ABERRATION OF LIGHT The aberration of light is a phenomenon in which light appears to be slanted (the angle at which the light appears to be coming is different than the angle at which it's actually coming) if the observer is in motion. (An analogous situation is watching rain fall from a moving train - although a raindrop is falling straight to the ground, it will appear to fall at a slant, and the slant increases as the train's speed increases.) The aberration of light is due to the fact that light travels at a finite speed (as do the raindrops). The aberration of light was discovered and demonstrated by James Bradley (an English amateur astronomer) in 1725. The discovery of the aberration of light was one of the first proofs that the speed of light was finite; it also provided an early estimate of the speed of light (Bradley calculated that the time it took for light to travel from the Sun to the Earth was 8 minutes and 12 seconds).

ABLATION Ablation is a cooling process in which heat is carried away from an object (like a falling meteorite) as the flow of air blows away the hot, melted or vaporized outer layers of the object. Some spacecraft use ablative heat shields; their outer surface is coated with heat-dissipating material (like a phenolic epoxy resin, a reinforced plastic) that burns rapidly, and then falls away during re-entry.

Star Absolute Magnitude Apparent Magnitude Distance from Earth (light-years) The Sun +4.8 -26.72 0.000016 Sirius +1.4 -1.46 8.6 Canopus -2.5 -0.72 74 Rigel Kentaurus +4.4 -0.27 4.3 Arcturus +0.2 -0.04 34 Vega +0.6 0.03 25 Capella +0.4 +0.08 41 Rigel -8.1 +0.12 900 Betelgeuse -7.2 +0.7 1,500 ABSOLUTE MAGNITUDE Absolute magnitude is a measure of the inherent brightness of a celestial object. This scale is defined as the apparent magnitude a star would have if it were seen from a standard distance of 32.6 light-years (10 parsecs). The lower the number, the brighter the object. Negative numbers indicate extreme brightness.

ABSOLUTE ZERO At absolute zero, a hypothetical temperature, all molecular movement stops. All actual temperatures are above absolute zero. Absolute zero would occur at -273.16°C, -459.69°F, or 0 K.

ABSORPTION LINES Absorption Lines are dark lines superimposed over a bright continuous absorption spectrum. Each dark line is formed as a cooler gas absorbs photons emitted by a particular element from a hotter source.

ABSORPTION SPECTRUM An absorption spectrum (also called a dark-line spectrum) consists of dark absorption lines superimposed on a bright continuous spectrum. An absorption spectrum is created when light from an incandescent source passes through a cooler gas that absorbs photons. Each different element and molecule absorbs light at a unique set of frequencies. Astronomers can determine the composition of gases in stars by looking for their characteristic frequencies (this is called spectroscopy).

ACCELERATION Acceleration is the rate of change of velocity. When an object is moving at a constant velocity (or if it is standing still) it has zero acceleration. An object whose velocity is increasing has a positive acceleration.

ACCRETION DISK An accretion disk is a flat disk of gas and dust in space that surrounds a newborn star, black hole, or other enormous object that is growing by attracting matter to it with its gravitational field.

ACHONDRITIC METEORITE An achondritic meteorite is a stony meteorite, coarsely crystallized, with large bits of various minerals visible to the naked eye.

ACID RAIN Acid rain is polluted and harmful to the environment. Acid rain may have been a component of the K-T extinction.

ACRUX Acrux (also known and alpha Crux) is a first magnitude star in the Southern Hemisphere constellation Crux (the Southern Cross).

ACTIVE GALACTIC NUCLEI Active galactic nuclei (AGN) are galaxies that have a massive black hole at the galactic center (nucleus). These galaxies produce huge amounts of energy (at all wavelengths of the electromagnetic spectrum). They produce more energy than the just the total energy emitted from all of the stars in the nebula. AGN may be related to starburst galaxies.

ACTIVE GALAXY An active galaxy is a galaxy that produces huge amounts of energy; they produce more energy than the total energy emitted from all of the stars in the nebula. There may be at least three types of active galaxies, including Seyfert galaxies, quasars, and blazars (although these three may be the same type of galaxy viewed from different distances and perspectives).

