fundamental plane (spherical coordinates)

{{Short description|Plane of reference that divides the sphere into two hemispheres}}

{{For|the concept in extragalactic astronomy|Fundamental plane (elliptical galaxies)}}

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.{{citation|title=It's ONLY Rocket Science: An Introduction in Plain English|first=Lucy|last=Rogers|publisher=Springer|year=2008|isbn=9780387753782|page=136|url=https://books.google.com/books?id=JTFGAAAAQBAJ&pg=PA136}}.

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

Astronomical coordinate systems have varying fundamental planes:{{cite book |title=A Compendium of Spherical Astronomy with Its Applications to the Determination and Reduction of Positions of the Fixed Stars |first=Simon |last=Newcomb |publisher=Macmillan |year=1906 |page=[https://archive.org/details/acompendiumsphe00newcgoog/page/n116 92] |url=https://archive.org/details/acompendiumsphe00newcgoog}}.

• The horizontal coordinate system uses the observer's horizon.
• The Besselian coordinate system uses Earth's terminator (day/night boundary).{{cite web |url=https://eclipse.gsfc.nasa.gov/SEcat5/beselm.html |title=NASA - Besselian Elements of Solar Eclipses |website=NASA |access-date=13 August 2022}} 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.