Use a great circle to find the shortest distance between two points on a globe. The intersection of a plane passing through the center of a sphere forms a great circle at the surface. Spheres are not flat, so the shortest distance between two points does not define a line.
Ships and airplanes use great circles to plan their route between two locations. Traveling this course requires constant redirection as the heading continues to change unless the plane is flying due north or south. The lines of longitude form half a great circle that connect with another longitude line 180 degrees around the globe. The equator is the only line of latitude that forms a great circle.
The circumference of a great circle around the earth is approximately 24,854 miles. The equator is slightly longer as the earth is not a perfect sphere.
At times, planes and ships plan routes that do not follow the shortest path due to other factors. When traveling with the jet stream, the forces of the winds make it worthwhile to choose a slightly longer path. However, when traveling into the jet stream, the wind resistance decreases fuel economy. The captain of the vessel often chooses a great circle path that avoids the wind resistance to increase economy.