Geocentric coordinates

Geocentric coordinates are an "Earth-centered" system of locating objects in the solar system in three-dimensions along the Cartesian X, Y and Z axes. They are differentiated from "topocentric" coordinates which use the observer's location as the reference point for bearings in altitude and azimuth. Both systems, however, share a common difficulty in that the Earth is constantly moving, which requires the addition of a time component to fix objects.

For nearby stars astronomers use heliocentric coordinates, with the center of the Sun as the origin. The reference plane can either be aligned with the Earth's equator, the plane of the ecliptic or the plane of the Milky Way galaxy. The distances involved are so great compared to the relative velocities of the stars that for most purposes the time component can be neglected.