=============
Great Circles
=============

         **Jason Harris**

         Consider a sphere, such as the Earth, or the :doc:`Celestial
         Sphere  <ai-csphere>`. 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 *Great Circle*. 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.

         Some examples of great circles on the celestial sphere include:
         the :doc:`Horizon  <ai-horizon>`, the :doc:`Celestial
         Equator  <ai-cequator>`, and the
         :doc:`Ecliptic  <ai-ecliptic>`.