ON THE PRECESSION CONSTANT - VALUES AND CONSTRAINTS ON THE DYNAMICAL ELLIPTICITY - LINK WITH OPPOLZER TERMS AND TILT-OVER-MODE

The luni-solar precession, derived by theoretical considerations from the precession of the equator, is one of the most important parameters for computing not only precession but also nutations, due to its rela tion to the dynamical flattening. In this paper, we review the numeric al values of this parameter, from the geodynamical point of view as we ll as the astronomical point of view, from the observational point of view as well as from the theoretical point of view. In particular, we point out a difference of about 1 percent between the global Earth dyn amical flattening derived from the astronomical observations and the v alues derived from the different geophysical computations. The nutatio n amplitudes depend on the Earth dynamical flattening and this depende nce is amplified by a resonance at an important normal mode, the Tilt- Over-Mode (TOM). Since the astronomical point of view as well as the g eophysical one are confronted, we also take the opportunity to make th e link between the TOM and the expressions of the nutations of the dif ferent axes which, in turn, are related with one another by the Oppolz er terms. Both, the Oppolzer terms and the TOM originate from a refere nce frame tilt effect. In writing the link between the nutational moti ons of the different axes, and so, in writing the Oppolzer terms, we a lso make the link with the precessional motion.