ADAMS, JOHN C. John Couch Adams (1819-1892) was an English astronomer and mathematician who, at 24 years old, predicted the existence of the planet Neptune (Le Verrier also predicted its existence, independently).

ADAPTIVE OPTICS Adaptive optics (AO) is a process in which distortions (like those from the Earth's atmosphere) are removed from a telescope's image in real time. First, a wavefront sensor uses a reference star to measure the distortions that are occurring. The distortions are then removed with a phase corrector. AO was recently installed on the Keck II telescope in Hawaii,

ADRASTEA Adrastea is one of Jupiter's 16 moons, and the second-closest to Jupiter. Adrastea is 12 miles (20 km) in diameter and orbits 80,000 miles (129,000 km) from Jupiter, within its main ring. Adrastea and the first moon, Metis, are probably the source of the dust in this ring. Adrastea has a mass of 1.91 x 1016kg. It orbits Jupiter in 0.29826 (Earth) days; this is faster than Jupiter rotates on its axis. Adrastea was discovered by D. Jewitt and E. Danielson (Voyager 2) in 1979.

AEROLITE An aerolite is a meteorite that is consists mostly of stony matter.

AFTERGLOW 1. An afterglow is a wide arc of glowing light that can sometimes be seen high in the western sky at twilight; it is caused by fine particles of dust scattering light in the upper atmosphere. 2. An afterglow (also called postluminescence) is lingering radiation that remains after an event like the big bang (whose afterglow is the the cosmic microwave background radiation) or a gamma ray burst (which has an x-ray afterglow).

AIMPOINT An aimpoint is a point in the plane of a planet's orbit that a spacecraft aims for (either for a fly by of that planet or to enter that planet's orbit).

AIRY, GEORGE Sir George Bidell Airy (1801-1892) was the director of Greenwich Observatory/Astronomer Royal of England from 1835 to 1881. Airy installed a transit (a precise surveying device) at Greenwich, England, which was used to define the zero degree meridian of the Earth (zero-degrees longitude). A crater on Mars about 5 degrees south of the equator and on what is defined as Mars' prime meridian (zero-degrees longitude) is call Airy. A small crater within this crater (which is called Airy-0) is where the meridian line (zero-degrees longitude) crosses. A crater on the moon is also named for him (latitude 18.1 degrees, longitude 354.3 degrees, diameter 36 km). Airy is supposed to have stated incorrectly that Charles Babbage's new "analytical engine" (the predecessor of the computer) was "worthless," effectively ending Babbage's government funding.

Planet Albedo Mercury 0.12   Venus 0.59   Earth 0.39   Mars 0.15   Jupiter 0.44   Saturn 0.46   Uranus 0.56   Neptune 0.51   Pluto 0.5   ALBEDO An object's albedo is the ratio of the amount of solar radiation reflected from an object to the total amount it receives. An object with a high albedo is shinier than an object with a low albedo. A white, completely reflecting object has an albedo of 1.0; a black object with no reflectivity has an albedo of 0.0 (zero).

ALBIREO Albireo (beta Cygni) is a third magnitude star in the constellation called Cygnus (Albireo is the head of the swan and the second-brightest star in Cygnus). Albireo is a double star; the primary (Albireo A, a K3II bright giant) is bright yellow and the companion (Albireo B) is blue. Albireo is roughly 386 ± 26 light years from us.

ALDEBARAN Aldebaran is the brightest star in the constellation Taurus (it is one of the bull's eyes) and is the 13th brightest star in the sky. Aldebaran is seen along the ecliptic. Aldebaran means "the Follower" in Arabic (since it seems to follow the stars in the Pleiades. Aldebaran is an giant, old, orange star that is cooler than the Sun; it is under 4,000 Kelvin (the Sun is 5,800 Kelvin). Aldebaran is about 40 times as big as the Sun.

