Great Circles

Consider a sphere, such as the Earth, or the
Celestial Sphere. 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 Horizon, the Celestial Equator, and the Ecliptic.