Astrometry of Pluto and Saturn with the CCD meridian instruments of Bordeaux and Valinhos^*

¹ Observatoire de Bordeaux, BP 89, 33270 Floirac, France
² Departamento de Astronomia, Universidade de São Paulo, Av. Miguel Stéfano 4200, 04301-904 São Paulo, Brazil

Corresponding author: M. Rapaport, rapaport@observ.u-bordeaux.fr

Received: 3 October 2001
Accepted: 21 November 2001

Abstract

The accuracy of CCD meridian circles observations performed at the Bordeaux and Valinhos Observatories confirms the improvement represented by the DE405 ephemeris when compared to DE403 for Saturn and Pluto. In this work, we also use published positions obtained with the Flagstaff CCD meridian circle. We compare the DE403 and DE405 ephemerides and show that for Pluto and Saturn, large differences appear (100 mas and 60 mas respectively). The CCD meridian observations of Pluto prove that the DE405 ephemeris better represents the observations than does DE403. In the case of Saturn, the comparison has to be done on the satellite positions since the planet is too bright to be observed with the meridian circles. First we compare the two current major theories (Vienne & Duriez 1995; Harper & Taylor 1993, hereafter VD95 and HT93) of the eight first satellites of Saturn. We show in the case of Iapetus that there is an oscillation in the residuals coming from HT93 and that VD95 better represents the observations. We conclude on the necessity to continue ground-based observations of the outer planets and of their satellites to discriminate between proposed theories and ephemerides.