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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.^{[1]}

For a geographic coordinate system of the Earth, the fundamental plane is the Equator.

Astronomical coordinate systems have varying fundamental planes:^{[2]}

- The horizontal coordinate system uses the observer's horizon.
- The Besselian coordinate system uses Earth's terminator (day/night boundary).
^{[3]}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.

**^**Rogers, Lucy (2008),*It's ONLY Rocket Science: An Introduction in Plain English*, Springer, p. 136, ISBN 9780387753782.**^**Newcomb, Simon (1906).*A Compendium of Spherical Astronomy with Its Applications to the Determination and Reduction of Positions of the Fixed Stars*. Macmillan. p. 92..**^**"NASA - Besselian Elements of Solar Eclipses".*NASA*. Retrieved 13 August 2022.