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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2010.06.007 Byline: Anthony Mallama (a)(b), Chris Stockdale (c), Bruce A. Krobusek (d), Peter Nelson (e) Keywords: Orbit determination; Io; Europa; Ganymede; Callisto Abstract: Astrometric satellite positions are derived from timings of their eclipses in the shadow of Jupiter. The 548 data points span 20years and are accurate to about 0.006arcsec for Io and Europa and about 0.015arcsec or better for Ganymede and Callisto. The precision of the data set and its nearly continuous distribution in time allows measurement of regular oscillations with an accuracy of 0.001arcsec. This level of sensitivity permits detailed evaluation of modern ephemerides and reveals anomalies at the 1.3year period of the resonant perturbations between Io, Europa and Ganymede. The E5 ephemeris shows large errors at that period for all three satellites as well as other significant anomalies. The L1 ephemeris fits the observations much more closely than E5 but discrepancies for the resonant satellites are still apparent and the measured positions of Io are drifting away from the predictions. The JUP230 ephemeris fits the observations more accurately than L1 although there is still a measurable discordance between the predictions and observations for Europa at the resonance period. Author Affiliation: (a) Raytheon Information Solutions, 5700 Rivertech Court, Riverdale, MD 20737, USA (b) Department of Mathematics and Statistics, University of Maryland University College, 3501 University Blvd. East, Adelphi, MD 20783, USA (c) Hazelwood Observatory, 8 Matta Drive, Churchill, Vic. 3842, Australia (d) Farmington Observatory, 5950 King Hill Drive, Farmington, NY 14425, USA (e) Ellinbank Observatory, 1105 Hazeldean Road, Ellinbank 3820, Australia Article History: Received 10 January 2010; Revised 31 May 2010; Accepted 3 June 2010