<sect1 id="ai-greatcircle">
<sect1info>
<author
><firstname
>Jason</firstname
> <surname
>Harris</surname
> </author>
</sect1info>
<title
>Great Circles</title>
<indexterm
><primary
>Great Circles</primary>
<seealso
>Celestial Sphere</seealso>
</indexterm>
<para
>Consider a sphere, such as the Earth, or the <link linkend="ai-csphere"
>Celestial Sphere</link
>. The intersection of any plane with the sphere will result in a circle on the surface of the sphere. If the plane happens to contain the centre of the sphere, the intersection circle is a <firstterm
>Great Circle</firstterm
>. Great circles are the largest circles that can be drawn on a sphere. Also, the shortest path between any two points on a sphere is always along a great circle. </para
><para
>Some examples of great circles on the celestial sphere include: the <link linkend="ai-horizon"
>Horizon</link
>, the <link linkend="ai-cequator"
>Celestial Equator</link
>, and the <link linkend="ai-ecliptic"
>Ecliptic</link
>. </para>
</sect1>