Great circle

A great circle is the intersection of a sphere with a plane through its center. It divides the sphere into two equal hemispheres and gives the shortest surface path between two points.

Sphere: A three-dimensional surface whose points are all equidistant from a center. A great circle is defined by slicing this surface with a central plane.
Small circle: A circle formed where a sphere meets a plane that does not pass through its center. Unlike a great circle, its radius is smaller than the sphere’s radius.
Great-circle navigation: Navigation that plans routes along great-circle arcs on an approximately spherical Earth. It uses the shortest surface routes defined by great circles.
Equator: The great circle around Earth equally distant from the North and South Poles. It is Earth’s most familiar example of a great circle.
Plane: A flat, two-dimensional surface extending infinitely in three-dimensional space. A plane through the sphere’s center cuts out a great circle.
Spherical distance: The length of the shortest path between two points constrained to a sphere’s surface. For non-antipodal points, this path follows the great-circle arc joining them.
Orthodrome: A great-circle route between two points on a sphere. The term names the route followed when traveling along a great circle.
Meridian: A half-circle running from one geographic pole to the other, paired with its opposite half to form a great circle. A meridian and its opposite together lie on a plane through Earth’s center.
Radius: A line segment from a circle’s or sphere’s center to its boundary. The radius fixes the great circle’s radius and helps define its central angles.
Great-circle distance: The shortest surface distance between two points on a sphere, measured along their great-circle arc. It is the arc-length quantity computed from a great circle.

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