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.