies according to the size, density, composition, and thermal structure of the planet or satellite. On Earth, the top of the asthenosphere is around 100 km below the surface, but on the Moon it lies 800 km below the surface, more than half-way to the centre.

Astigmatism

astigmatism definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy astigmatism A defect of a lens or an optical system in which the focal length along one diameter differs from that along another. Typically, as the focus is approached, the image of a star appears first as a short straight line; then, at the best focus, as a small circle (the least circle of confusion or focal circle); then, once the best focus is passed, as a line perpendicular to the first. Astigmatism may be caused by distortion of an optical component, often as a result of mechanical stress; or it may be inherent in an optical design, in which case its effects increase with distance from a system's optical axis. It can occur in the human eye as well as in optical instruments.

Astration

astration definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy astration The cycle in which interstellar material forms into stars, is enriched with heavy elements as a result of nuclear reactions, and is then returned to interstellar space via stellar winds, planetary nebulae, or supernovae.

Astrobiology

astrobiology definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy astrobiology The scientific study of the possibility of life elsewhere in the Universe; also known as exobiology. Astrobiology embraces branches of biology (for example microbiology, biochemistry, and ecology) in addition to astronomy. Considerations include assessment of the likely conditions required for the origin and development of life, and the means of detecting such life. Investigations such as the study of life in hostile environments on Earth, the search for water on Mars and sampling of the soil, the study of organic molecules in meteorites and comets, the synthesis of prebiotic compounds, and the various SETI programmes based on radio astronomy can be considered part of astrobiology.

Astrobleme

astrobleme definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy astrobleme An eroded impact crater on Earth, identifiable by its geological structure and highly shocked rocks; the name means 'star wound'. About 180 terrestrial impact structures are known. Evidence of an impact origin includes signs of high-pressure shock waves, such as *shatter cones, shock-induced minerals (e.g. *coesite and *stishovite), and microscopic linear patterns in quartz called shock lamellae. The largest are Vredefort in South Africa (diameter 300 km), Sudbury in Ontario, Canada (250 km), *Chicxulub in Mexico (170 km), Popigai in Russia (100 km), and Manicouagan in Quebec, Canada (100 km). Virtually all are on land, but Montagnais (diameter 45 km) is on the continental shelf off Nova Scotia, and Chicxulub extends into the Gulf of Mexico from the Yucatán Peninsula. The oldest, Suavjärvi (in Russia) and Vredefort, are just over 2 billion years old, but the majority (about 60%) are less than 250 Myr old. Web Link: http://www.passc.net/EarthImpactDatabase/index.html Database of terrestrial impact structures.

Astrochemistry

astrochemistry definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy astrochemistry The study of chemical reactions that occur naturally in space. Molecules are able to form in space at very low temperatures (e.g. 20 K) and at pressures far lower than are achievable in a laboratory on Earth. Many chemical species that are unstable on Earth exist in space, and are detectable by their spectral lines at radio, infrared, optical, and ultraviolet wavelengths. There are two broad types of formation process: chemistry in gas clouds (including *photodissociation) and chemistry on the surfaces of dust grains. Many astrochemical reactions involve only gases. In some cases atoms or molecules in the gas may become ionized by the passage of *cosmic rays, and the subsequent reactions between ions and molecules are more rapid as a result. In the other main process, dust particles act as catalysts, providing a surface on which atoms, ions, and molecules can stick and then react. The dust also acts as a shield and prevents starlight from breaking up the molecules again. Heating of the dust by newly formed stars may release complex molecules from the grains back into the gas cloud and drive a new series of chemical reactions. See also Interstellar Molecule.

Astrodynamics

astrodynamics definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy astrodynamics The branch of *celestial mechanics that deals with the motions of artificial satellites and space probes. Astrodynamics involves planning and controlling the trajectories of spacecraft, by techniques such as *gravity assist.

Astrograph

astrograph definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy astrograph A telescope specifically designed for photographing comparatively wide areas of sky. Traditionally, an astrograph is a refractor with a lens corrected to give its best images at blue wavelengths, to which early photographic emulsions were sensitive. The astrograph is mounted together with a visual refractor of similar focal length for accurate tracking on a guide star. Today, *Schmidt cameras are used instead, typically having fields of view of 6°. See also Normal Astrograph.

Astrolabe

astrolabe definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy astrolabe An ancient device for measuring the altitudes of stars, like a simple sextant. A basic astrolabe consists of a disk hung vertically, with a sight (the alidade) which is pivoted so that it may be pointed at a chosen star. The local time can then be read off the face (the tablet) of the astrolabe. Different tablets can be used for different latitudes. Sophisticated modern versions are used for high-precision measurements of star positions; see Danjon Astrolabe; Prismatic Astrolabe.

Astrology

astrology definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy astrology The supposed influence of the relative positions of the planets on people's personalities and events in their lives. In its modern form astrology is a pseudoscience, but in ancient times astrology and astronomy were intertwined. Often, the motive for keeping observational records was astrological. Ancient Chinese records of celestial events, from which the fortunes of entire dynasties were divined, are now of great value in the study of historical eclipses, novae, and comets.

Astrometric Binary

astrometric binary definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy astrometric binary A binary star in which the presence of an invisible or unresolved component is inferred from irregularities in the proper motion of the visible component. Similarly, a known binary star may prove to be an astrometric multiple star. Some astrometric binaries have subsequently been resolved by *speckle interferometry. Components calculated to have extremely low mass may be either faint red dwarfs, brown dwarfs, or possibly even planets.

Astrometry

astrometry definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy astrometry The measurement of positions, parallaxes, and proper motions on the sky. It can be divided broadly into two categories: global and small-field astrometry. Global astrometry is concerned with mapping and cataloguing positions and motions over large areas of sky. It was traditionally based on optical observations made with meridian instruments and astrolabes. Currently, optical interferometers are being developed to achieve greater accuracy. A stellar reference frame of bright stars is derived from such observations, and interpolation to fainter stars is achieved by photographic surveys. The accuracy of all ground-based optical astrometry is limited by thermal and mechanical instabilities in the telescopes, but also mainly by uncertainties in the amount of atmospheric refraction. The *Hipparcos satellite was launched to overcome these limitations. Global astrometry at radio wavelengths is carried out by interferometers, with both short and very long baselines. In small-field astrometry, relative positions are measured within the area observable with long-focus telescopes, by means of photographic plates or, more recently, CCDs. Its main aims are to measure relative proper motions and trigonometric parallaxes of stars, to discover *astrometric binaries, and to identify faint optical counterparts of objects detected at other wavelengths.

Astronavigation

astronavigation definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy astronavigation The technique of position-finding by reference to celestial bodies. An observer uses a sextant to observe the altitude of a celestial body, such as the Sun, the Moon, a planet, or a bright star. From several observations of the same object at different times, or of different objects at the same time, the observer's position can be calculated. Traditional astronavigation on Earth has been largely superseded by methods using artificial satellites or radio beacons.

Astronomical Triangle

astronomical triangle definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy astronomical triangle A triangle on the celestial sphere in which the three sides are formed by the arcs of great circles. Such a triangle is called a spherical triangle in spherical trigonometry (the geometry of figures drawn on a sphere).

