1 <sect1 id="ai-cequator">
4 <firstname>Jason</firstname>
5 <surname>Harris</surname>
8 <title>The Celestial Equator</title>
9 <indexterm><primary>Celestial Equator</primary>
10 <seealso>Equatorial Coordinates</seealso>
13 The <firstterm>Celestial Equator</firstterm> is an imaginary <link
14 linkend="ai-greatcircle">great circle</link> on the <link
15 linkend="ai-csphere">celestial sphere</link>. The celestial equator
16 is the fundamental plane of the <link linkend="equatorial">Equatorial
17 Coordinate System</link>, so it is defined as the locus of points
18 with Declination of zero degrees. It is also the projection of the
19 Earth's equator onto the sky.
22 The Celestial Equator and the <link linkend="ai-ecliptic">Ecliptic</link>
23 are set at an angle of 23.5 degrees in the sky. The points where they
24 intersect are the Vernal and Autumnal <link
25 linkend="ai-equinox">Equinoxes</link>.