Astronomical Dictionary
Letter: A


Dictionary of Astronomical Definitions and Terms.
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Absolute Magnitude
The apparent magnitude that a star would possess it if were placed at a distance of 10 parsecs from Earth. In this way, absolute magnitude provides a direct comparison of the brightness of stars. The apparent magnitude of a star is based upon its luminosity and its distance. If all stars were placed at the same distance then their apparent magnitudes would only be dependent on their luminosities. Thus, absolute magnitudes are true indicators of the amount of light each star emits.
Our Sun's absolute magnitude is +4.9. The most luminous star, known as S Doradus, has an absolute magnitude of about -8.9. The faintest dwarf stars have a magnitude of about +21.
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Accretion
An accumulation of dust and gas into larger bodies such as stars, planets and moons, or as discs around existing bodies. Past a certain speed, accretion becomes gravitational collapse.
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Albedo
The total ratio of sunlight reflected by a planet or body compared to what it receives from the Sun. Planets with dense atmospheres have higher albedos than those with no atmospheres. A value of 1 represents a perfectly reflecting (white) surface, whilst a value of zero represents a perfectly absorbing (black) surface. Some typical albedos are: The Earth - 0.39; The Moon - 0.07; Venus - 0.59.
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Aphelion
The point in the orbit of a planet or comet when it is furthest from the center of the Sun. (Opposite of perihelion).
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Apogee
Similar to aphelion. The point in the orbit of the moon furthest from the center of the earth. (opposite of perigee).
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Apoapsis
The point in an orbit when a planet is farthest from any body other than the Sun or the Earth.
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Arc Minute
A measure of angular separation. Arc is measured in a number of different units designed for various purposes: radians, grads, revolutions or circles, hours, degrees, etc. The most common unit of arc in astronomy and astrology is the degree, of which there are 360 in a full circle.
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Asteroid
(Also "planetoid") These are rocky bodies, the vast majority of which orbit the Sun between Mars and Jupiter. It is thought that there must be around 100,000 in all. The largest asteroid is Ceres which has a diameter of 579 miles. The smallest detected asteroids have diameters of several hundred feet. Together with comets and meteoroids, asteroids make up the minor bodies of the solar system. They are considered to be left over planetesimals from the formation of our solar system. The gravitational pull of Jupiter is thought to have stopped the members of the asteroid belt from forming a planet. Most asteroids orbit the Sun in a belt between Mars and Jupiter between 2.2 and 3.2 au. 4,000-odd asteroids have been formally numbered and named. The average eccentricity of their orbit is .15 and their average orbital inclination about 10?. There are millions of asteroids, most of them in a belt of such bodies orbiting between Mars and Jupiter. The outermost asteroid was thought to be Chiron with a 51-year orbital period until asteroid 1992AD was noted (on January 9, 1992 by David L. Rabinowitz) with a period 93 years, perihelion at approximately 8.7 au, aphelion at 32 au (extending far enough to cross the orbits of Saturn, Uranus, and Neptune), a maximum inclination to the ecliptic of 25?, and magnitude 17.
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Ascending Node
The point in the orbit of an object, when it crosses the ecliptic, (or celestial equator) whilst moving south to north. The ascending node is also called the north node.
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Astronomical Unit
A system of measurement used within our solar system based on the mean distance between the Earth and the Sun 92,957,130 miles (149,600,000 km). The relation of planets and other bodies to the Sun is expressed using the AU as a measuring stick. For example, the mean distance of Saturn from the Sun is 9.54 au; the semimajor axis of the Earth's orbit is 1.000000031 au.
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Aurora
The Northern Lights. The brilliant displays of colored light in the sky at extreme northern and southern latitudes. The auroral displays are the result of gases in the upper atmosphere being stimulated by bombardment from charged particles and electrons which originate in the Sun--perhaps from solar flares. These solar particles find their way through the Earth's magnetic field at the poles, and funneling down, interact with atoms and ions in the Earth's atmosphere. It gives rise to the "Northern Lights", or Aurora Borealis, in the Northern Hemisphere, and the Aurora Australis in the Southern Hemisphere.
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Atlas
In Greek mythology, brother of Prometheus and grandfather of Hermes (Mercury). Condemned to stand forever supporting the heavens on his shoulders. The Atlantic Ocean is named for him.
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Atla
In Norse mythology, a giantess, mother of Heimdall.
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Arachnoid
Spider or cobweblike feature on the surface of Venus, typically having a diameter of about 100-km and a central volcanic structure surrounded by a complex network of lineaments.
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Apollo
In Greek mythology, one of the twelve Olympian gods. God of prophecy, healing, archery, music, youth, plastic arts, science and philosophy.
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Aphrodite
One of the twelve Greek Olympian gods. Goddess of Love (Roman name, Venus), daughter of Zeus and Dione .
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Antipodal point
The opposite point with respect to any given point.
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Anorthosite
A type of igneous rock composed almost entirely of feldspar, a group of minerals that make up about 60% of the Earth's crust.
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Altimetry
The measurement of elevation or altitude.
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Albedo
The ratio of the amount of solar radiation reflected from an object to the total amount incident upon it.
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Akna
In Native American traditions from Mexico and from the Arctic, "Moon" (Wife of the Sun) and "The Mother" (Goddess of Childbirth), respectively.
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Adams
John Couch Adams (1819-1892) English astronomer. One of the discoverers of the planet Neptune.
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Achondrite
A stony meteorite, coarsely crystallized, with sizable fragments of various minerals visible to the naked eye.
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Aa
A basaltic lava with a rough, jagged surface.
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