Astronomical Unit

astronomical unit definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy astronomical unit (AU) A unit of length, formerly the mean distance of the Earth from the Sun, but now defined more technically, as below. In the original definition of the AU by C. F. *Gauss, the Earth's mean distance from the Sun (i.e. the semimajor axis of the Earth's orbit) was given by the precise form of Kepler's third law as found by Newton: n2a3 = k2 (m + mE), where n is the Earth's mean motion (in radians per day), a is the semimajor axis of the Earth's orbit (in AU), m and mE are the masses of the Sun and Earth respectively (in solar masses), and k is the *Gaussian gravitational constant. The astronomical unit is now defined as the distance from the Sun of a massless particle moving in a circular orbit around the Sun with an orbital period of one Gaussian year of 2π/k ephemeris days. The Earth's mean distance from the Sun is 1.000 000 031 AU, where 1 AU = 149 597 870 km.

Astronomy

astronomy definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy astronomy The study of the space beyond the Earth and its contents; phenomena in the Earth's upper atmosphere that have their origin in space, such as aurorae and meteors, are also included. Before the application of the telescope to astronomy early in the 17th century, astronomy was concerned purely with measuring the positions and movements of celestial bodies, a branch now known as *astrometry. The realization in the 17th century that *gravitation governed the movements of celestial bodies led to mathematical methods for calculating orbits, now known as *celestial mechanics. In the 19th century the development of spectroscopy, by which the composition of bodies could be determined by analysing their light, marked the birth of *astrophysics. The recognition in the 20th century that galaxies exist outside our own and the discovery of the expansion of the Universe gave rise to modern *cosmology, which deals with the origin and evolution of the Universe.

Astrophotography

astrophotography definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy astrophotography The application of photography in astronomy. Images can build up for long periods on a photographic emulsion or *charge-coupled device (CCD), revealing stars and other objects invisible to the eye, and are recorded permanently, allowing their positions and brightnesses to be measured accurately. Astrophotography began in earnest in 1883, when A. A. *Common photographed the Orion Nebula with a 36-inch (0.91-m) reflector, recording stars which could not be seen visually with the same instrument. Astronomers were quick to exploit the new medium, and for the next hundred years it was the primary means of making optical observations. Its application to *spectroscopy was equally important. In astrophotography, a photographic emulsion or CCD is placed at the focus of a telescope, instead of an eyepiece. The emulsion may be on a glass plate or film. A shutter is required for making the exposure, which can last many minutes or even hours. During the exposure the telescope must be guided accurately on the object being photographed (see Guider). Conventional emulsions suffer from *reciprocity failure, which limits effective exposure times to less than 30 min. Emulsions specifically designed for astrophotography are available with reduced reciprocity failure. Alternatively, conventional emulsions can be *hypersensitized to improve their long-exposure performance. Colour photography is possible using conventional colour film, but the best results with long exposures come from three-colour photography. Separate exposures are made on hypersensitized black-and-white emulsions, each filtered to record only red, green, or blue light. The separate exposures are then combined in the darkroom to give a full-colour picture. Photographic emulsions have now been almost entirely replaced for professional purposes by CCDs, which have a greater and more linear sensitivity to light.

Astrophysics

astrophysics definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy astrophysics The study of the physical nature of the Universe and the objects in it, notably stars, galaxies, and the composition of the space between them. Astrophysics originated in the application of spectroscopy to the study of starlight in the 19th century. It complements the traditional branches of astronomy, *astrometry and *celestial mechanics, which are concerned with the positions and motions of objects. Observational astrophysics interprets the electromagnetic and gravitational radiation emitted by celestial objects. Theoretical astrophysics attempts to explain the processes involved, which can lead to new understanding of the behaviour of matter under conditions not encountered on Earth. For example, nuclear physics had to develop before energy generation inside stars could be understood, and study of objects such as white dwarfs and neutron stars has helped confirm predictions about the behaviour of matter under extreme compression and intense gravitational fields. Astrophysics can also probe the extremely tenuous gas between the stars, where complex molecules are formed and high-energy particles called cosmic rays move close to the speed of light. It also addresses ultimate questions about the origin of the Universe, the conditions shortly after its creation, and the origin of the chemical elements. Closer to home, astrophysics deals with matters such as the environment of the planets and the effects of the solar wind, which could explain short-term changes in our weather and long-term changes in climate. Many areas of physics are involved in such studies, including spectroscopy, plasma physics, atomic physics, and relativity. In recent years, astrophysical advances have resulted from observations by satellites in space, which allow astronomers to study the Universe at all wavelengths from radio waves to gamma rays.

Astrum

astrum definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy astrum A star-shaped feature consisting of a pattern of cracks radiating from a focus on the surface of Venus; pl. astra The term is not a geological one, and has not yet been officially allotted to any feature.

Asymptotic Giant Branch Star

asymptotic giant branch star definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy asymptotic giant branch star (AGB star) A star that occupies a strip in the Hertzsprung-Russell diagram that is almost parallel to, and just above, the giant branch. Stars evolve from the *horizontal branch to the asymptotic giant branch when they have exhausted the helium in their core and are burning it in a shell (see Helium Shell Flash). At the end of the AGB stage, stars with between about 1 and 8 solar masses undergo extensive mass loss through a vigorous stellar wind, exposing the core and leaving the star with a surrounding envelope of gas and dust. A star in such a stage is known as a post-AGB star, or a *protoplanetary nebula.

Ataxite

ataxite definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy ataxite A class of iron meteorite which is nickel-rich, containing more than 12% nickel. The ataxites contain discrete spindles of kamacite (a nickel-poor iron-nickel alloy), fringed by taenite (up to 50% nickel), forming an octahedral arrangement (see Octahedrite). A mixture of kamacite and taenite, called plessite, fills the spaces between the spindles. Ataxite meteorites do not show an obvious *Widmanstätten pattern, nor can the fine striations known as *Neumann lines be seen. The *Hoba West meteorite is an ataxite.

Atmosphere

atmosphere definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy atmosphere The gaseous envelope surrounding an astronomical body. Several planets (including the Earth) retain considerable atmospheres because of their strong gravity. Motions of the gas within planetary atmospheres in response to heating, coupled with rotational forces, generate weather systems. The planetary satellites Titan and Triton also possess atmospheres. Pluto has a 'seasonal' atmosphere, which forms when the planet is close to perihelion, condensing out at aphelion. See also Solar Atmosphere; Stellar Atmosphere.

Atmospheric Extinction

atmospheric extinction definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy atmospheric extinction The loss of starlight in passing through the Earth's atmosphere. Most of the loss arises from *Rayleigh scattering by molecules of nitrogen and oxygen. At certain wavelengths there is *selective absorption from molecules of oxygen, ozone, and water vapour. Particles of dust and industrial pollutants can also contribute to extinction by *Mie scattering. Atmospheric extinction is proportional to the *airmass and the atmospheric pressure. At sea level and in a perfectly clear sky a star 80° from the zenith appears 1 mag. fainter than it would at the zenith. This in turn is 0.3 mag. fainter than if no atmosphere were present. These figures apply only to visual observations; in blue light, the extinction is greater, while in red light it is less. This is why the Sun and Moon in particular appear red when near the horizon.

