Coordinate Systems

Abstract

Astronomy uses a whole series of different coordinate systems to specify the positions of stars and planets. Here we must distinguish between heliocentric coordinates, which are based on the Sun, and geocentric coordinates, which are centred on the Earth. Ecliptic coordinates refer the position of a point to the plane of the Earth’s orbit, while equatorial coordinates are measured relative to the position of the Earth’s or celestial equators. The slow shift of this reference plane because of precession means that the equinox of the coordinate system that is being used also has to be taken into account. Although it would obviously be possible to decide, once and for all, to use a single, fixed coordinate system, every system has specific advantages that make it particularly suitable for certain purposes.