If that plane passes through the centre of the sphere (so that the centre of the circle is also the centre of the sphere), the circle is called a great circle. The mean radius of the Earth, in the sen...If that plane passes through the centre of the sphere (so that the centre of the circle is also the centre of the sphere), the circle is called a great circle. The mean radius of the Earth, in the sense of the radius of a sphere having the same volume as the actual geoid, is a 3√a2c=6371.00 km.