A
great ellipse is an
ellipse passing through two
points on a
spheroid and having the same
center as that of the spheroid. Equivalently, it is an ellipse on the
surface of spheroid and centered at the
origin, or the curve formed by intersecting the spheroid by a plane through its center. For points which are separated by less than about a quarter of the circumference of the earth, about , the length of the great ellipse connecting the points is close (within one part in 500,000) to the
geodesic distance. The great ellipse therefore is sometimes proposed as a suitable route for marine navigation.