Earth rotation varies continuously in both its rate and its axis of orientation, with respect to either crust-fixed or space-fixed reference frames. Its study links the fields of Geodesy, Astronomy and Geophysics. Rotational fluctuations occur on all observable time scales, from subdaily to decadal and longer, and are measured by geodetic and astronomical methods. Interpretation of the variations reveals subtle features of the Earth system involving a wide range of geophysical and astronomical effects, which are causing those rotational fluctuations. These include external tidal forcing by the sun, moon, and planets, interactions between the solid Earth and its surficial fluid envelopes (atmosphere, oceans, and hydrosphere), and internal processes within the solid Earth. Those causes fall into three categories: mass redistribution (or matter term excitation), relative motion (or motion term excitation) and torque, and can be converted to the rotational excitations through the responses of the stratified, deformable Earth. Theoretical developments relating Earth rotation variations with specific geophysical causes have a long and rich history, providing unique information about the rheology of the planet. But the high accuracy of modern geodetic data presents some challenges for the fullest theoretical understandings, such as the frequency-dependent rotational responses of the Earth, the nonpassive responses of the oceans to the Earth’s rotational variations and the dynamic-barometer effect in the ocean-atmosphere-ocean interactions.

Measurements of the Earth’s time-varying rotation have been provided traditionally by optical astrometry and more recently by the space geodetic techniques of satellite and lunar laser ranging (SLR and LLR), very long baseline interferometry (VLBI), global navigation satellite systems (GNSS), and Doppler orbitography and radio positioning integrated by satellite (DORIS). With the launch of the GRACE twin gravity satellites in March 2002 and the densification of the global GNSS ground tracking network, new opportunities for studying the Earth’s rotation have opened. GRACE directly observes surface mass redistribution while the global network of GNSS receivers senses the associated load deformations of the Earth’s surface and its effect on the Earth’s rotation. While all the above-mentioned techniques can only provide the position of the celestial intermediate pole, the emerging ring laser gyroscopes are directly sensitive to the terrestrial position of the instantaneous rotation pole of the Earth and thus may contribute to better understanding ultra-high-frequency rotational variations, particularly those occurring on subdaily to daily time scales. Combining independent observations of the Earth’s rotation, gravity, and shape enable greater insight into the common processes causing them over a very broad band.

This joint science symposium, organized by Commission 19/A2 (Rotation of the Earth) of the International Astronomical Union (IAU), Commission 3 (Earth Rotation and Geodynamics) of the International Association of Geodesy (IAG), the International Earth Rotation and Reference Systems Service (IERS), and National Natural Science Foundation of China (NSFC), and hosted by Wuhan University, Shanghai Astronomical Observatory and the Institute of Geodesy and Geophysics, will be a forum for assessing our current ability to observe the Earth’s time varying rotation, for assessing our current understanding of the causes of the observed variations, for assessing the consistency of Earth rotation observations with global gravity and shape observations, for exploring methods of combining Earth rotation, gravity, and shape observations to gain greater understanding of the mass load acting on the surface of the solid Earth, and for identifying improvements in the global geodetic observing system needed to further our understanding of the Earth’s variable rotation.