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.
Global Locations of Great CirclesGreat circles are easily identified on a globe based on the lines of latitude and longitude. Each line of longitude, or meridian, is the same length and represents half of a great circle. This is because each meridian has a corresponding line on the opposite side of the Earth. When combined, they cut the globe into equal halves, representing a great circle. For example, the Prime Meridian at 0° is half of a great circle. On the opposite side of the globe is the International Date Line at 180°. It too represents half of a great circle. When the two are combined, they create a full great circle which cuts the Earth into equal halves.
The only line of latitude, or parallel, characterized as a great circle is the equator because it passes through the exact center of the Earth and divides it in half. Lines of latitude north and south of the equator are not great circles because their length decreases as they move toward the poles and they do not pass through Earth's center. As such, these parallels are considered small circles.
Navigation with Great CirclesThe most famous use of great circles in geography is for navigation because they represent the shortest distance between two points on a sphere. Due to the earth's rotation, sailors and pilots using great circle routes must constantly adjust their route as the heading changes over long distances. The only places on Earth where the heading does not change is on the equator or when traveling due north or south.
Because of these adjustments, great circle routes are broken up into shorter lines called Rhumb lines which show the constant compass direction needed for the route being traveled. The Rhumb lines also cross all meridians at the same angle, making them useful for breaking up great circles in navigation.
Appearance on MapsTo determine great circle routes for navigation or other knowledge, the gnomic map projection is often used. This is the projection of choice because on these maps the arc of a great circle is depicted as a straight line. These straight lines are then often plotted on a map with the Mercator projection for use in navigation because it follows true compass directions and is therefore useful in such a setting.
It is important to note though that when long distance routes following great circles are drawn on Mercator maps, they look curved and longer than straight lines along the same routes. In reality though, the longer looking, curved line is actually shorter because it is on the great circle route.
Common Uses of Great Circles TodayToday, great circle routes are still used in long distance travel because they are the most efficient way to move across the globe. They are most commonly used by ships and aircraft where wind and water currents are not a significant factor though because currents like the jet stream are often more efficient for long distance travel than following the great circle. For example in the northern hemisphere, planes traveling west normally follow a great circle route that moves into the Arctic to avoid having to travel in the jet stream when going the opposite direction as its flow. When traveling east however, it is more efficient for these planes to use the jet stream as opposed to the great circle route.
Whatever their use though, great circle routes have been an important part of navigation and geography for hundreds of years and knowledge of them is essential for long distance travel across the globe.