Fundamental plane (spherical coordinates)

For the concept in extragalactic astronomy, see Fundamental plane (elliptical galaxies).

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;
• the horizontal coordinate system uses the observer's horizon; and
• the Besselian coordinate system uses the Terminator (day/night boundary). This system is a rectangular (x,y,z) system (Cartesian coordinates)

See also

• Plane of reference