A great circle is the largest possible circle that can be drawn on the surface of a sphere, such as the Earth. It represents the shortest path between two points on the sphere. For example, when flying from one city to another, airplanes often follow a great circle route to minimize travel distance. The equator is the most well-known example of a great circle, as it divides the Earth into the Northern and Southern Hemispheres. Other great circles can be formed by any plane that passes through the center of the sphere, including lines of longitude, which connect the North and South Poles.