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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 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>
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