Atmospheric Refraction

atmospheric refraction definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy atmospheric refraction The displacement in apparent direction of a celestial object caused by the refraction of its light in passing through the Earth's atmosphere. Refraction increases the observed altitude of an object. For an object on the horizon, the amount is about 34′ (i.e. just over half a degree). According to a simple atmospheric model, the refraction at zenith distances up to about 45° is proportional to the tangent of the zenith distance, but the exact figure depends on the atmospheric pressure, temperature, and humidity. See also Horizontal Refraction.

Atmospheric Window

atmospheric window definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy atmospheric window A range of wavelengths to which the Earth's atmosphere is relatively transparent, so that astronomical observations can be made from the ground. The major windows are in the visible, infrared, and radio parts of the spectrum. The visible window extends from about 0.3 to 0.9 μm. There are *infrared windows at several wavelengths between about 1.25 and 30 μm, and beyond 300 μm. The *radio window extends from about 1 cm to 30 m.

Atom

atom definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy atom The smallest part of a chemical element that can take part in a chemical reaction. An atom is composed of a nucleus made up of protons and neutrons, surrounded by electrons orbiting at different *energy levels. The number of protons in the nucleus of an element's atom is referred to as the atomic number of the element or atom; the total number of protons and neutrons is the mass number. In a neutral atom the number of protons is equal to the number of electrons.

Atomic Clock

atomic clock definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy atomic clock A device that generates a signal at a precisely known frequency which is locked on to some fundamental atomic resonance. Caesium-based frequency standards are the most basic since the *second is defined in terms of properties of the caesium atom (see Atomic Time). Atomic clocks achieve an accuracy of about one part in 1014, equivalent to a second in 3 million years. Slightly greater accuracy can be achieved, over short periods of time at least, by hydrogen maser devices which provide a signal at the frequency corresponding to the 21-cm radio line.

Atomic Time

atomic time definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy atomic time The time-scale used for all precise timekeeping, including civil time. It is based upon atomic frequencies and is the most accurate and consistent available today. The fundamental unit is the SI *second, which is defined in terms of a particular spectral line of the caesium-133 atom. The frequency of this microwave line is adopted as precisely 9 192 631 770 Hz. The SI second is the basis for International Atomic Time (TAI). TAI was officially introduced by international agreement in January 1972, but had been available since 1955. When TAI was officially introduced, the length of the SI second was the same as that of a second in *Ephemeris Time, the time-scale then used in astronomy. The two time-scales differed by a fixed amount (ET = TAI + 32.184s). Strictly speaking, however, the two definitions of the second were conceptually different. This difference was removed in 1984 when Terrestrial Dynamical Time (now known simply as *Terrestrial Time) was introduced to replace Ephemeris Time. Terrestrial Time has the SI second as its basic unit and differs from TAI only by the constant offset mentioned above. TAI is the basis not just of astronomical time-scales but also of civil timekeeping. Broadcast time signals use *Coordinated Universal Time (UTC), which is TAI with an offset of a whole number of seconds. This offset requires occasional adjustment by the insertion of leap seconds.

Attenuation

attenuation definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy attenuation 1 The reduction in strength of an electromagnetic wave by absorption or scattering in the medium through which it passes; for example, the extinction of starlight by interstellar dust. 2 The deliberate reduction of the signal strength in a radio receiver to prevent following stages of amplification being overloaded.

Aubrite

aubrite definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy aubrite A class of calcium-poor achondrite meteorite; also known as enstatite achondrites. Aubrites consist almost entirely of the silicate mineral enstatite. They resemble the *enstatite chondrites, to which they are probably related. Aubrites have large grain sizes (sometimes exceeding several centimetres), which suggests that they formed within cooling magmas; alternatively, they may have originated through processes occurring in the solar nebula. Almost all known aubrites are *breccias. They are named after a meteorite that fell at Aubres, France, on 1836 September 14.

Augmentation

augmentation definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy augmentation The increase in the apparent diameter of a celestial body as seen from the surface of the Earth compared with the apparent diameter it would have if seen from the centre of the Earth.

Aurora

aurora definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy aurora An emission of light from the Earth's high atmosphere, caused principally by oxygen atoms or nitrogen molecules that are excited by electrons accelerated within the magnetosphere. The visible aurora is dominated by the green (557.7 nm wavelength) and red (630 nm) emissions of oxygen, occurring respectively at altitudes of 100 km and above about 400 km, and red (661-686 nm) nitrogen emissions at about 95 km. Violet purple (391.4 nm) nitrogen emissions are sometimes seen in the sunlit uppermost parts of aurorae at altitudes of 1000 km. Popularly known as the northern lights (or southern lights in the southern hemisphere), the aurora takes a number of characteristic forms. These may range from a glow, low over the northern horizon (from which the aurora borealis—'northern dawn'—takes its name), through arcs and bands, which may be homogeneous, or may show vertical rays. Isolated rays and patches of auroral light may also be seen. Most spectacular of all is the corona, a perspective effect whereby rays appear to converge on a single region of the sky almost overhead during a particularly intense storm. During strong activity, the rays and other structures move, causing a 'curtain' effect, and there may often be rapid changes in brightness. The aurora is seen from the southern hemisphere as the aurora australis, a mirror-image of activity present at the same time over the opposite hemisphere of the Earth. Auroral activity is present more or less continuously around the high-latitude *auroral ovals. Observers at lower latitudes, such as in the British Isles, southern United States, or Australasia, see auroral activity only when the magnetosphere is disturbed by violent solar events. Mid-latitude aurorae are usually triggered by solar flares or *coronal mass ejections, and are most likely around times of high sunspot activity. Aurorae also occur on Jupiter, Saturn, Uranus, and Neptune. Web Link: http://earthobservatory.nasa.gov/IOTD/view.php?id=44348&src=fb

Auroral Oval

auroral oval definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy auroral oval A high-latitude ring of more or less permanent aurora, girdling the geomagnetic pole at a distance of 2000-2500 km under quiet geomagnetic conditions. There are two ovals, one in either hemisphere, each the mirror-image of the other. They are displaced such that the dayside edge is closer to the pole than that on the night side. Under disturbed geomagnetic conditions, the auroral ovals brighten, broaden, and expand towards the equator, particularly on the night side, so the aurora becomes visible at lower latitudes. The auroral ovals remain relatively fixed in space above the rotating Earth.

Australite

australite definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy australite A tektite from the largest strewn field of tektites in the world, which covers the whole of southern Australia, including Tasmania. Estimates of the *ablation age of australites are in the range 600 000-750 000 years. The 1-km Mount Darwin crater in western Tasmania has a similar age (730 000 years), but it seems too small to be their source.

Autocorrelation

autocorrelation definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy autocorrelation A mathematical operation used in signal processing, especially as the first stage in obtaining the spectrum of a radio source. The autocorrelation function of a signal is a measure of the degree to which successive segments of the signal resemble each other. See Autocorrelator.

Autocorrelator

autocorrelator definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy autocorrelator A digital device used in radio astronomy to make a spectrum of a radio source.

Autoguider

autoguider definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy autoguider A *guider that causes a telescope to follow a guide star automatically by feeding control signals to the telescope's drive system. It may use a photomultiplier which views the star through the centre of a rotating mask, or a *charge-coupled device (CCD) with software which monitors the position of the guide star. Some CCDs use a system known as track and accumulate in which software compensates for drift of the image when superimposing several short exposures to give the effect of one long one. The autoguider may be attached to either a *guide telescope or an *off-axis guider.

