<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>
