CORRECTION TO THE LUNI-SOLAR PRECESSION FROM VERY LONG-BASE-LINE INTERFEROMETRY

The redetermination of the luni-solar precession is performed by using VLBI catalogues of extragalactic radio sources containing positions a t observation epochs which cover 9 years. The positions of the sources have been determined by the VLBI network of the Crustal Dynamics Proj ect of NASA from measurements covering consecutive half year and one y ear intervals between 1981 and 1989. In the course of their reduction the IAU recommended precession terms were applied. On the assumption o f an imperfection of the luni-solar precession it is expected that the positions of each source obtained at different epochs but uniformly r educed to the reference system J2000.0 exhibit an apparent proper moti on. - We attributed this motion tentatively to precession and solved f or a correction of the luni-solar precession by making a least squares fit to the position differences between homonymous sources in the set of catalogues. The 18.6 yr terms of nutation entered the process as p arameters the values of which were set by choice from the reservoir of recent determinations. Twenty-five sources contributed to the analysi s with the effect that the annual and semi-annual catalogues contain 1 6 sources on the average. The analysis confirmed the slightly overesti mated IAU value of the luni-solar precession yielding a mean correctio n of -3.59 +/- 1.14 mas yr-1 from the annual and -3.84 +/- 1.16 mas yr -1 from the semi-annual catalogues. We discuss these secular terms and their dependence on the adopted nutation terms.