ALDRIN, EDWIN E., JR. Edwin Eugene Aldrin, Jr. (1930- ) was the second man to walk on the moon. Also known as "Buzz" or "Dr. Rendezvous," Aldrin was the Lunar Module Pilot (July 16-24, 1969) on NASA's Apollo 11 mission. He and Neil Armstrong (the first man on the moon) were on the moon for about 2 hours, collecting rock samples and doing lunar surface experiments. Aldrin's first space mission was Gemini 12, which launched on November 11, 1966; he and Command Pilot James Lovell orbited Earth for 4 days. Aldrin has written many books about his life and his career as an astronaut.

ALGOL Algol (Beta Persei) is a star in the constellation Perseus. Algol was the first known eclipsing binary star. It has a magnitude that ranges from 2.1 to 3.4; the period is 2.9 days.

ALNITAK Alnitak (Zeta Orionis) is a star at the eastern end of the Orion's belt (in the constellation Orion). Alnitak means belt. Alnitak is spectral Type O9.7Ib. It is also known as Alnitah. The other two stars in Orion's belt are Alnilam and Mintaka.

ALPHA Alpha is the first letter in the Greek alphabet. Alpha often denotes the brightest star in a constellation.

ALPHA CENTAURI Alpha Centauri is the star system that is closest to the Earth. The dimmest star in the system, Proxima Centauri (Alpha Centauri C), is the closest star to us (other than our sun). The stars Alpha Centauri A and Alpha Centauri B are close binary stars.

ALMUCANTAR An almucantar (also called a "parallel of altitude") is a circle on the celestial sphere that is parallel to the horizon. It consists of all points at a given altitude - if two points are on the same almucantar, they have the same altitude.

ALTAIR Altair (alpha Aqu) is a fist-magnitude star in the (in the constellation Aquila (the eagle); Altair is the 12th brightest star in the sky. Altair means "flying eagle" in Arabic. Altair is in the Summer Triangle, which it forms with Vega and Deneb.

ALTITUDE Altitude is a measurement in mapping astronomical objects on the celestial sphere (the sky as visible from Earth). Altitude is the angle of the object from the observer's horizon. If an object is on the horizon, its altitude is 0 degrees. If it is at the observer's zenith, its altitude os 90 degrees. To find an object in the sky, two coordinates are needed, its azimuth and its altitude.

ALT-AZIMUTH TELESCOPE MOUNT An Alt-Azimuth telescope mount is a "U"-shaped device that allows the telescope to move both up and down (Altitude) and left to right (Azimuth).

ALVAREZ THEORY OF EXTINCTION This theory is that a large asteroid, meteor, or comet hit the Earth 65 million years ago, causing huge atmospheric and geologic disruptions, leading to a mass extinction which killed the dinosaurs and many other species.

ALVAREZ, LUIS Luis Alvarez (1911-1988) was a physicist who, with his son Walter Alvarez (a geologist), hypothesized that the a huge asteroid hit Earth the Earth 65 million years ago, causing a mass extinction. The Alvarez Theory of Extinction is widely accepted. Luis Alvarez received a Nobel Prize in physics (1968) for his work on subatomic particles.

AMALTHEA Amalthea (J-V) is the third-closest moon of Jupiter and the largest of Jupiter's smaller moons. It is a small, red, elongated, cratered moon (with two very large craters); it has the fastest speed of any known moon. It is potato-shaped, with dimensions: 145 x 91 x 83 miles (232 x146 x134 km). Its mass is 7.17x1021kg. Amalthea's mean distance from Jupiter is 181,300 miles. It takes Amalthea about 12 hours to orbit Jupiter; like most of Jupiter's moons, it is in synchronous orbit. It was discovered and named in 1892 by the American astronomer Edward Emerson Barnard.

AMOR ASTEROID Amors are asteroids which orbit between the orbits of Earth and Mars. They were named for the asteroid Amor, #1221. Eros (#433) is an Amor asteroid.

