Any circle, the plane of which passes through the centre of the earth, the imaginary core, projecting into space to intersect the ecliptic. Those most used in Astrology are the Horizon, the Equator and the Ecliptic. A LESSER CIRCLE does not pass through the centre of the earth but can be at any point. Lesser Circles define the TROPICS. On a sphere, meridians of longitude are great circles. The following are examples of great circles on the celestial sphere: Meridian Prime Vertical Celestial Equator.