VLBI Observations of Nearly Diurnal Free Wobble

Theories of the Earth’s nutation show that the couplings between the core andmantle can give rise to an additional rotational normal mode,except for the Chandlerwobble(CW),due to the ellipsoidal core-mantle boundary and the fluid core of the Earth.The normal mode has often been referred to as the nearly diurnal free wobble(NDFW)with a period of nearly one sidereal day as viewed from the terrestrial reference system.The nutation of this mode relative to inertial reference system is retrograde free corenutation(RFCN).The theoretical value of the period is about 460 sidereal days.