astronomical coordinate systems


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astronomical coordinate systems.

A coordinate system is a method of indicating positions. Each coordinate is a quantity measured from some starting point along some line or curve, called a coordinate axis. There are four basic systems of astronomical coordinates: the equatorial coordinate systemequatorial coordinate system,
the most commonly used astronomical coordinate system for indicating the positions of stars or other celestial objects on the celestial sphere. The celestial sphere is an imaginary sphere with the observer at its center.
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, the altazimuth coordinate systemaltazimuth coordinate system
or horizon coordinate system,
astronomical coordinate system in which the position of a body on the celestial sphere is described relative to an observer's celestial horizon and zenith.
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, the celestial or ecliptic coordinate systemecliptic coordinate system,
an astronomical coordinate system in which the principal coordinate axis is the ecliptic, the apparent path of the sun through the heavens. The ecliptic poles are the two points at which a line perpendicular to the plane of the ecliptic through the
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, and the galactic coordinate systemgalactic coordinate system,
astronomical coordinate system in which the principal axis is the galactic equator (the intersection of the plane of the Milky Way with the celestial sphere) and the reference points are the north galactic pole and the zero point on the galactic
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. These systems are based on three common principles: (1) all stars are considered to be located on the inner surface of the celestial spherecelestial sphere,
imaginary sphere of infinite radius with the earth at its center. It is used for describing the positions and motions of stars and other objects. For these purposes, any astronomical object can be thought of as being located at the point where the line of sight
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, the imaginary sphere centered on the earth and representing the entire sky; (2) each coordinate axis is a great circle on the celestial sphere; and (3) coordinate measurements of an object to be located are made along two great circles, one a coordinate axis and the other perpendicular to it and passing through the object. Measurements are made either in degrees or in hours.
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