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A great circle divides the sphere in two equal hemispheres

A great circle is a circle on the surface of a sphere that has the same circumference as the sphere, dividing the sphere into two equal hemispheres. Equivalently, a great circle on a sphere is a circle on the sphere's surface whose center is the same as the center of the sphere. A great circle is the intersection of a sphere with a plane going through its center. A great circle is the largest circle that can be drawn on a given sphere.

Great circles serve as the analog of "straight lines" in spherical geometry. See also spherical trigonometry and geodesic.

The great circle on the spherical surface is the path with the smallest curvature, and, hence, an arc (an orthodrome) is the shortest path between two points on the surface. The distance between any two points on a sphere is known as the great-circle distance. The great-circle route is the shortest path between two points on a sphere; however, if one were to travel along such a route, it would be difficult to manually steer as the heading would constantly be changing (except in the case of due north, south, or along the equator). Thus, Great Circle routes are often broken into a series of shorter Rhumb lines which allow the use of constant headings between waypoints along the Great Circle.

When long distance aviation or nautical routes are drawn on a flat map (for instance, the Mercator projection), they often look curved. This is because they lie on great circles. A route that would look like a straight line on the map would actually be longer.

On the Earth, the meridians are on great circles, and the equator is a great circle. Other lines of latitude are not great circles, because they are smaller than the equator; their centers are not at the center of the Earth -- they are small circles instead. Great circles on Earth are roughly 40,000 km in length, though the Earth is not a perfect sphere; for instance, the equator is 40,075 km.

Some examples of great circles on the celestial sphere include the horizon (in the astronomical sense), the celestial equator, and the ecliptic.

Great circle routes are used by ships and aircraft where currents and winds are not a significant factor. For aircraft traveling westerly between continents in the northern hemisphere these paths will extend northward near or into the arctic region, while easterly flights will often fly a more southerly track to take advantage of the jet stream.

Airline great circle track at top, jet stream track at bottom

• Jet stream
• Lune
• Small circle
• Gnomonic map projection
• Rhumb line
• Geodesic
• Great-circle navigation
• Great-circle distance
• Great ellipse

• Great Circle – from MathWorld Great Circle description, figures, and equations. Mathworld, Wolfram Research, Inc. c1999
• Great Circle Mapper Interactive tool for plotting great circle routes.
• Blue Marble Mapper Draws Great Circle routes between airports using the NASA Blue Marble as the base map.
• Great Circle Calculator deriving (initial) course and distance between two points.
• Great Circle Distance Graphical tool for drawing great circles over maps. Also shows distance and azimuth in a table.