MICROARCSECOND INSTABILITY OF THE CELESTIAL REFERENCE FRAME

The fluctuation of the angular positions of reference extragalactic ra dio and optical sources under the influence of the irregular gravitati onal field of visible Galactic stars is considered. It is shown that t hese angular fluctuations range from a few up to hundreds of microarcs econds. This leads to a small rotation of the celestial reference fram e. The non-diagonal coefficients of the rotation matrix are of the ord er of a microarcsecond. The temporal variation of these coefficients d ue to the proper motion of the foreground stars is of the order of one microsecond per 20 years. The celestial reference frame can therefore be considered inertial and homogeneous only to microarcsecond accurac y. Astrometric catalogues with microarcsecond accuracy will be unstabl e, and must be re-established every 20 years.