PRECESSION AND NUTATION FROM THE ANALYSIS OF POSITIONS OF EXTRAGALACTIC RADIO-SOURCES

Corrections to the Earth's precession and nutation have been derived f rom VLBI observations of extragalactic sources carried out by JPL's De ep Space Network between 1978 and 1994. The analysis is based on the s ource right ascensions and declinations given in annual position catal ogues referring to the J2000.0 equator and equinox. These catalogues r esult from the reduction of the VLBI observables by adopting the 1976 IAU convention on precession and one of the following nutation models: 1980 IAU Theory of Nutation, ZMOA 1990-2 and KSNRE. Differences of th e J2000.0 positions of a source obtained at different epochs suggest t he presence of imperfections in the precession and nutation terms. In contrast to the commonly practiced direct solutions, corrections to th e luni-solar precession and the 18.6 yr nutations in longitude and obl iquity are determined by a least squares fit to the differences of pos itions of individual sources at different epochs. Using the 1980 IAU a nd KSNRE models gives sizable, largely similar corrections. The ZMOA 1 990-2 model, on the other hand, is characterized by small corrections to the nutation terms. Each of the three data sets associated with one of the nutation models provides a solution in right ascension (RA) as well as in declination (Dec). The Dec solution is self-sufficient, wh ereas the RA solution requires some a priori knowledge of the precessi on and nutation quantities that are to be determined. The self-suffici ent declination solutions for the 1980 IAU, the ZMOA 1990-2 and the KS NRE models yield almost identical corrections of the luni-solar preces sion, which are in turn -3.1 +/- 0.2 mas yr(-1), -3.1 +/- 0.1 mas yr(- 1) and -3.3 +/- 0.2 mas yr(-1). For the 18.6 yr nutations in longitude and obliquity the IAU and KSNRE models yield -5.4 +/- 1.1 mas, 3.8 +/ - 0.3 mas and -6.7 +/- 0.8 mas, 3.3 +/- 0.1 mas, while the ZMOA model gives 0.0 +/- 0.4 mas and 0.5 +/- 0.2 mas, respectively.