Flattening of the Earth: further from hydrostaticity than previously estimated

The knowledge of the gravitational potential coefficients J 2 and J 4 of a hydrostatic Earth model is necessary to deal with non-hydrostatic properties of our planet. They are indeed fundamental parameters when modelling the 3-D density structure or the rotational behaviour of our planet. The most widely used values computed by Nakiboglu need to be updated for two reasons. First, we have noted a mistake in one of his formulae. Secondly, the value of the inertia ratio I /M R 2 chosen at the time of PREM is not any more the best estimate. Both corrections slightly but significantly reduce the hydrostatic J 2 value: the dynamical flattening of the Earth is even further from hydrostaticity than previously thought. The difference between the polar and equatorial radii appears to be 113 1 m (instead of 98 m) larger than the hydrostatic value. Moreover, uncertainties upon the hydrostatic parameters are estimated.