Autoionization

autoionization definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy autoionization A process by which an excited electron within an atom or molecule imparts sufficient energy to another electron in the same atom or molecule for it to escape. It is the inverse of *dielectronic recombination. See also Bound-Free Transition.

Averted Vision

averted vision definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy averted vision A technique used to see faint objects visually. It involves looking to one side of an object's position, rather than directly at it, so that light falls on to the outer part of the retina, which is more sensitive than the centre of vision. To avoid the risk of the object falling on the eye's blind spot, it should be kept on the side of the field of view nearest the nose.

Axion

axion definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy axion A hypothetical elementary particle which has been proposed as a candidate for *dark matter in the Universe. Axions are expected to have been produced abundantly in the early phases of the hot Big Bang. Although the axion is predicted to have a very tiny mass (about 10−11 of the mass of an electron) and might therefore be expected to be a form of *hot dark matter, it interacts so weakly with radiation that it is, in fact, a plausible candidate for *cold dark matter.

Axis, Rotation

axis, rotation definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy axis, rotation The imaginary line around which a body spins. The axis of rotation joins a body's north and south poles.

Azimuth

azimuth definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy azimuth (A) The direction to a celestial object measured in degrees, clockwise from north around the observer's horizon. Azimuth is 0 ° for an object due north, 90 ° due east, 180 ° due south, and 270 ° due west.

Ångström, Anders Jonas

Ångström, Anders Jonas definition from A Dictionary of Astronomy - https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.oxfordastronomy Ångström, Anders Jonas (1814-74) Swedish physicist and astronomer. He noted that the dark *Fraunhofer lines in the Sun's spectrum had the appearance of a reversed emission spectrum. In 1861 he began an intensive study of the solar spectrum, confirming the presence of hydrogen in the Sun. In 1868 he published an atlas of the solar spectrum and measurements of the wavelengths of over a thousand spectral lines. These measurements were expressed in units of 10−10 m, a length subsequently named the angstrom unit in his honour.

Alnair

The star Alpha Gruis, magnitude 1.7. It is a B7 dwarf, 101 l.y. away.

Alphard

The star Alpha Hydrae, magnitude 2.0. It is a K3 giant, 180 l.y. away.

Advanced Technology Solar Telescope

A 4-m reflector for solar studies being built at an altitude of 3055 m on Haleakala, Hawaii, by a consortium of over 20 US institutions led by the National Solar Observatory. Using adaptive optics it will be able to resolve features on the Sun as small as 30 km across at wavelengths from the near ultraviolet to the infrared (0.3-28 μm). When opened in 2014 or later it will be the largest solar telescope in the world.

Akari

A Japanese infrared astronomy satellite, known before its launch in 2006 February as ASTRO-F; the name means 'light' in Japanese. Akari carries a 0.69-m telescope equipped with two main instruments: the Far-Infrared Surveyor (FIS) to make an all-sky survey in the wavelength range 50-180 μm; and the InfraRed Camera (IRC) for targeted observations in the near and medium infrared range, 1.7-25.6 μm. Akari's observations extend and improve those of the Infrared Astronomical Satellite (IRAS).

Advanced Composition Explorer

A NASA spacecraft to study the composition of the solar wind and cosmic rays, launched 1997 August. It is stationed in a halo orbit around the L1 Lagrangian point 1.5 million km sunwards of the Earth. ACE carries nine instruments to sample low-energy particles of solar origin and high-energy particles from the Galaxy. From its position upstream in the solar wind it provides advance warning of geomagnetic storms.

A Band

A broad Fraunhofer line in the Sun's spectrum at around 759 nm, due to absorption by oxygen in the Earth's atmosphere. Because the oxygen is in molecular form, the A band is actually a group of close, regularly spaced lines over the range 759-768 nm, unresolved at low resolution.

All-Sky Camera

A camera with a field of view that includes all, or nearly all, of the sky on one frame. The images from such cameras are circular, and have the zenith at the centre and the horizon around the edge. They are used in particular for meteor and fireball 'patrols'. A simple design of all-sky camera consists of a convex mirror which reflects the image of the sky to a camera above it, pointing downward. More advanced designs use ultra-wide-angle lenses.

Abell Catalogue

A catalogue of 2712 rich clusters of galaxies published in 1958 by the American astronomer George Ogden Abell (1927-83) from inspection of the Palomar Observatory Sky Survey photographs. The catalogue had well-defined criteria for selection of the clusters (see Abell Cluster). A later extension to the southern sky (published 1989) was based on photographs taken with the United Kingdom Schmidt Telescope in Australia.

Adiabatic Process

A change or process in which no heat enters or leaves a system, as occurs for example in an expanding or contracting gas cloud. An adiabatic change is usually accompanied by a rise or fall in the temperature of the system; ionization of atoms or dissociation of molecules may also occur.

Achondrites

A class of stony meteorite usually (though not always) lacking the tiny, rounded inclusions known as chondrules found in chondrites. Achondrites make up about 9% of all meteorite falls. They consist principally of one or more of the minerals plagioclase, pyroxene, and olivine. The main distinction between achondrites and chondrites is that the achondrites have different abundances of calcium and similar elements, and almost no metal or sulphide. Achondrites are thought to have crystallized from a magma in the same way as terrestrial rocks. The achondrites are divided into five main classes.

Abell Cluster

A cluster of galaxies listed in the Abell Catalogue. To appear in the catalogue, a cluster must satisfy selection criteria which include containing more than 50 galaxies and having a dense concentration (richness). The clusters are classified as regular (R) or irregular (I) in appearance, ranked in increasing richness from 1 to 5, and increasing distance from 1 to 6. The approximate frequency of Abell clusters is one per 2.4 × 105 cubic megaparsecs.

Almagest

A compendium of astronomical and mathematical knowledge written by Ptolemy in about ad 150. It incorporates the star catalogue compiled by Hipparchus, upon whose work Ptolemy may have drawn for other parts of the book. It is the most complete surviving treatise on ancient astronomy, and contains descriptions of the 48 Greek constellations on which our present-day constellation system is based. Its original Greek title was Syntaxis; Almagest, the name it acquired when translated into Arabic in about ad 820, means 'the greatest'.

Absorption Line

A dark feature in the spectrum of a star, formed by cooler gas in the star's outer layers (the photosphere) that absorbs radiation emitted by hotter gas below. The Fraunhofer lines in the solar spectrum are the best-known examples. Each line is a unique signature of the element or molecule that forms it, which enables the chemical composition of the star to be determined.

Albedo Feature

A feature on a planet that is markedly darker or brighter than its surroundings. Albedo features do not always correspond to a topographic or geological feature, as differences in albedo can arise from variations in surface composition as well as topography. For example, one of the darkest features on Mars seen through a telescope is Syrtis Major, but spacecraft have found no distinctive differences in topography or type of terrain between it and its surroundings. In general, though, albedo features are related to different terrain types, as on the Moon where the dark maria are plains of solidified lava and the bright spots are relatively recent impact craters.

Advance of Perihelion

A gradual turning of the major axis of an orbit in the same direction as the body moves along the orbit; also known as apsidal motion. An advance of perihelion means that the longitude of perihelion of the orbit increases. In the Solar System, the effect is caused mostly by the gravitational attractions of the planets. A similar effect, the advance of pericentre, is seen in the orbits of binary stars, caused by the oblateness of the stars themselves. In the 19th century, a small unexplained advance of Mercury's perihelion of some 43″ per century was attributed to an undiscovered planet within the orbit of Mercury (see Vulcan). This effect is now known to be caused by the curvature of space near the Sun, as predicted by the general theory of relativity.

