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.