The shortest route between two points on a sphere, formed by the intersection of the sphere and a plane through its center.
In practice
In practice, an OOW plans a great-circle passage by computing a series of intermediate waypoints at regular intervals of longitude, connecting them with rhumb-line legs, thus approximating the great circle. Modern ECDIS systems with MSC.232(82) performance standards can calculate and display great-circle routes automatically, simplifying passage planning considerably. The chief limitation is that great circles at high latitudes curve toward the poles, potentially traversing ice-prone or storm-track areas, so the optimum route often lies between the great circle and the rhumb line, taking weather routing into account.
Regulatory detail & full definition
A great circle is the circle formed by the intersection of a sphere with a plane passing through its centre. On the surface of the earth, a great circle represents the shortest distance between two points—the geodesic—making it the theoretically optimal route for long ocean passages. Bowditch (American Practical Navigator) covers great-circle sailing in detail, noting that the required course changes continuously as the vessel proceeds along the arc, whereas a rhumb line course remains constant.
In practice, an OOW plans a great-circle passage by computing a series of intermediate waypoints at regular intervals of longitude, connecting them with rhumb-line legs, thus approximating the great circle. Modern ECDIS systems with MSC.232(82) performance standards can calculate and display great-circle routes automatically, simplifying passage planning considerably. The chief limitation is that great circles at high latitudes curve toward the poles, potentially traversing ice-prone or storm-track areas, so the optimum route often lies between the great circle and the rhumb line, taking weather routing into account.