Celestial coordinates are any system of coordinates that can be used to give the position of an object, such as a star, on the celestial sphere. A particular system is defined by the chosen point of observation (the origin), the plane of reference, and whether the coordinates are spherical or rectangular.
As an origin, geocentric coordinates use the center of Earth, topocentric coordinates use a specific point on Earth's surface, and heliocentric coordinates use the center of the Sun. This choice of origins can be combined with a variety of reference planes. (1) Equatorial coordinates, the most commonly used in astronomy, may be geocentric or topocentric, and take the celestial equator as their reference plane. (2) Ecliptic coordinates, may be geocentric or heliocentric, and refer to the ecliptic. (3) Horizontal coordinates are topocentric and refer to the observer's horizon. (4) Galactic coordinates are specified relative to the plane of the Galaxy. In all these systems, spherical coordinates dominate but for some special purposes rectangular coordinates are occasionally used.