ANALEMMA The analemma is a figure-8-shaped diagram that shows the declination of the sun (the angle that the sun is from the equator), for each day in the year. If you took a snapshot of the the sun at the same time each day (from the same location), the Sun would make a figure shaped like an analemma in a year (this is because the Earth is tilted on its axis and because it doesn't orbit the Sun in a perfect circle).

ANANKE Ananke is Jupiter's thirteenth moon. Ananke is 12.5 miles (20 km) in diameter and orbits 13,100,000 miles (21,200,000 km) from Jupiter. Ananke has a mass of 4 x 1016kg. It orbits Jupiter in 631 (Earth) days and is in a retrograde orbit (orbiting opposite to the direction that Jupiter rotates). Very little is known about Ananke. Ananke was discovered by S. Nicholson in 1951.

ANDROMEDA Andromeda (the princess, the daughter of Cassiopeia in mythology) is a northern hemisphere constellation. Alpha Andromedae (called Sirrah, meaning "Horse's navel") is the same as Delta Pegasi (from the constellation Pegasus).

ANDROMEDA GALAXY The Andromeda Galaxy (also known as M31 and NGC 224) is the closest major galaxy. It is a spiral galaxy (like our galaxy) and is in the Local Group. It can just be seen with the naked eye in the constellation Andromeda.

ANGSTROM An Angstrom is equal to one ten billionth (1 x 10-10) of a meter. This unit of measure is named for the Swedish physicist Anders J. Ångström.

ANGULAR DISTANCE Angular distance is the measure of an arc (a segment of the circumference of a circle). Angular distance measures the proportion of a circle that the arc in question consists of. Angular distance is expressed in degrees, radians, arc minutes (one-sixtieth of a degree), or arc seconds (one-sixtieth of a minute). For example, latitude is the angular distance north or south of the equator measured in degrees, and longitude is the angular distance east or west of the prime meridian (which goes through Greenwich, England), measured in degrees.

ANGULAR MOMENTUM Angular momentum is a momentum-like quantity associated with a circular motion around an axis of rotation. Essentially, it is the amount of spin an object has. Formally, Angular momentum = mass x velocity x radius (from point that the object is spinning or orbiting around). Angular momentum is conserved (in the absence of an outside force).

ANISOTROPY Anisotropy is a state in which a physical characteristic (like the temperature of the cosmic background radiation) varies in value along axes in different directions - a physical measurement made in one direction differs from the measurement made in another direction. For example, the cosmic microwave background radiation (the radiant heat left over from the Big Bang) is anisotropic.

ANNULAR ECLIPSE An annular eclipse is a type of solar eclipse. During an annular eclipse, the Sun looks like an "annulus" or ring. The ring is visible when the Moon does not entirely cover the disc of the Sun during the eclipse. This type of eclipse happens when the Sun is at perihelion (closest to the Earth, making the solar disc appear larger) and the Moon is at apogee (farthest from the Earth, making it look smaller).

ANORTHOSITE Anorthosite is a type of rock found on the moon, on lunar highlands. Anorthosite is composed of aluminum and calcium silicates.

ANTARES (Alpha Scorpii) Antares (meaning "Rival of Mars") is the brightest star in Scorpius, one of the constellations in the zodiac. Antares is a M1.5Iab variable red supergiant star that is about 520 light-years from Earth and is about 230 times as big as the Sun. This incredibly massive, old, low-temperature (3500 K) star is the 15th brightest star in the sky; it has a visual (apparent) magnitude of +0.96 (var.) and an absolute magnitude of -5.2.

ANTENNA An antenna (plural antennae) is a conductor that can transmit or receive radio waves. Radio telescopes are antennae (or a series of antennae).

ANTIMATTER Antimatter is matter that is composed of the counterparts of ordinary matter. For example, positrons are the antimatter counterpart of electrons; positrons have a positive charge instead of a negative charge. The existence of an antimatter version of the electron was predicted by Paul Dirac in 1930. Carl Anderson, in 1932, found positron tracks that were produced by cosmic rays in a cloud chamber. Antimatter is also produced in some radioactive decays. A particle and its antimatter particle will annihilate when they meet, and energy is released (according to Einstein's E = mc2). Antiparticles have the same mass as their counterpart, but the opposite charge. There seems to be very little antimatter in the universe.

