Damping of the Earth's Free Nutation

Abstract

THE ‘free nutation’ of the Earth is a motion of the Earth's instantaneous axis of rotation around its mean position. The poles (intersections of the axis with the Earth's surface) describe changing orbits with the ‘Chandler period’ of about fourteen months. Every six or seven years the amplitude of these orbits reaches a maximum of up to ten metres and then decreases almost to zero. Following Jeffreys¹, Bondi and Gold² have considered that the free nutation has a characteristic damping time of the order of ten periods. They find that such heavy damping cannot arise from internal motion of the core or from relative motion between core and mantle, and conclude that it results from non-elastic behaviour of the mantle.