great circle A line on the surface of a sphere representing the circumference of a circle whose centre is coincident with the centre of that sphere. In making a stereographic projection, a horizontal (equatorial) projection of the sphere (i.e. as a plane at right angles to the N—S plane) is a primitive circle of given radius, and also a great circle. All other great circles are similar to lines of longitude. A line joining the points on the conceptual sphere at right angles to the horizontal plane will project on a stereogram as a straight line passing through the centre of the projection, with the N and S poles as its diameter, both of which plot coincidentally in the centre of the horizontal plane. See PLANE OF PROJECTION.

great circle Circle on a spherical surface, the centre of which is coincident with the centre of the sphere. On the celestial sphere, the Equator is a great circle, as are all meridians. The shortest distance between any two points on a sphere, great circles are used for mapping aircraft routes.