Any system for locating the positions of objects on the celestial sphere. Various systems are defined, according to the chosen point of observation (the origin) and the plane of reference they use. Geocentric coordinates give the position of an object as it would be seen from the centre of the Earth, the most commonly used point of origin. Heliocentric coordinates, sometimes used for objects in the Solar System, give the position as would be seen from the centre of the Sun; a variant system, barycentric coordinates, gives the position as would be seen from the centre of mass of the Solar System. Topocentric coordinates give positions as seen from a specific point on Earth. There are several different reference planes for celestial coordinates. Equatorial coordinates, the most common system, use the celestial equator as the reference plane; their origin can be geocentric or topocentric. Ecliptic coordinates take the ecliptic as their reference plane; they can be geocentric or heliocentric in origin. Horizontal coordinates give positions with respect to an observer's horizon, and are topocentric. Galactic coordinates are given with reference to the plane of the Galaxy. See also rectangular coordinates; spherical coordinates.
Subjects: Astronomy and Astrophysics.