Astronomical Dictionary
Letter: C


Dictionary of Astronomical Definitions and Terms.
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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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Culmination
A general term for the hightest and lowest points a planet or celestial body reaches in the sky relative to an observer. For northern observers, this occurs when the object is due South. For southern observers when it is due North..
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Culmination
A general term for the hightest and lowest points a planet or celestial body reaches in the sky relative to an observer. For northern observers, this occurs when the object is due South. For southern observers when it is due North..
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Cuspate
Shaped like a cusp; a sharp projection of material.
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Crust
The outermost layer of the lithosphere.
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Crater
An approximately circular depression, sometimes surrounded by a raised rim. Craters are typically formed by explosion during meteorite impact.
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Chondrite
A stony meteorite, composed of finely crystallized material.
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Chasma
A large canyon.
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Charon
In Greek mythology, ferryman of the River Styx, who carried the dead to the underworld. Each dead person was buried with a coin in his mouth or on his eyelids to pay for the crossing.
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Cerberus
In Greek mythology, the three-headed dog that guards the entrance to the underworld.
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Centaur
In Greek mythology, a being with the head, arms, and torso of a man, and the body and legs of a horse. The personification of wisdom and beastliness: the two natures of humankind.
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Cassini
Gian Domenico Cassini (1625-1712) Astronomer born in Italy, later a naturalized French citizen. Discovered four of Saturn's satellites, observed a dark division in Saturn's ring (the Cassini Division).
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Carson
Rachel Carson (1907-1964) American biologist and author.
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Candor
Candor Chasma - from the Latin candor, meaning "blaze" or "the white" from its appearance.
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