<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>