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The diagrams and text below explain how the various phenomena (Eclipses, Occultations, Transits and Shadow Transits) of the Gaililean satellites of Jupiter take place.

An ECLIPSE of a satellite occurs when the satellite moves through the SHADOW of Jupiter. On the diagrams below a satellite will disappear into eclipse as it reaches the point labelled E in its orbit. It will reappear at the point labelled E'.

An OCCULTATION of a satellite occurs when the satellite moves BEHIND Jupiter as seen from the Earth and so becomes hidden by the planet. Points at which the satellite will disappear into occultation are labelled O and where they reappear are labelled O' on the diagrams. Satellites always disappear into occultation at the west side of Jupiter and come out from behind the planet at the east side.

Jovian Phenomena before Opposition [13k]

A TRANSIT of a satellite occurs when it moves in FRONT of Jupiter as seen from the Earth. When in transit over Jupiter's disk the satellite will be virtually impossible to see. Satellites move into transit at points marked with a T and emerge from transit at points marked T' on the diagrams. A satellite in transit always moves from east to west across the face of Jupiter.

A SHADOW TRANSIT occurs when the shadow of a satellite, cast by the Sun, moves across Jupiter's disk. The shadow may be seen through a telescope as a small black dot on Jupiter. Shadow transits start at the point S and end at S' on the diagram. Shadow transits also move from east to west across Jupiter.

As will be appreciated from the diagrams, eclipse events usually take place some distance from the edge or limb of Jupiter, but occultations necessarily take place at the limb of Jupiter. This makes eclipses easier to watch than occultations, since eclipses mostly appear to take place some distance from Jupiter and there is no difficulty seeing the satellite close to the bright disk of the planet.

The diagrams show the direction of the Sun compared to the direction of the Earth before and after opposition. Also shown are the direction of the shadow of Jupiter and the region hidden by Jupiter.

Before opposition the shadow is to the west of Jupiter as seen from the Earth. As a result eclipses occur before occultations. The two inner satellites, Io and Europa, emerge from eclipse after the occultation of the satellite has started, hence the reappearance from eclipse cannot be seen and neither can the disappearance into occultation. i.e. Only the disappearance from eclipse and the reappearance from occultation are visible. Ganymede and Callisto are sufficiently far out from Jupiter for us to be able to see the satellites reappear from eclipse before they are occulted, at least for part of the orbit.

Jovian Phenomena after Opposition [13k]

After opposition the situation is reversed; occultations occur before eclipses, and in the case of Io and Europa, only the disappearance into occultation and the reappearance from eclipse are visible. Before opposition the shadow of a satellite will start moving across the face of Jupiter before the satellite itself starts to transit the planet, but after opposition the order is again reversed with the transit starting first.

As Jupiter moves towards opposition, the angle between the directions from the Earth and Sun get smaller and smaller. As the angle gets smaller so the time between an eclipse and occultation will get less, as will the time between a shadow transit and the transit of the satellite.

At opposition, when Jupiter is on the opposite side of the Earth to the Sun, the three bodies are in line. As a result, close to opposition eclipses and occultations occur very close in time to each other, as do the two types of transits. At opposition the times will coincide.

After opposition the situation is reversed. The angle between the direction of the Sun and Earth increases for about three months, and then it starts decreasing again before Jupiter moves behind the Sun.

This page was last updated on 11 March 1999.
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