<sect1 id="ai-greatcircle">
<sect1info>
<author>
<firstname>Jason</firstname>
<surname>Harris</surname>
</author>
</sect1info>
<title>Great Circles</title>
<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 center 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 Celestial Equator, and the <link
linkend="ai-ecliptic">Ecliptic</link>.
</para>
</sect1>