Tidal friction is known to change the spin period of the bodies and their relative distance.
For instance, in the Earth-Moon system the rotation rate of the Earth decreases and the relative distance increases. Let m , I and be the mass, moment of inertia and spin angular velocity of the bodies orbiting around one another on circular orbits at relative distance 2 r and orbital angular velocity n . We refer here to the case of equal masses and parallel rotation and revolution vectors.