ANTIPODAL POINT A point that is on the opposite side of a celestial body from a given point is an antipodal point. For example, the North and South Poles are antipodal points on the Earth.

APASTRON The apastron is the point of greatest separation of two stars, as in a binary star system orbit.

APERTURE The aperture is the size of a telescope's main lens (or mirror).

APHELION For an object (a planet or comet) orbiting the Sun, the aphelion is the point in its orbit which is farthest from the sun.

APOAPSIS For an object orbiting a celestial body, the apoapsis is the point in its orbit which is farthest from that body.

APOGEE For an object orbiting the Earth, the apogee is the point in its orbit which is farthest from the Earth.

APOLLO OBJECT or APOLLO ASTEROID Apollo asteroids have an orbit that crosses the orbit of the Earth and have a period longer than 1 year. They have a semimajor axes greater than 1 astronomical unit (au), and perihelion distances less than 1.017 au. These asteroids were named for asteroid Apollo, #1862. Geographos is an Apollo.

APOLLO MISSIONS NASA's Apollo missions (1961 to 1975) were the only missions to send people to the moon. Apollo 11's LEM or LM (Lunar Excursion Module or Lunar Module) landed on the moon on July 20, 1969. Apollo 12 to 17 continued lunar exploration.

APOLLO 11 NASA's Apollo 11 (1969) was the first mission to send people to the moon. The spacecraft launched from Cape Kennedy, Florida, USA, on July 16, 1969, from the Kennedy Space Center Launch Complex 39A. Apollo 11's LEM or LM (Lunar Excursion Module or Lunar Module, dubbed the Eagle) separated from the orbiter and landed on the moon on July 20, 1969, with Neil Armstrong and Buzz Aldrin. Michael Collins was in the command-service module (CSM), the orbiter (which was dubbed the Columbia). The LM landed at the Mare Tranquillitatis (Sea of Tranquility). Armstrong and Aldrin were on the moon's surface for 21 hours, 38 minutes, 21 seconds. They took off in the LM and rendezvoused with the command-service module (CSM); the Descent stage of the LM remained on Moon, at the Sea of Tranquillity landing site. Apollo 11 landed in the Pacific Ocean on July 24, 1969. The ship carried 21.7 kilograms of Moon rocks back to the Earth. The command module "Columbia" is on display at the National Air and Space Museum in the Milestones of Flight gallery, Washington, D.C., USA.

Star Absolute Magnitude Apparent Magnitude Distance from Earth (light-years) The Sun +4.8 -26.72 0.000016 Sirius +1.4 -1.46 8.6 Canopus -2.5 -0.72 74 Rigel Kentaurus +4.4 -0.27 4.3 Arcturus +0.2 -0.04 34 Vega +0.6 0.03 25 Capella +0.4 +0.08 41 Rigel -8.1 +0.12 900 Betelgeuse -7.2 +0.7 1,500 Altair +2.3 +0.77 16 Deneb -7.2 +1.25 1,500 Proxima Centauri +15.5 +11.05 (var.) 4.3 APPARENT MAGNITUDE Apparent magnitude is a measure of the brightness of a celestial object as seen from Earth. The lower the number, the brighter the object. Negative numbers indicate extreme brightness. The full moon has an apparent magnitude of -12.6; the sun's is -26.8. We can see objects up to 6th magnitude without a telescope. Apparent magnitude is abbreviated m. This system of rating the brightness of celestial objects was developed by the Greek astronomer Hipparchus in 120 B.C.

APPARENT MOTION Apparent motion is the observed motion of astronomical obejcts from Earth. The apparent motion might be due to the movement of the object or themovement of the Earth. For example, the apparent motion of a star is caused only by the motion of the Earth (and not by the motion of the star).

APPARENT TIME Apparent time is the local time, determined by the angle of the sun.

