Relativity in Celestial Mechanics and Astrometry

The driving idea, in organizing this Symposium on "Relativity in Celestial Mechanics . and Astrometry" was that, in recent years, the ac­ curacy of several Astrometric techniques has so much increased that re­ lativistic effects are no more marginally mentioned in academic presen­ tations but have become an intrinsic part of the interpretation of all the positional data. Techniques such as V. L. B. I. , space probes, lunar lasers, etc . . . have greatly contributed to reduce the accuracy level to an angular equivalent of a few milliarc-seconds. HIPPARCOS will extend this trend to stellar astrometry and new proposals aim at gaining ano­ ther one or two orders of magnitude in observing precision. In paral­ lel, great progress has also been made in the theories of motion of ce­ lestial bodies and more rigourous treatments within the frame of general relativity become necessary. The definition of reference frames and of various coordinate times become also a major problem in the intercompa­ rison of observations produced by different methods. This Symposium gave to people using relativistic corrections in reducing their measurements or in constructing various theories of mo­ tion a chance to exchange their experience in the field and to confront it with theoreticians in general relativity. The Symposium was approved by the IAU in 1983 and it was organi­ zed by the Institute of Theoretical Astronomy of the USSR Academy of Sciences on May 28-31, 1985.