Great Circles have a radius of 90 degrees measured along the circumference of the sphere. The equator of the Earth and meridians of longitude are great circles. A plane passing through the center of the sphere cuts the sphere in a great circle. Small Circles have a radius not equal to 90 degrees. Parallels of latitude are small circles. A plane not passing through the center of the sphere cuts the sphere in a small circle. A pole to such a plane, hoever, does. A circle of radius r degrees also has a radius of 180-r degrees. Great circles divide the sphere into equal halves. The importance of great circles in geological applications of spherical projections is that they can represent planes. The center of a great circle is called its pole. If you know a great circle, you can find its pole, and if you know the pole, you can find the great circle. Thus it is possible to represent a plane by a single point. This fact is extensively used in advanced projection techniques. The perimeter of equatorial plane is called primitive circle.