Knowledge of the lunar librations at the Lunar Laser Ranging experiment epoch

A renovated knowledge of the lunar librations is exposed in the present pap er: This updating derives both from progress of observational techniques of the Moon's motions and fi om theoretical methods of celestial dynamics. Fi rst is presented a summary of classical libration families such as the appa rent librations, the physical librations, ol the called fr ee librations. A mbiguities and redundancies of this cutting out are discussed On the one ha nd lunar librations ala strongly overlapped and datum of their branching is not immediate. On the other hand, new librations have been made into evide nce. Starting from a causal analysis of libration nature, the necessary arr angement of the lunar librations is first based on the modern knowledge of dynamical systems. It takes then into account the post-Newtonian gravitatio n acquisitions in use of reference systems required for dealing with rotati on motions combined with translation ones. By the way of the new families o f lunar librations, namely the potential and kinetical librations, the pape r gives the libration amplitudes discovered at the joint epoch of the LLR e xperiment and elaborated methods of numerical integrations of the coupled s pin-or-bit motion of the Moon The paper presents also a description of the lunar. rides If takes stock on the I-elation between the accurate study of the Moon's librations and the knowledge of its internal structure. Finally, it takes into account the lunar spin-or-bit interaction whereby the mechan ism of the secular acceleration of the Moon is marched by a secular slowing -down of its rotation. (C) 2000 Academie des sciences/Editions scientifique s Pt medicales Elsevier SAS.