Hg. Walter et Oj. Sovers, PRECESSION AND NUTATION FROM THE ANALYSIS OF POSITIONS OF EXTRAGALACTIC RADIO-SOURCES, Astronomy and astrophysics, 308(3), 1996, pp. 1001-1008
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.