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.