A great circle is defined as any circle drawn on a globe (or other sphere) with a center that includes the center of the globe. Thus, a great circle divides the globe into two equal halves. Since they must follow the circumference of the Earth to divide it, great circles are about 40,000 kilometers (24,854 miles) in length along meridians. At the equator though, a great circle is a little bit longer as the Earth is not a perfect sphere.
In addition, great circles represent the shortest distance between two points anywhere on the Earth's surface. Because of this, great circles have been important in navigation for hundreds of years but their presence was discovered by ancient mathematicians.