Astrometry of the Galilean satellites from mutual eclipses and occultations

Abstract

Twelve eclipses and occultations of Io were recorded with a CCD camera during the mutual eclipse and occultation season of 1991. In 11 of these events Io was covered by Europa, and in the twelfth it was covered by Callisto. These observations give high quality astrometric information that should be useful in connection with infrared determinations of the locations of Io's volcanoes made by other investigators during the occultations. Differential astrometric positions were determined by fitting the photometry to a model light curve that is based on the photometric properties of the eclipsed or occulted satellite, as well as the apparent motion of the satellites during the event. The astrometric results agree with Lieske's E-3 ephemeris predictions at a 1 σ level of about 13 milliarcsec in orbital latitude. However, an 80-milliarsec longitude residual, seen in the Europa-Io events, is many times larger than the uncertainty. This mean longitude residual can be applied to the E-3 ephemeris in order to use it for Europa-Io events that were not observed photometrically but that need astrometric calibration. Evidence for the longitude residual in previous mutual event astrometry is briefly discussed.