Report of the IAU Working Group on ‘Nonrigid-Earth Nutation Theory’

Abstract : The last precession-nutation model adopted by the International Astronomical Union (IAU) in 1980 is the nutation series built on Wahr's Earth transfer function for the nutations of an oceanless elastic Earth (Wahr, 1979, 1981), and on Kinoshita's rigid-Earth, precession-nutation series (Kinoshita, 1977; Kinoshita et al., 1979). The resulting precession and nonrigid-Earth nutation series have been used since that time and have been compared with observations. This comparison, which has been done by different teams all over the world, shows that the theoretical series must be improved to meet observational precision. A Working Group (WG) was set up to examine the possibility of adopting a new nonrigid-Earth nutation series and to study the existing possibilities. On the one hand, the rigid-Earth nutation series have been improved (three new series) and the mutual differences have been shown to be less than a few hundreds of microarcseconds. On the other hand, new Earth transfer functions have been derived based on additional physical considerations within the Earth. However, the problem with these transfer functions is that there is no reliable, independent information about the geophysical parameters needed to improve a theoretical model. Instead, the discrepancies with the nutation observations themselves are used to infer those parameters. Recent fits of geophysical parameters to the observed nutations have provided a series that is suitable for practical use, and is also a source of important information on the physics of the Earth's interior. This paper reviews the recent work of the WG and establishes the reasons and criteria for the choice of the new model IAU 2000, which is proposed for adoption at the next IAU General Assembly.