Any system of coordinates with their origin at the centre of the Earth. For bodies in the Solar System, geocentric coordinates differ slightly from the coordinates as actually measured by an observer on the surface of the Earth (topocentric coordinates). Various forms of geocentric coordinates are defined: geocentric equatorial coordinates, the most common, have the celestial equator as their reference plane, while geocentric ecliptic coordinates are referred to the ecliptic. Rectangular coordinates can also be geocentric in origin.