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The Celestial Equator
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         **Jason Harris**

         The *Celestial Equator* is an imaginary :doc:`great
         circle <ai-greatcircle>` on the :doc:`celestial
         sphere <ai-csphere>`. The celestial equator is the
         fundamental plane of the :ref:`Equatorial Coordinate
         System <ai-skycoords-equatorial>`, so it is defined as
         the locus of points with Declination of zero degrees. It is
         also the projection of the Earth's equator onto the sky.

         The Celestial Equator and the :doc:`Ecliptic <ai-ecliptic>`
         are set at an angle of 23.5 degrees in the sky. The points
         where they intersect are the Vernal and Autumnal
         :doc:`Equinoxes <ai-equinox>`.