# Astronomical coordinate systems

**Astronomical** (or **celestial**) **coordinate systems** are organized arrangements for specifying positions of satellites, planets, stars, galaxies, and other celestial objects relative to physical reference points available to a situated observer (e.g. the true horizon and north to an observer on Earth's surface).[1] Coordinate systems in astronomy can specify an object's position in three-dimensional space or plot merely its direction on a celestial sphere, if the object's distance is unknown or trivial.

Spherical coordinates, projected on the celestial sphere, are analogous to the geographic coordinate system used on the surface of Earth. These differ in their choice of fundamental plane, which divides the celestial sphere into two equal hemispheres along a great circle. Rectangular coordinates, in appropriate units, have the same fundamental (*x, y*) plane and primary (*x*-axis) direction, such as an axis of rotation. Each coordinate system is named after its choice of fundamental plane.