The Earth and Moon are held together by gravitational attraction, which is balanced at the common centre of gravity by an equal and opposite centrifugal force. Away from the centre of gravity the strength of the gravitational force changes as the distance to the Moon varies. The centrifugal force, however, remains constant everywhere on Earth. Therefore, except at the centre of gravity, the gravitational and centrifugal forces are not the same and this inequality creates a residual force. These three forces are depicted in the following diagram.