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0001 <sect1 id="ai-greatcircle">
0002 <sect1info>
0003 <author>
0004 <firstname>Jason</firstname>
0005 <surname>Harris</surname>
0006 </author>
0007 </sect1info>
0008 <title>Great Circles</title>
0009 <indexterm><primary>Great Circles</primary>
0010 <seealso>Celestial Sphere</seealso>
0011 </indexterm>
0012 <para>
0013 Consider a sphere, such as the Earth, or the
0014 <link linkend="ai-csphere">Celestial Sphere</link>.  The intersection
0015 of any plane with the sphere will result in a circle on the surface of
0016 the sphere.  If the plane happens to contain the center of the sphere,
0017 the intersection circle is a <firstterm>Great Circle</firstterm>.
0018 Great circles are the largest circles that can be drawn on a sphere.
0019 Also, the shortest path between any two points on a sphere is always
0020 along a great circle.
0021 </para><para>
0022 Some examples of great circles on the celestial sphere include: the
0023 <link linkend="ai-horizon">Horizon</link>, the
0024 <link linkend="ai-cequator">Celestial Equator</link>, and the
0025 <link linkend="ai-ecliptic">Ecliptic</link>.
0026 </para>
0027 </sect1>