What is Celestial coordinate system?
[[File:Ecliptic equator galactic anim.gif|thumb|This animation shows how to get the coordinates of an object in the sky]] A '''celestial coordinate system''' is a [[coordinate system]] that helps [[astronomer]]s specify the location of objects in the sky. These objects may be [[star]]s, [[planet]]s or even [[satellite]]s. There are several such coordinate systems listed below. All of them are [[Spherical coordinate system|spherical]]. The [[Fundamental plane (spherical coordinates)|fundamental plane]] divides the [[celestial sphere]] into two equal [[Celestial sphere|hemispheres]] and defines the baseline for the latitudinal coordinates, similar to the [[equator]] in the [[geographic coordinate system]]. The poles are located at ±90° from the fundamental plane. The primary direction is the starting point of the longitudinal coordinates. == Coordinate systems == These are common coordinate systems used by the astronomical community: === Horizontal system === The [[horizontal]] system is based on the position of the observer on Earth. It is a useful coordinate system for finding and tracking objects for observers on Earth. It is based on the position of stars relative to an observer's true or ideal horizon. === Equatorial system === The equatorial coordinates are based on the position of stars relative to the Earth's [[equator]]. Imagine the Earth's equator extended out to an infinite distance. The equatorial describes the sky as seen from the [[solar system]]. ''Modern star maps almost exclusively use equatorial coordinates''. === Eclyptic system === The fundamental plane is the plane of the [[Earth's orbit]], also called the ecliptic plane. There are two versions: the geocentric ecliptic coordinates centred on the Earth, and heliocentric ecliptic coordinates centred on the centre of mass of the solar system. The geocentric ecliptic system was the principal coordinate system for ancient astronomy and is still useful for computing the apparent motions of the Sun, Moon, and planets.<ref>Aaboe, Asger. 2001 ''E