Abstract
At the end of the 20th century, the VLBI observations put in evidence inconsistencies at the level of a few milliarcseconds in the conventional models that represented the position of the celestial pole. The International Astronomical Union (IAU), jointly with the International Union of Geophysics and Geodesy (IUGG), established the IAU/IUGG Working Group on Nutation for a non-rigid Earth to elaborate a new model that fits the observations better. The WG activities concluded with the elaboration of the precession-nutation model IAU 2000, based on the transfer function of Mathews et al. (2002). In this paper, we present the characteristics of the model, introduce the new concepts and discuss the transformation between the terrestrial and the celestial frames.
