Fundamental plane (spherical coordinates)

The fundamental plane in a spherical coordinate system is a plane of reference that divides the sphere into two hemispheres. The geocentric 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.

Astronomical coordinate systems have varying fundamental planes:
 * The horizontal coordinate system uses the observer's horizon.
 * The Besselian coordinate system uses Earth's terminator (day/night boundary). This is a Cartesian coordinate system (x, y, z).
 * The equatorial coordinate system uses the celestial equator.
 * The ecliptic coordinate system uses the ecliptic.
 * The galactic coordinate system uses the Milky Way's galactic equator.