Secular variation of the obliquity of the ecliptic from a core-mantle coupling

The Earth is assumed to consist of a rigid core and a rigid mantle coupled by a non-isotropic friction. It is shown that if the core rotates slightly faster than the mantle, then the observed secular change of the obliquity of the ecliptic can be accounted for without giving rise to an unacceptably high rate of spin-down of the Earth.