<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>