Fundamental plane

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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.

The fundamental plane also refers to a relationship between the effective radius, average surface brightness and central velocity dispersion of stars in elliptical galaxies. The relationship can be used to aid the understanding of galaxy formation and the relative distances between clusters.

Fundamental plane

See also