ON THE NUTATIONS OF A MORE REALISTIC EARTH MODEL

The last adopted nutation series corresponds to an elliptical uniformly rotating earth. It is now well known that when comparing the theory with Very Long Baseline Interferometry (VLBI), Lunar Laser Ranging (LLR) or astronomical observations, relatively large residuals appear. This paper reviews and evaluates different possibilities to explain these differences: new rigid nutations are used in the convolution for computing nutations series (the elastic nutation amplitudes are computed at each frequency by multiplying the ‘ratios’ with rigid earth results); the mantle inelasticity is introduced in the earth model; the core flattening [and thus the Free Core Nutation (FCN)] is changed. Among these effects, the only very significant one is the effect of a change in the core flattening. It explains the discrepancy between recent data and the amplitude adopted by the International Astronomical Union (IAU) for the annual component. These are still some differences to be explained for the semi‐annual and the fortnightly components. Other effects are also introduced such as a pressure change at the Core‐Mantle Boundary (CMB). The sensitivity of the nutation amplitudes to a change of pressure at the CMB can be estimated. However, in order to perform an exact computation including this latter effect it would be necessary to know the pressure variations at tidal periods. This pressure is unknown. This is the reason why it could not be introduced in a new nutation series computation. Copyright © 1990, Wiley Blackwell. All rights reserved