On the Damping of Free Nutation of the Earth

Abstract

The free (420^d) nutation is known to be heavily damped, the characteristic damping time being less than ten periods. The origin of this damping is unknown, but must be due to dissipative, non-rigid-body movements of the Earth. The view, which has been widely held, that this may arise from the relative motion between the liquid core of the Earth and the mantle is shown to be based on an error. When account is taken of its small moment of inertia, the core cannot be held responsible for this damping, nor can a limit be derived for its viscosity. This confines the origin of the damping to a non-elastic behaviour of the mantle.