APPARENT SOLAR DAY An apparent solar day is the period of time between two successive passages of the sun across a meridian.

AQUARIUS [Abbreviation: Aqr] Aquarius (the water bearer), abbreviated Aqr, is a constellation of the zodiac. This faint constellation is seen along the ecliptic. The sun passes through Aquarius from mid-February until mid-March. There are many interesting celestial objects in this large constellation, including globular clusters (M2 and M72), planetary nebulae (NGC 7009 and NGC 7293), the star group M73, a double star (zeta Aqr). Aquarius is the radiant for the meteor showers: March Aquarids, Delta Aquarids, Eta Aquarids, and Iota Aquarids. Alpha Aquarii (the brightest star in Aquarius) is called Sadalmelik (meaning the Lucky One of the King) and beta Aquarii (the second-brightest star in Aquarius) is named Sadalsuud (meaning the Luckiest of the Lucky); they are twin supergiants.

ARACHNOIDS Arachnoids are round networks of fractures in Venus' crust. They are called arachnoids because they loook a bit like spider webs (spiders are arachnids).

ARC An arc is a segment along the circumference of a circle.

ARC MINUTE An arc minute is a measure of angularity and is equal to one-sixtieth of a degree (there are 360 degrees in a circle).

ARC SECOND An arc second is equal to one-sixtieth of an arc-minute.

ARCTURUS (Alpha Boötis) Arcturus is the brightest star in the constellation Boötes (the herdsman). It is a red giant (spectral type K1.5IIIp) that is the fourth brightest star in the sky. Arcturus is 34 light-years from Earth. It has an apparent magnitude of -0.04 and an absolute magnitude of 0.2.

ARECIBO DISH The Arecibo Dish is the world's largest radio telescope. The 1,000 ft (305 m) dish is built into a natural hollow in the hills of Puerto Rico.

ARIEL Ariel is the brightest and one of the largest of the 18 moons of Uranus. It is a moon with many long rift valleys, canyons, and craters. It has a diameter of 720 miles (1158 km) and orbits Uranus at a mean distance of 120,000 miles (190,930 km). It revolves around Uranus in 50.5 Earth hours. Its mass is 1.27x1021 kg. It was discovered in 1851 by William Lassel, an amateur English astronomer.

ARIES [Abbreviation: Ari] Aries (the ram) is a constellation of the zodiac located along the ecliptic between Pisces and Taurus. This tiny, faint constellation is seen just north of the ecliptic. The brightest star in Aries (Alpha Ari) is Hamal, the second-brightest star (Beta Ari) is Sharatan, the third-brightest star (Gamma 2 Ari) is Mesarthim, and the fourth-brightest is Botein (Delta Ari)

ARISTARCHUS Aristarchus of Sámos (310?BC-250?BC) was an ancient Greek astronomer who was the first person to propose a heliocentric model of the Solar System. Aristarchus realized that the Earth rotates on its axis and revolves around the Sun. He estimated how far the Sun and the moon are from the Earth and how big the Moon and the Sun are. Archimedes and Plutarch wrote of Aristarchus' works. He also calculated a relatively precise value for the length of the solar year. Aristarchus invented a bowl-shaped sundial whose pointer cast shadows in the middle of the bowl. A lunar crater was named for Aristarchus (pictured above); it is located on the NW edge of the Oceanus Procellarum. This huge, circular crater is 25 miles (40 km) in diameter and 2.2 miles (3.6 km) deep (from rim to floor).

ARISTOTLE Aristotle (384-322 BC) was a Greek philosopher who theorized about astronomy. Using only philosophical speculation (he did no scientific observations), Aristotle believed that the universe is spherical, finite, and centered around the Earth. Aristotle, like many others of his time, believed that the circle was the "perfect" shape, so the universe must be spherical, and all the orbits in it must also be circular. He also believed that celestial bodies were composed of ether (in addition to the four other basic elements believed to exist at that time, earth, air, fire, water). Aristotle's ideas were adopted by the Church and were not tested for over a thousand years, until Galileo's experiments demonstrated errors in Aristotle's writings.

