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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.