Fundamental plane (spherical coordinates)

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; andthe horizontal coordinate system uses the observer's horizon.
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