ARMILLARY SPHERE An armillary sphere is an instrument from the 1500s that was used to determine the relative positions of the celestial equator, the ecliptic, the planetary orbits, etc. This device consisted of a series of concentric rings.

ARMSTRONG, NEIL Neil Alden Armstrong (1930- ) was the first person to walk on the moon. He piloted NASA's Apollo 11 mission, which took off on July 16, 1969. Armstrong and Edwin E. (Buzz) Aldrin, Jr., landed on the moon on July 20, 1969, in the lunar module (landing in the Mare Tranquilitatis), while Michael Collins orbited the moon in the command module. Upon his first step on the moon, Armstrong said, "One small step for man, one giant leap for mankind." Armstrong and Aldrin explored the Moon's surface for about 2 hours. Years earlier, in 1966, Armstrong and David R. Scott performed the first successful docking of two vehicles in space on the Gemini 8 mission.

D'ARREST, H.L. Heinrich Louis d'Arrest (1822-1875) was a Danish astronomer and the co-discoverer of Neptune (in 1846), with Galle.

ASTERISM An asterism is a collection of stars (within a constellation) that forms an apparent pattern from Earth. Some familiar asterisms include the Big Dipper, the Little Dipper, Pleiades, Trapezium, and the Summer Triangle.

Asteroid Name and Number Diameter (km) Mass (kg) Discoverer, Date of Discovery 1. Ceres 960 x 932 8.7x1020 Piazzi, 1801 2. Pallas 570 x 525 x 482 3.18x1020 Heinrich Olbers, 1802 3. Juno 240 2.0x1019 K. Harding, 1804 4. Vesta 530 3.0x1020 H. Olbers, 1807 ASTEROID An asteroid is a large rocky object or very small planet (planetoid). Most asteroids orbit the Sun in the asteroid belt between Mars and Jupiter. A few asteroids approach the Sun more closely. An asteroid impact with the Earth may have caused the extinction of the dinosaurs. The largest asteroid and first one ever discovered (by Giuseppe Piazzi on January 1, 1801) is Ceres, whose mass is equal to over one-third of the 2.3 x 1021 kg estimated total mass of all the 3,000 catalogued asteroids.

ASTEROID BELT The asteroid belt is a doughnut-shaped concentration of asteroids orbiting the Sun between the orbits of Mars and Jupiter (2.2 - 3.3 A.U.). This belt may have once been a planet (in the same orbit) that was destroyed by a collision with a huge asteroid or comet.

ASTHENOSPHERE The asthenosphere is a part of the upper mantle that exhibits plastic (flowing) properties. It is located below the lithosphere (the crust and rigid upper mantle), between about 100 and 250 kilometers deep.

ASTROBIOLOGY Astrobiology is the science that addresses the question of whether or not there is life in space, and if so, then what it its origin, distribution, and evolution.

ASTROLABE An astrolabe is an instrument that was used to determine the altitude of objects (like the sun) in the sky. It was first used around 200 B.C. by astronomers in Greece. The astrolabe was replaced by the sextant.

ASTROMETRY Astrometry is the precise measurement of the position and motion of astronomical objects, often with respect to standard star catalogues.

ASTRONAUT Astronauts are people who go into space.

ASTRONAUTICS Astronautics is the science of the design, construction, and operation of spacecraft.

ASTRONOMER An astronomer is someone who studies astronomy.

ASTRONOMER ROYAL The Astronomer Royal is appointed by the Queen (or King) of England to be custodian of the Greenwich Observatory. The first Astronomer Royal, John Flamsteed, was appointed in 1675 by King Charles II .

ASTRONOMICAL UNIT An astronomical unit, or AU, is equal to the mean (average) distance from the Earth to the Sun, about 93 million miles (150 million km). It takes a beam of light about 8.3 minutes to travel 1 AU.

