<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 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
<link linkend="ai-cequator">Celestial Equator</link>, and the
<link linkend="ai-ecliptic">Ecliptic</link>.
</para>
</sect1>