An excitation mechanism for the free 'core nutation'

Summary. The Earth is believed to possess a free nutational mode due to its rotating, elliptical, fluid core, with an eigenfrequency of approximately (1 + 1/460) cycle sidereal day^?1 as seen from the sidereally rotating Earth. This free 'core nutation' has not yet been undisputably observed. Furthermore, there has been considerable doubt that any known mechanism could excite this mode to an observable level. We show here that diurnal atmospheric and oceanic loading of the Earth's surface provide an efficient excitation mechanism which depends critically on the physical damping of the mode. Possible effects of the mode on geodetic measurements are discussed. We also consider the effects of 'wobble' and 'nutation' on astrometric observations.