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Mutual Events

Apostolos Christou, Jonathan McAuliffe, Ignacio Ugarte Urra, Babulakshmanan Ramachandran, and others have participated in the international Mutual Events observing campaign, in collaboration with Arto Oksanen (Nyrölä Observatory, Jyväskylä, Finland), to observe and reduce the lightcurves of so-called `Mutual Events' of the Galilean satellites of Jupiter (see Figure 4). These phenomena, which involve passages of one satellite directly in front of or behind another, or into the shadow of another satellite, are visible from Earth during a mutual event `season' lasting for a period of approximately one year roughly every six years, and provide unique astrometric information on the Jovian system. For example, the timing of an event provides an accurate measurement of the relative position of the two satellites in space, and the detailed light curve gives information on the albedo distribution across the two satellites, improving knowledge of the photometric centre of the satellite and so improving the reduction of regular astrometric observations. Accurate measurements of the positions of the Galilean satellites will allow their respective tidal accelerations to be determined, which in turn provides a clue to the nature of the planetary and satellite interiors, information which is not otherwise accessible (`photometric geophysics'). All this work has been carried out from Armagh using relatively small telescopes, including the Planetarium 10-inch Schmidt-Cassegrain. A total of 17 eclipse or occultation events involving Jupiter's Galilean satellites were fully observed and calibrated. Final reduction of the Armagh and Nyrölä data will be carried out in-house.