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 <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>