<sect1 id="ai-cequator"> <sect1info> <author ><firstname >Jason</firstname > <surname >Harris</surname > </author> </sect1info> <title >The Celestial Equator</title> <indexterm ><primary >Celestial Equator</primary> <seealso >Equatorial Coordinates</seealso> </indexterm> <para >The <firstterm >Celestial Equator</firstterm > is an imaginary <link linkend="ai-greatcircle" >great circle</link > on the <link linkend="ai-csphere" >celestial sphere</link >. The celestial equator is the fundamental plane of the <link linkend="equatorial" >Equatorial Coordinate System</link >, 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. </para> <para >The Celestial Equator and the <link linkend="ai-ecliptic" >Ecliptic</link > are set at an angle of 23.5 degrees in the sky. The points where they intersect are the Vernal and Autumnal <link linkend="ai-equinox" >Equinoxes</link >. </para> </sect1>