ASTRONOMIC SCINTILLATION Atmospheric scintillation is the twinkling of stars (fluctuation of intensity) seen through a planet's atmosphere. Scintillation in caused when the star's light is distorted by the Earth's atmosphere. Scintillation is greater for bright stars that are low on the horizon. It is also known as stellar scintillation.

ASTRONOMY Astronomy is the scientific study of space, including the planets, stars, galaxies, comets, and nebulae.

NASA'S ASTRONOMY PICTURE OF THE DAY

ASTROPHYSICS Astrophysics is a branch of astronomy that studies the physical properties of celestial bodies and other matter and energy in space.

ASYMPTOTIC GIANT BRANCH The Asymptotic Giant Branch is the upper right hand corner of the Hertzsprung-Russell (H-R) diagram that represents stars in which Helium burning no longer occurs in their cores - red giants. As the carbon core of these old star collapses, the outer layers expand, and the star becomes huge red giants.

ATEN ASTEROID Atens are asteroids that are always closer to the Sun than the Earth is; they have a period shorter than 1 year (the semi-major axis is smaller than Earth's). They were named for asteroid Aten, #2062. Ra-Shalom is an Aten asteroid.

ATLAS Atlas is one of the smallest of the 18 moons of Saturn. Atlas is a shepherd satellite for Saturn's A-ring. It has a radius of about 18.5x17.2x13.5 km. Atlas orbits at about 137,670 km from Saturn. Its orbital period is 0.6019 earth days. It was discovered by R. Terrile/Voyager 1 in 1980.

ATMOSPHERE The atmosphere is the mixture of gases that surrounds a planetary object, moon or star. The Earth's atmosphere is mostly nitrogen; the Sun's atmosphere is mostly hydrogen gas.

ATOM All matter is made up of tiny atoms. An atom is the smallest part of an element that has the properties of that element. Atoms consist of a nucleus (containing protons and neutrons) and orbiting electrons.

ATOMIC NUMBER An atom's atomic number of an element is the number of protons in the nucleus of one of its atoms. The atomic number indicates the place of an element in the periodic table of the elements.

ATOMIC WEIGHT An atom's atomic weight measures the total of the mass of the protons, neutrons, and electrons. The atomic weight is roughly equal to the number of protons and neutrons, with only a little bit added by the mass of the electrons.

AU Planet Distance from the Sun (measured in AU) Mercury 0.39 Venus 0.723 Earth 1.0 Mars 1.524 Jupiter 5.203 Saturn 9.539 Uranus 19.18 Neptune 30.06 Pluto 39.53 AU AU stands for "astronomical unit." An AU is equal to the mean (average) distance from the Earth to the sun, about 93 million miles (150 million km = 500 light-seconds).

AURORA Auroras are beautiful undulating sheets of light in the near-polar sky. They are caused by gases that become excited after being hit by solar particles. Most auroras are 100 to 250 km above the ground.

AUTUMNAL EQUINOX Equinoxes are days in which daytime and nighttime are of equal duration. The two yearly equinoxes occur when the Sun crosses the celestial equator. The autumnal equinox occurs around September 21; the vernal (spring) equinox occurs around March 21.

Axial Tilt   Planet Axial Tilt (Obliquity to the Ecliptic) Mercury 2° Venus 2° Earth 23.45° Mars 24° Jupiter 3.1° Saturn 26.7° Uranus 97.9° Neptune 28.8° Pluto 57.5° AXIAL TILT Axial tilt, or obliquity to the ecliptic, is the tilt of a planet's axis from perpendicular to the plane of the ecliptic. Another way of looking at it is the angle between the plane of the planet's orbit and that of the equator.

AXIS An axis is an imaginary straight line around which an object, like a planet, turns. The Earth's axis is a line that goes through the North and South Poles.

AZIMUTH Azimuth is a measurement in mapping astronomical objects on the celestial sphere (the sky as visible from Earth). Azimuth is the angle of the object from the observer's north point (projected onto the horizon). If an object is due north, its azimuth is 0 degrees. If it is due east, its azimuth is 90 degrees, etc. To find an object in the sky, two coordinates are needed, its altitude and its azimuth.