Absorption Edge

A limiting feature in a series of absorption lines from a single element (e.g. hydrogen), representing the wavelength at which the element becomes ionized. In the Balmer series of hydrogen, the lines become closer together as the series limit of 364.6 nm is reached, forming an edge at that wavelength (see Balmer Limit). Similarly, in the Lyman series the lines become closer towards the Lyman limit of 91.2 nm.

Absorption Coefficient

A measure of the decrease in intensity of radiation as it passes through a medium. It is the reciprocal of the distance required to reduce the radiation to 1/e of its original value (where e is a constant equal to 2.718).

Alpha Capricornid Meteors

A meteor shower of generally low activity (maximum ZHR 10) between July 3 and August 15. There appear to be several maxima, the principal one around July 30. Alpha Capricornid meteors are often long, slow, and bright. At maximum, the radiant lies at RA 20h 28m, dec. −10°.

Allende Meteorite

A meteorite of carbonaceous chondrite type, which fell near Pueblito de Allende, northern Mexico, on 1969 February 8, scattering thousands of fragments over an area of 48 × 7 km. The parent body probably weighed over 30 tonnes. More than 2 tonnes of CV3-type material were collected, the largest piece weighing 110 kg. The formation age of this material is 4.6 billion years, making it some of the oldest primordial planetary material to be recovered.

Aeon

A period of 109 years (i.e. a billion years); US eon.

Aliasing

A phenomenon in which a digitized signal is observed to contain spurious low-frequency components. These occur if the original signal is sampled at a rate insufficient to record the highest frequencies present (undersampling).

Almanac

A publication, usually issued yearly, listing predicted dates and times of forthcoming celestial phenomena and positions of celestial objects, along with other information of interest to astronomers, navigators, and surveyors. Examples are The Astronomical Almanac and The Nautical Almanac.

AI Velorum Star

A pulsating variable star of short period (0.04-0.2 days) that closely resembles a Delta Scuti star, although with a greater amplitude (0.3-1.2 mag. or more) and somewhat higher luminosity. The AI Velorum stars may be the older of the two groups. They are also called high-amplitude Delta Scuti stars.

Alpha Cygnid Meteors

A putative meteor shower with ill-defined activity limits, emanating throughout July and August from an apparently stationary radiant at RA 21 h 00m, dec. +48° near Deneb. Observed rates seldom exceed 1-3 meteors/h, and there are doubts as to the shower's authenticity.

Algonquin Radio Observatory

A radio observatory in Algonquin Park near Lake Traverse, Ontario, site of a 45.7-m antenna opened in 1966. It was operated by the National Research Council of Canada until 1991 and then jointly by the Geodetic Survey Division of Natural Resources Canada, Ottawa, and the Space Geodynamics Laboratory of York University, Toronto. Since 2008 it has been operated by Thoth Technology Inc. of Kettleby, Ontario. It is now used primarily for geodetic and astrometric Very Long Baseline Interferometry (VLBI).

Allen Telescope Array

A radio telescope array at Hat Creek Radio Observatory, California, designed to search for extraterrestrial intelligence while simultaneously carrying out astronomical observations. The ATA is jointly owned and operated by the SETI Institute of Mountain View, California, and the University of California, Berkeley. When completed it will consist of 350 antennas, each of 6.1 m diameter, which can be combined electronically to form 16 different beams, each having the sensitivity of a single 114-m dish and the resolution of a 700-m dish. The ATA observes at wavelengths from 3 cm to 60 cm simultaneously. Its first 42 antennas began operation in 2007. Two intermediate stages, with 98 and 206 antennas, will enter service before the full array is completed. The ATA takes its name from the Paul G. Allen Foundation, which provided funding.

Abell Radius

A radius of about 2 megaparsecs within which at least 50 galaxies of a particular range of brightness must be found if the cluster is to qualify as an Abell cluster.

A-Class Asteroid

A rare class of asteroid that has both a moderately high albedo (0.13-0.35) and an extremely reddish spectrum at wavelengths shorter than 0.7 μm. Strong absorption in the near-infrared is interpreted as indicating the presence of the mineral olivine. Members of this class include (246) Asporina, diameter 60 km, and (446) Aeternitas, diameter 45 km.

Alidade

A simple sighting device for measuring altitudes. It consists of a bar pivoted so as to swing in a vertical plane and be aligned with a celestial object. The object's altitude can then be read off from a scale. Alidades were often incorporated in ancient position-measuring instruments, such as astrolabes.

ALEXIS

A small US Department of Energy satellite carrying six telescopes to map the soft X-ray and extreme ultraviolet background over the whole sky. ALEXIS (Array of Low-Energy X-ray Imaging Sensors) was launched in 1993 April. The mission ended in 2005 April.

Almucantar

A small circle on the celestial sphere that is parallel to the horizon. All objects on an almucantar are at the same altitude at a given time.

Alexandra Family

A small family of asteroids at a mean distance of 2.6-2.7 AU from the Sun, with inclinations of 11-12°. The family is unusual in that its members are of varying composition (classes C, G, and T). The family is named after the C-class (54) Alexandra, 166 km in diameter, discovered in 1858 by the German astronomer Hermann Mayer Salomon Goldschmidt (1802-66). Alexandra's orbit has a semimajor axis of 2.713 AU, period 4.47 years, perihelion 2.18 AU, aphelion 3.25 AU, and inclination 11°.8.

Agglutinate

A small object consisting of impact glass and fragments of minerals or rocks, all welded together into an aggregate. Agglutinates are produced by the impact of micrometeorites into the lunar regolith or other planetary surface.

Aerosol

A small particle, either solid or liquid, suspended in an atmosphere. Aerosols cause atmospheric extinction. Some properties of the particles, such as their mean size and oblateness, may be inferred from the way in which an aerosol layer scatters light.

Active Prominence

A solar prominence with very rapid motion (up to 2000 km/s), often associated with a flare. Active prominences are located at low latitudes on the Sun, where sunspots and active regions are usually found. The main categories include loop prominences, coronal rain, surge prominences, sprays, and arch filament systems.

Absorption Spectrum

A spectrum that consists primarily of dark absorption lines, created when light from a hot source passes through cooler material. The spectra of normal, cool stars such as the Sun fall into this category.

Ae Star

A star of spectral type A which exhibits emission lines in its spectrum (hence the suffix 'e'). Usually these are sharp emission lines of hydrogen, and are superimposed on an otherwise normal spectrum. These lines arise in a surrounding expanding shell or disk of material. Ae stars are young stars still in the process of formation. See also Herbig Ae/Be Star.

Accretion Disk

A structure that forms around a compact object (e.g. a white dwarf, neutron star, or black hole) when matter flows towards it. Accretion disks are found in interacting binary stars, and are assumed to exist in active galactic nuclei and quasars. In a binary, mass lost from the secondary star can form a disk of gas around the compact object. The disk may have a hot spot (1) where the stream of material hits its outer edge. Material is fed from the inner edge of the disk through a boundary layer (which may radiate as much energy as the disk itself) on to the compact object. When the compact object has an extremely strong magnetic field, as in an AM Herculis star, the material may form an accretion column over each magnetic pole, rather than a disk. The gravitational energy that is released can cause high ultraviolet or X-ray luminosities, and may accelerate jets of material from the disk to very high speeds.

