  The masses distribution inside the Earth governs the behaviour of the
rotation axis in the Earth (polar motion), as well as the Earth rotation rate
(or equivalently, length of day). This masses distribution can be measured from
space owing to artificial satellites, the orbitography of which provides the
Earth gravity field determination. Then, the temporal variations of the Earth
gravity field can be related to the variations of the Earth Orientation
Parameters (EOP) (with the Inertia Tensor). Nowadays, owing to the satellite
laser ranging (SLR) technique and to the new gravimetric satellite missions
(such as CHAMP or GRACE), the temporal variations of the low degree
coefficients of the Earth gravity field (i.e. Stokes coefficients) can be
determined. This paper is one of the first study using gravity variations data
in the equations already established (e.g. Lambeck 1988) and linking the
variations of the length of day and of the C20 Stokes coefficient (or, linking
the polar motion and the C21 and S21 coefficients). This paper combines the
Earth rotation data (mainly obtained from VLBI and GPS measurements) and the
Earth gravity field variations ones (e.g. Lageos I and II data, or GRACE data),
in order to complete and constrain the models of the Earth rotation. The final
goal is a better Earth global dynamics understanding, which possible
application can be the constraint on the couplings into the Earth system (e.g.
core-mantle couplings).
