<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>