Active Optics

A system that compensates for the deforming effects of gravity on a telescope's mirrors, maintaining their surface accuracy and alignment. The image of a guide star is examined as the telescope tracks it across the sky. Actuators behind the mirrors control movable supports to preserve the mirrors' shape and alignment. The first large telescope to employ active optics was the New Technology Telescope at the European Southern Observatory.

Alfvén Wave

A transverse wave that occurs in a region containing a magnetic field and a plasma. The ionized and therefore highly conducting material of the plasma is said to be 'frozen in' to the magnetic field and is forced to take part in its wave motion. At least part of the energy heating the Sun's corona is thought to be provided by Alfvén waves propagating from the outer layers of the Sun. The waves are named after H. O. G. Alfvén.

Algol Star

A type of eclipsing binary with periods of constant (or almost constant) brightness between well-defined eclipses; abbr. EA. This feature is an indication that the system is a detached binary or semidetached binary. The secondary minimum may be invisible. Periods range from 0.2 to 10 000 days, and the amplitudes may reach several magnitudes. If mass transfer occurs, the material is accreted directly on to the hot star, not via an accretion disk. Mass transfer via an accretion disk does occur in the W Serpentis stars, which have wider separations and may be pre-Algol stars. The terms W Serpentis star and Serpentid are sometimes used to include both groups of stars.

Alpha Cygni Star

A type of supergiant pulsating variable star exhibiting non-radial pulsation; abbr. ACYG. The spectral types are Be-Ae Ia, and the optical amplitude is approximately 0.1 mag. Multiple pulsation frequencies are superimposed, giving rise to light-curves that often appear highly irregular. The periods range from a few days to several weeks.

AGK

Abbr. for Astronomischen Gesellschaft Katalog, a series of catalogues of star positions. AGK1 covered most of the sky, observed by meridian circles around the world; it was published between 1890 and 1954. AGK2 (1951-8) was a repetition of AGK1, with additional stars, from the north pole to −2° declination observed photographically from Hamburg and Bonn in about 1930. It contains 181 581 stars, mostly to about magnitude 9 plus some fainter stars. AGK2A (published 1943) contains positions of 13 747 reference stars for AGK2, mostly of 8th and 9th magnitudes, derived from meridian observations made from Germany and Pulkovo, Russia. AGK3 (published 1975) was a reobservation of AGK2; the prefix AGK was retained even though the catalogue was not published under the auspices of the Astronomischen Gesellschaft. AGK3R contains the positions of 21 499 reference stars for AGK3, observed in an international programme from about a dozen observatories.

Aitken, Robert Grant

American astronomer. At Lick Observatory he carried out a vast survey of double stars, initially with William Joseph Hussey (1862-1926), discovering over 3100 new pairs. In 1923 he was the first to see the companion of Mira. His New General Catalogue of Double Stars was published in 1932 (see Aitken Double Star Catalogue).

Adams, Walter Sydney

American spectroscopist, born in Syria. He was the first to detect systematic differences in the spectra of giant and dwarf stars. From 1914, with the German astronomer Arnold Kohlschütter (1883-1969), Adams developed methods of establishing the surface temperature, luminosity, and distance of stars from their spectra. In 1918 he showed that the density of the white dwarf Sirius B is 50 000 g/cm3. Adams also carried out spectrographic studies of the atmospheres of Mars and Venus.

AGILE

An Italian satellite for high-energy astrophysics, launched in 2007 April to observe at energies of 30 MeV-50 GeV and 10-40 keV; the name is an acronym of Astro-rivelatore Gamma a Immagini LEggero. AGILE carries three main instruments: a Gamma-Ray Imaging Detector (GRID) with an exceptionally large field of view, covering the energy range 30 MeV-50 GeV across one-fifth of the entire sky simultaneously; a hard X-ray coded-mask imaging detector, Super-AGILE, observing at 10-40 keV; and a caesium iodide Mini-Calorimeter (MC), a non-imaging detector for the 0.25-200 MeV range, providing spectral and timing information of transient events. The satellite's targets include gamma-ray bursts, AGNs, solar flares, and pulsars.

Active Region

An area on the Sun where magnetic fields emerge through the photosphere into the chromosphere and corona. Active regions on the photosphere include sunspots and faculae. Their counterparts higher in the chromosphere are plages. Also in the chromosphere are dark fibrilles and filaments. Active regions in the corona are areas of enhanced density and temperature sometimes called coronal condensations. Other examples of active regions are areas on the photosphere where sunspots have faded, and X-ray bright points in the corona.

Aberration, Optical

An imperfection or error in the image produced by a lens, mirror, or optical system. There are six types of aberration: chromatic aberration, spherical aberration, coma (see Coma, Optical), field curvature, distortion, and astigmatism. Chromatic aberration is not present in images formed by mirrors. All can be corrected to a greater or lesser extent by suitable optical design.

Adaptive Optics

An optical design that can rapidly counteract the effects of atmospheric seeing on an image. This may be done by deforming a mirror in the light path of a telescope to keep a star's image as point-like as possible. The system may use as a reference a real star, or an artificial star produced by shining a laser up through the layers of air that are causing the bad seeing. Any extended objects in the field, such as galaxies, will also be sharpened. This technique can increase the resolution of a ground-based telescope by a factor of 40.

Adonis

Asteroid 2101, the second of the Apollo group to be discovered, by the Belgian astronomer Eugène Joseph Delporte (1882-1955) in 1936, when it passed within 0.015 AU (2.2 million km) of the Earth. It was not seen again until 1977. Adonis is about 1 km in diameter. Its orbit has a semimajor axis of 1.875 AU, period 2.57 years, perihelion 0.44 AU, aphelion 3.31 AU, and inclination 1°.3.

Achilles

Asteroid 588, the first Trojan asteroid to be discovered, by Max Wolf in 1906. It is a member of the group of Trojans at the L4 Lagrangian point 60° ahead of Jupiter. Achilles is a D-class asteroid of diameter 135 km. Its orbit has a semimajor axis of 5.195 AU, period 11.84 years, perihelion 4.43 AU, aphelion 5.96 AU, and inclination 10°.3.

Achromatic

Describing a lens consisting of two or more optical components (elements), intended to correct for chromatic aberration. Commonly used as the objective of small refractors, the achromatic lens (or achromat) was invented in 1729 by the English optician Chester Moor Hall (1703-71) and first manufactured commercially by J. Dollond in 1758. It has one element of crown glass and another of flint glass. The dispersion (1) of the crown glass compensates for the chromatic error of the flint glass, while still leaving some refractive power. The two-element design is termed an achromatic doublet. It is practically impossible to correct all wavelengths of light, however, and most lenses adopt a compromise, bringing two particular wavelengths to a common focus, thus reducing the false colour. A lens that corrects for more than two wavelengths is termed an apochromatic lens.

Afocal

Describing an optical system in which an image is transferred without bringing it to a focus. Afocal photography, for example, involves pointing a camera, focused on infinity, into the eyepiece of a telescope whose image also appears at infinity. When an image is transferred as a beam of parallel light rays, the beam is termed an afocal beam.

