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.^[1]

For a geographic coordinate system of the Earth, the fundamental plane is the Equator.

Astronomical coordinate systems have varying fundamental planes:^[2]

• The horizontal coordinate system uses the observer's horizon.
• The Besselian coordinate system uses Earth's terminator (day/night boundary).^[3] 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.

• Plane of reference