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What Are Jovian Satellite Phenomena?

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As well as total lunar, grazing, and minor planet occultations, the RASNZ Occultation Section also co-ordinates observation of the eclipses of Jupiter's four Galilean satellites. (Io, Europa, Ganymede and Callisto).

Eclipses are one of the four types of phenomena Jupiter's satellites can undergo - Eclipses, Occultations, Transits and Shadow Transits. Eclipses are the easiest events to time because they usually occur some distance from the bright limb of Jupiter.


Jupiter casts a shadow which stretches behind it in space, and eclipses occur when a satellite passes into the planet's shadow. This happens to the three inner satellites every orbit - for example, eclipses of Io occur every 42 hours. Callisto, the outermost of the four satellites, goes through periods of being eclipsed each orbit at 16 day intervals. These periods last nearly three years. After this follows a similar interval when Callisto passes either above or below Jupiter's shadow and so is not eclipsed.

When a satellite moves into eclipse its light dims until the satellite disappears from view. A few hours later it emerges from the other side of the shadow, and as it does so it brightens and becomes visible again. Thus eclipses of Jupiter's satellites differ from occultations in that the events are not instantaneous. Rather, the disappearance or reappearance takes place over a matter of a few minutes.


Timing when Jupiter's satellites enter and leave eclipse provides important information on the position of the satellites in their orbits. This data has been used by Dr Jay Lieske at the Jet Propulsion Laboratory to check on his ephemeris of the satellites. In turn, Dr Lieske's ephemeris has been an important part of planning for the various NASA spacecraft missions to Jupiter, including the current Galileo mission.


Limited predictions for Jovian satellite events visible from New Zealand this month are available here.

The Jovian satellite eclipse program is co-ordinated by Brian Loader, who computes predictions based on the ephemeris produced by Dr Jay Lieske. If you would like full predictions for your site (or further information about Jovian eclipses) please email Brian the following information:

For Brian's snail mail address, click here.


The basic method of timing Jovian satellite eclipses is the same as for total, grazing and planetary occultations. However, because a Jovian event takes some time to occur, when you observe a disappearance the "last speck" time is recorded - that is, the time when the satellite finally disappears from view. For a reappearance the first speck as the satellite emerges from the shadow is timed. While other occultation timings are required to tenth of a second precision, timings of Jovian events to the nearest second are adequate.

The actual time a particular observer sees a Jovian eclipse occur depends on a large number of factors, mostly beyond the control of the observer. Not least amongst these is the aperture of the telescope being used. In general, with a larger aperture disappearances will be timed later and reappearances earlier. However Jovian eclipses can be observed with almost any size instrument; even down to telescopes with only 5 cm of aperture.

Jovian satellite eclipse predictions cover one Jovian apparition, which is the time from one conjunction of Jupiter with the Sun to the next, a period of about 13 months. At the end of each apparition, eclipse timings should be returned to Brian Loader.


A few months after the end of each Jovian observing season Brian carries out reductions of timings made by observers in Australia and New Zealand. The results are sent back to the observers, as well as being forwarded to Dr John Westfall of the Association of Lunar and Planetary Observers (ALPO) in California and Dr Lieske at JPL. Dr Westfall acts as co-ordinator for observing these events in many other parts of the world. Results from the RASNZ Occultation Section are added to others he receives, and the reductions he carries out on the combined observations are eventually published in "The Strolling Astronomer", the ALPO journal.

Results from the RASNZ Occultation Section contribute a significant portion of the observations Dr Westfall receives.

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