Airy, George Biddell

English astronomical administrator and geophysicist. During his tenure as the seventh Astronomer Royal, the Royal Observatory at Greenwich became a model of efficiency for positional astronomy. He belittled pure research, however, which made Greenwich a late starter in the fields of spectroscopy and astrophysics. The transit telescope he installed at Greenwich in 1851 now defines the position of 0° longitude on Earth. Airy's only significant astronomical discovery was of irregularities in the orbits of Venus and the Earth. In 1854, by making gravity measurements at the top and bottom of a mineshaft, he estimated the Earth's mass.

Adams, John Couch

English mathematical astronomer. In 1843 he began to calculate the orbit of a new planet whose gravitational effects would explain why Uranus did not follow its predicted path, but he did not complete his calculations or publish them. In 1846 the new planet, subsequently named Neptune, was sighted by J. G. Galle as a result of independent calculations by U. J. J. Le Verrier. Adams and Le Verrier were eventually both credited with predicting Neptune's existence, although it is now clear that the credit belongs mainly to Le Verrier. Adams' later work included calculation of the Moon's secular acceleration, and of the orbital elements of the Leonid meteor swarm.

Aitken Double Star Catalogue

Popular name for the New General Catalogue of Double Stars Within 120° of the North Pole by R. G. Aitken, published in 1932, containing measurements of 17 180 double stars. It succeeded the General Catalogue of Double Stars published in 1906 by the American observer Sherburne Wesley Burnham (1838-1921). Double stars are often referred to by their ADS number as listed in Aitken's catalogue.

Alfvén, Hannes Olif Gösta

Swedish physicist. In the 1930s he developed a theory of sunspot formation based on the idea that under certain conditions a magnetic field can be 'frozen in' to a plasma. In 1942 he proposed that waves (now called Alfvén waves) can propagate through a plasma under conditions similar to those found in the Sun's atmosphere. His work inaugurated the study of magnetohydrodynamics, for which he was awarded a share of the 1970 Nobel Prize in Physics.

Acceleration of Free Fall

The acceleration experienced by an object falling freely in a gravitational field, also known as the acceleration due to gravity. Its mean value at the Earth's surface is 9.807 m/s2; this varies slightly with latitude, because the Earth is not perfectly spherical. On any body the acceleration due to gravity can be found from the formula g = GM/R2, where M is the mass of the body, R is its radius, and G is the universal gravitational constant.

Airmass

The pathlength of starlight through the Earth's atmosphere, expressed in relation to the value at the zenith. Airmass is 1 at the zenith and approximately 2 at an altitude of 60°. It is approximately the secant of the zenith distance, but rises more slowly as the star approaches the horizon, becoming 1% less than the secant at 17° altitude.

Alpha Crucis

The brightest star in the constellation Crux, also called Acrux. It is a double star consisting of a B0.5 subgiant, magnitude 1.3, and a B1 dwarf, magnitude 1.7. To the naked eye they appear as a single star of magnitude 0.8. Alpha Crucis is 322 l.y. distant.

Absolute Magnitude

The brightness that a star would have if it were at a distance of 10 parsecs in perfectly clear space without interstellar absorption. Absolute magnitude is usually deduced from the visual magnitude measured through a V filter, and is then written MV. If defined for another wavelength it carries a different subscript (U, B, etc.). When radiation at all wavelengths is included it becomes the absolute bolometric magnitude, Mbol. The Sun has an absolute magnitude of +4.8. Most other stars range between −9 (supergiants) and +19 (red dwarfs).

Active Galactic Nucleus

The central region of a galaxy which emits far more radiation than can be produced by stars alone. A galaxy with such a nucleus is known as an active galaxy. The first active galaxies were discovered in 1943 by C. K. Seyfert, who noticed that some spirals have an exceptionally bright core; these are now termed Seyfert galaxies. Quasars, discovered in the 1960s, appear to be a more luminous version of Seyferts in which the nucleus is so bright that the surrounding galaxy can hardly be seen at all. About 10% of active galaxies have the additional characteristic of being strong radio sources, or 'radio-loud'. The radio emission from such radio galaxies is often from regions well outside the galaxy itself known as lobes, although in some cases there are radio jets pointing back towards the centre of the galaxy, indicating that it is the nucleus which is the ultimate source of the energy.

Airy Disk

The central spurious disk of the image of a star formed by a telescope. Because of diffraction, even with perfect optics a star's image is never point-like, but consists of a central disk, the Airy disk, surrounded by several fine diffraction rings. All telescopes of given size have the same size of Airy disk, which gets smaller with increasing aperture. The size of the Airy disk is given approximately in radians by 1.22λ times the f/number, where λ is the wavelength of the light. The size of the Airy disk limits the resolving power of a telescope, although in apertures larger than about 100 mm the Airy disk is often smaller than the false disk caused by seeing. It is named after G. B. Airy.

Alpha Centauri

The closest star to the Sun, also known as Rigil Kentaurus. It is actually a triple system, consisting of a bright binary with a period of 80 years and a faint red dwarf 2° away called Proxima Centauri. The binary consists of a G2 dwarf of magnitude −0.01 and a K1 dwarf of magnitude 1.3. To the naked eye they appear as a single star of magnitude −0.28, the third-brightest in the sky. The binary pair lie 4.32 l.y. from the Sun, 0.09 l.y. farther than Proxima Centauri.

Afterglow

The electromagnetic radiation often observed after a gamma-ray burst (GRB). As a shockwave of gas from a GRB ploughs into the surrounding gas and dust, kinetic energy is converted into radiation, creating the afterglow. The emission is synchrotron radiation from electrons accelerated in magnetic fields within the shock and can be observed at X-ray, optical, and radio wavelengths in succession as the shockwave expands and cools. The afterglow appears first hours to weeks after the initial GRB is detected, and later may be detected at longer wavelengths. It can last about a week as seen at X-ray wavelengths, for several weeks at optical wavelengths, and up to a year at radio wavelengths.

Alpha Particle

The nucleus of a helium-4 atom. It consists of two protons and two neutrons and hence is positively charged. Alpha particles are emitted by the nuclei of atoms in a process of radioactive decay known as alpha decay.

Allegheny Observatory

The observatory of the University of Pittsburgh, at an altitude of 380 m in Pittsburgh, Pennsylvania, founded in 1860 but relocated on its present site, Riverview Park, in 1912. Its main instrument is the 0.76-m Thaw refractor, in operation since 1914 but fitted with a new objective lens in 1985. Other instruments include the 0.74-m Keeler astrometric reflector, opened in 1906 but given new optics in 1992.

Adams Ring

The outermost of Neptune's rings, named after J. C. Adams. It lies 62 930 km from Neptune's centre, and is about 20 km wide. It contains four denser sections, or ring arcs, named Courage, Liberté, Egalité, and Fraternité.

Ablation Age

The period of time since the outer glassy layers of a tektite solidified following ablation during its re-entry into the Earth's atmosphere. The ablation ages of known tektites vary from about 600 000 to 35 million years.

Accretion

The process by which the mass of a body increases by the accumulation of matter, in the form of either gas or small solid bodies which collide with and adhere to the body. The bodies in the Solar System are thought to have grown by accretion; some stars are surrounded by an accretion disk.

