Coordinate System (astronomy)

Coordinate System (astronomy), in astronomy, any of the various mapping systems used to locate objects in the heavens, just as longitude and latitude are used to locate points on the surface of the earth. In astronomy, positions are described in terms of the celestial sphere (see Ecliptic) on which the stars and other celestial objects are imagined to lie for purposes of location, without concern for their actual varying distances from the earth.

In any particular astronomical coordinate system, one particular plane is used as a basic reference for determining position points. This plane cuts through the center of the earth in one way or another and extends out to the celestial sphere. In one system, for example, called the equatorial system, the reference plane is the earth's equator, and its extension out to the celestial sphere is called the celestial equator. For a given celestial object, a circle passing through the object and the celestial poles is called an hour circle. The hour circle used as the reference, in the same manner as 0° longitude on earth, is the one passing through the celestial poles and the point on the celestial equator where it is crossed by the ecliptic in March (the vernal equinox). The angle, west to east, from this reference hour circle to the celestial object's hour circle is the object's so-called right ascension, measured in hours. The angle from the equator to the object along the object's hour circle is called the object's declination, measured in degrees. Right ascension and declination compare to longitude and latitude on earth.

In the so-called ecliptic coordinate system, the ecliptic is the reference plane. In the horizontal, or horizon, system, the observer's horizon is the reference plane, and the coordinates are altitude and azimuth.