Fundamental plane (spherical coordinates)

The fundamental plane in a spherical coordinate system is a plane which divides the sphere into two hemispheres. The latitude of a point is then the angle between the fundamental plane and the line joining the point to the centre of the sphere.

For a geographic coordinate system of the Earth, the fundamental plane is the equator. Celestial coordinate systems have varying fundamental planes:


 * the equatorial coordinate system uses the celestial equator;
 * the ecliptic coordinate system uses the ecliptic;
 * the galactic coordinate system uses the Milky Way; and
 * the horizontal coordinate system uses the observer's horizon.