"Every six years the Sun (and the Earth) pass through the equatorial plane of the planet Jupiter. At those instances, Jupiter's four Galilean satellites: Io, Europa, Ganymede and Callisto - can cast shadows on, as well as occult each other as viewed from the Earth on numerous occasions during a period of 6-9 months. This is known as 'the mutual events season'.
A wordlwide network of amateur and professional astronomers record and time the mutual eclipses and occultations of the Galilean satellites as accurately as possible. The reason why such observations are interesting is because the tidal force that a planet exerts on a satellite and vice versa.
The same process takes place in the Earth-Moon system with the result that the Moon's orbit recedes from the Earth at a rate of several cm per year. This greatly affects the position of the satellite in long timescales (decades to centuries).
The mutual event timings are particularly sensitive to this longitudinal acceleration. Already, the acceleration of Io is evident in the data and continued monitoring through this coming season will help pin down the acceleration of the other three satellites.
The tide that Jupiter raises on Io is not constant but varies slightly as the satellite moves around its eccentric orbit. This flexing of the crust of Io by about 100m is converted, through mechanical stress and strain, into enormous amounts of heat (approx 1,000,000,000,000 kW) which manifests itself through active volcanism.