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A great circle passing through the Earth's rotational poles and through the zenith (the point directly overhead) of a location on the Earth.
Reference:
Explanatory Supplement to the Astronomical Almanac. 1992. K.P. Seidelmann, Ed., University Science Books, Mill Valley, California.
NOTE:
The list of defined terms to the left is by no means exhaustive with
respect to the general subject of solar system dynamics.
The terms listed are thought to be those more commonly used on our site
and thus appropriate for inclusion on this page.
