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 system, the altazimuth coordinate system, the celestial or ecliptic coordinate system, and the galactic coordinate system. These systems are based on three common principles: (1) all stars are considered to be located on the inner surface of the celestial sphere, 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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- celestial sphere - celestial sphere celestial sphere, imaginary sphere of infinite radius with the earth at its ...
- ecliptic coordinate system - ecliptic coordinate system ecliptic coordinate system, an astronomical coordinate system in which ...
- altazimuth coordinate system - altazimuth coordinate system altazimuth coordinate system or horizon coordinate system,astronomical ...
- equatorial coordinate system - equatorial coordinate system equatorial coordinate system, the most commonly used astronomical ...
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