On IAU and IAG collaboration in the new JWG on Consistent Improvement of Earth Rotation Theory

Over the last decade, the International Astronomical Union (IAU) and the International Association of Geodesy (IAG) have organised successive Joint Working Groups (JWGs) to study in depth the current theories and models of Earth rotation, which are of paramount importance for general geodesy and astronomy, and for positioning and navigation on Earth and in space in particular. Their activities have resulted in IAU and IAG resolutions encouraging the improvement of such theories and models, particularly with regard to the consistency and accuracy of the Earth Orientation Parameters (EOPs). In this communication, we recall some results of the past JWGs and introduce the new JWG on Consistent Improvement of Earth Rotation Theory (CIERT) and its planned activities.

The former IAU/IAG JWG on Improving Theories and Models of Earth Rotation (ITMER) showed that the unexplained variance of the precession/nutation (PN) variables, measured in terms of the WRMS of the observed celestial pole offsets (CPO), can be significantly reduced by fitting to the observations some corrections of the linear component of precession and of the theoretical amplitudes of some lunisolar and planetary nutation terms. Furthermore, the unexplained WRMS is reduced to less than 3 mm by supplementing the corrections with appropriate Free Core Nutation (FCN) models. However, not only because of the need to deepen theoretical knowledge, but also because of the large number of nutation frequencies involved and the consequent limitations of the fits, the use of semi-empirical nutation models is only a temporary solution.

It is therefore necessary, and a major goal of the new JWG, to derive complete theories that bring together the many advances made by different research groups in recent years in a way that is internally consistent and also coherent with the analysis of observational data. One of the most recent advances is the derivation of fully analytical corrections to planetary nutations, which are competitive since they allow the empirical fit to be limited to the lunisolar terms.

Acknowledgment. This research was supported partially by Spanish Projects PID2020-119383GB-I00 funded by Ministerio de Ciencia e Innovación (MCIN/AEI/10.13039/501100011033); SEJIGENT/2021/001, funded by Generalitat Valenciana; and the European Union—NextGenerationEU (ZAMBRANO 21-04).