A circle described on the surface of a sphere with its plane passing through the centre of the sphere. The shortest distance between any two points on a sphere lies along a great circle. On the Earth, lines of longitude lie on great circles.
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.
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.
On the Earth, the meridians are on great circles, and the equator is a great circle. Great circles on Earth are roughly 40,000 km in length, though the Earth is not a perfect sphere;
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.
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