Adrastea

The second-closest satellite of Jupiter, distance 128 980 km, orbital period 0.298 days; also known as Jupiter XV. Adrastea is 20 × 16 × 14 km in size. It was discovered in 1979 with the Voyager 2 spacecraft. It lies less than one Jovian radius above the planet's cloud tops and just within the outer rim of Jupiter's main ring. Material knocked off Adrastea by micrometeorite impacts is thought to contribute to the ring.

Aberration of Starlight

The small apparent difference between the observed direction of a star and its true direction (see diagram). It is due to the combined effect of the observer's motion across the path of incoming starlight and the finite velocity of light. The actual amount of displacement and its direction depend on the observer's speed and direction of motion. Aberration of starlight resulting from the Earth's orbital motion is termed annual aberration; the much smaller effect resulting from the Earth's rotation is diurnal aberration. Planetary aberration is a combined result of the observer's motion and the time taken for light to travel from a body in the Solar System to the observer.

Alcor

The star 80 Ursae Majoris, an A5 dwarf of magnitude 4.0, distance 82 l.y. It forms a naked-eye double with Mizar.

Alderamin

The star Alpha Cephei, magnitude 2.4. It is an A7 dwarf 49 l.y. away.

Achernar

The star Alpha Eridani, a B3 dwarf of magnitude 0.45, ninth-brightest in the sky, 139 l.y. away. Its name comes from Arabic and means 'river's end'.

Aldebaran

The star Alpha Tauri. It is a K5 giant that varies irregularly by about 0.1 mag. from its average brightness of magnitude 0.87. It appears to be a member of the Hyades star cluster but is in fact only 67 l.y. away, less than half the cluster's distance.

Albireo

The star Beta Cygni, one of the best-known double stars in the sky. It consists of a K3 bright giant of magnitude 3.1 and a B9.5 dwarf of magnitude 5.1. The stars show contrasting colours of orange and blue-green. The primary is also a close visual binary with a period of just over 200 years. Albireo lies 434 l.y. away.

Algol

The star Beta Persei, the first eclipsing binary to be discovered. Algol was found to be variable in 1669 by the Italian astronomer and mathematician Geminiano Montanari (1633-87), but the period was first determined by J. Goodricke in 1782-3. Algol varies from magnitude 2.1 to 3.4 in a period of 2.867 3043 days, although the period has varied slightly. The variable period and the emission lines that are sometimes detectable in its spectrum are evidence for mass transfer, indicating that the system is a semidetached binary. The eclipsing pair, which consists of a B8 dwarf and a K2 subgiant, is accompanied by a third component (Algol C), magnitude 4.7, having an orbital period of 1.862 years. The system is a faint X-ray source and also emits radio bursts. Algol lies 90 l.y. away.

Adhara

The star Epsilon Canis Majoris, magnitude 1.5. It is a B2 supergiant, 405 l.y. away, with a companion of magnitude 7.4.

Alnilam

The star Epsilon Orionis, magnitude 1.7, the middle star of the belt of Orion. It is a B0 supergiant, estimated to be about 2000 l.y. away.

Alioth

The star Epsilon Ursae Majoris. It is an A0 subgiant of magnitude 1.8 with strong lines of chromium in its spectrum. It is a variable of the Alpha2 Canum Venaticorum type, fluctuating by a few tenths of a magnitude with a period of 5.1 days. It lies 83 l.y. away.

Alcyone

The star Eta Tauri, magnitude 2.9, the brightest member of the Pleiades star cluster. It is a B7 giant 403 l.y. away.

Alkaid

The star Eta Ursae Majoris, also known as Benetnasch. It is a B3 dwarf of magnitude 1.9, lying 104 l.y. away.

Almach

The star Gamma Andromedae; also spelled Almaak and Alamak. It is a double star consisting of a K3 bright giant and a B9 dwarf, magnitudes 2.3 and 4.8 (combined magnitude 2.1). The fainter star is itself a close double with an orbital period of 64 years. Almach is 393 l.y. away.

Alhena

The star Gamma Geminorum, magnitude 1.9. It is a subgiant of type A1 lying 109 l.y. away.

Algieba

The star Gamma Leonis. It is a binary consisting of a K1 giant of magnitude 2.6 and a G7 giant of magnitude 3.5 (combined magnitude 2.0). The stars orbit each other with a period of 510 years, and lie 130 l.y. away.

Algenib

The star Gamma Pegasi, a B2 subgiant of magnitude 2.8, lying 392 l.y. away. Algenib is also an alternative name for the star *Mirfak (Alpha Persei).

Acamar

The star Theta Eridani, magnitude 2.9. It is a double star, consisting of an A5 subgiant and an A1 dwarf, magnitudes 3.2 and 4.3, 161 l.y. away.

Alnitak

The star Zeta Orionis, one of the three stars of the belt of Orion. It is a supergiant of type O9.5, magnitude 1.9, with a close companion of magnitude 3.9, a B0 giant that orbits it every 1500 years; the combined magnitude of the pair is 1.7. Alnitak is 740 l.y. away.

Aeronomy

The study of processes and phenomena in the Earth's upper atmosphere, from the mesopause to its uppermost limit which, for most practical purposes, can be taken to lie at an altitude of around 300 km. Among the processes studied are airglow, reactions in the chemosphere, and the formation of noctilucent clouds and the ionosphere.

Alfvén Surface

The surface of the region surrounding a neutron star within which ionized gas is pulled around by the star's magnetic field as it spins. It is named after H. O. G. Alfvén.

Aerobraking

The technique of using the atmospheric drag of a planet to modify the orbit of a spacecraft, thereby saving propellant. It was first used at another planet by the US Venus probe Magellan in 1993. Aerobraking lowered the high point of Magellan's orbit above Venus from 8500 to 600 km and shortened its orbital period from 195 to 94 min. Aerobraking can also be used to help space probes enter orbit around a planet, as with the Mars Global Surveyor craft in 1997.

Absorption

The transfer of energy from a photon to an atom or molecule. If the energy of the photon is just sufficient to raise an electron from one energy level to another, the result is an absorption line at a particular wavelength; this is called a bound-bound transition. Absorption at a wide range of wavelengths is called continuous absorption, and occurs when the absorbed photons have energies greater than the minimum needed to eject an electron completely; this is called a bound-free transition, or photoionization. Absorption may occur whenever radiation traverses a gas, for example the outer layers of a star, the Earth's atmosphere, a nebula, or interstellar space.

Ablation

The wearing away of the outer layers of a body by melting, erosion, vaporization, or some other process due to aerodynamic effects as the body moves at high speed through a planetary atmosphere. Ablation can affect natural bodies such as meteoroids, or artificial objects such as spacecraft. Ablation of a spacecraft's protective heat shield prevents overheating of the spacecraft's interior during atmospheric entry.

Absolute Zero

The zero point on the thermodynamic temperature scale, equal to −273.15° Celsius or −459.67° Fahrenheit. It is often stated that all motion of atoms and molecules ceases at absolute zero, but in fact a small amount of energy (the zero-point energy) still remains. Absolute zero is the coldest temperature theoretically possible, but it can never be attained in practice.

Advection

Transfer by a moving fluid, generally in a direction perpendicular to a gravitational field. The term is applied to both the shifting of planetary atmospheric gases by horizontal motion, and also to the resultant transfer of heat, for example from low to high latitudes. More recently, advection has also come to mean the transfer of heat vertically within a planetary body, for example by hot, molten material rising through the lithosphere of a planet.