Astronomical Dictionary


Dictionary of Astronomical Definitions and Terms.
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Definition


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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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Binary Star
A double star; most commonly, a physical binary, or two stars which revolve about a common center of mass. Binary stars are twins in the sense that they formed together out of the same interstellar cloud.
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Blue Moon
The second full moon in a calendar month, or the third full moon in a season containing four.
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Celestial Equator
The projection of the Earth's equator upon the celestial sphere. Right ascension is the longitude measurement eastward along this circle from a specified vernal point. declination is measured along a great circle perpendicular to the celestial equator and passing through a body.
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Celestial Sphere
The projection of space onto the night sky, an imaginary hollow sphere of infinite radius surrounding the Earth but centred on the observer. (First postulated by Ptolemy.) It is the basis of sky charts, and the celestial co-ordinate system. The coordinate system most commonly used is right ascension and declination. The sphere itself is split up into arbitrary areas known as constellations. It is a two-dimensional system, radial measurement ignored, all objects considered to be on its surface. Three- dimensional phenomena in astronomy such as parallax are treated by other means.
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Celestial Poles
The projection of the Earth's poles onto the celestial sphere. One of two points about which the Celestial Sphere appears to rotate.
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Chromosphere
The layer between the photosphere and the corona in the atmosphere of the Sun, or any other star, mainly composed of excited hydrogen atoms. It can be seen only during total eclipses extending from the photosphere to a height of 2000 km. Its temperature ranges from 4500?K at the bottom to 1,000,000?K at the top, where it merges with the outermost layer, the corona.
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Coma
(1) The luminous structure surrounding the head (solid portion) of a comet.
(2) A defect in an optical system which gives rise to a blurred, pear shaped, comet-like image.
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Comet
An icy object in independent orbit about the Sun; smaller than a planet, usually having a highly elliptical orbit extending out to beyond Jupiter. At long intervals, of up to ten thousand years, they move close to the Sun, which partially melts them and evaporates some of the gases. The solar wind catches these luminous gas particles, and blows them out into a long streaming tail, stretching away from the Sun for millions of kilometers. Comets have long fascinated man, and were believed to be omens of revolutions and other catastrophic events. Halley's comet, a famous periodic comet which orbits the Sun in about 75 years, made an appearance in l986.
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Conjunction
When two bodies appear to close together in the sky, i.e. they have the same Right Ascension. Mercury & Venus are said to be at Superior Conjunction when they are behind the Sun, and at Inferior Conjunction when they are in front of it. The outer planets are simply said to be at conjunction when they pass behind the Sun.
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Constellation
An arbitrary grouping of stars which form a pattern. The 12 zodiacal constellations, Orion, the Big Dipper, and the Pleiades are familiar to most people. The ancients named 48 constellations, mostly for objects or heroes of mythology. Now the sky is divided into 88 constellations. These vary in size and shape from Hydra, the sea monster, which is the largest at 1,303 square degrees, to Crux, the cross, which is the smallest at 68 square degrees.
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Corona
The outer layer, and hottest part, of the Sun's atmosphere; also the halo appearing around the black disk of the Moon during a total solar eclipse.
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Coronagraph
A special telescope which blocks light from the Sun's disc, thus creating an artificial eclipse, in order to study its atmosphere.
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Cosmic Ray
An extremely fast, energetic and relativistic (high speed) charged particle.
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Cosmos
The Universe: the word is derived from the Greek, meaning 'everything'.
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Astronomical [1] [2] [3] [4] [5] [6] [7] [8] [9]

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