Nov 10, 2010


WHEN: Various times and dates throughout November

THE FOUR LARGEST satellites of Jupiter are known as the Galilean moons after their discovery by Galileo Galilei in 1610. The moons appear to flit from one side of the planet to the other in an unceasing celestial ballet. As they get close to the planet, their discs and shadows create interesting events that can be seen with small telescopes. 

Io and Europa orbit the planet relatively quickly in periods of 1.77 and 3.55 days respectively; Europa's period being twice that of Io, This means that events between these moons and Jupiter take place fairly frequently.

The next moon out is Ganymede, the largest moon in the Solar System. With an orbital period of7.15 days, Ganymede takes twice as long as Europa to orbit Jupiter, so its events are less common. There are plenty of Ganymede-related events occurring on the evening of 3 ovember. As the sky darkens, the giant moon will be behind the planet, reappearing at 16:58 in an event known as an occultation reappearance (OcR). An hour later, Ganymede will dramatically fade from view as it enters Jupiter's shadow. This event, known as an eclipse disappearance (EcD), marks the start of an eclipse of Ganymede that will continue until 20:58, when an eclipse reappearance (EcR) restores the moon to view. As an added bonus, at 19:43, Jupiter's innermost moon, Io, can also be seen coming out of eclipse. Use the charts below to find the events on specific dates and times. 

Being so large, Ganymede's shadow is impressive when it crosses Jupiter's disc, and you'll have a chance to see it on the evening’s of 21 and 28 November. Ganymede's shadow transit will be underway as the sky darkens on the 21st, and the event comes to an end at 19:1J. with a shadow transit egress (ShE). 

On the 28th, Ganymede itself can be seen exiting off Jupiter's disc at 18:05, an event known as a transit egress (TrE). This is followed by the appearance of Ganymede's shadow on the disc, a shadow transit ingress (ShI), at 20:21. The shadow remains visible until it leaves the disc at 23:J 2; a shadow transit egress (ShE). 

The outermost Galilean moon, Callisto, takes 16.6 days to orbit Jupiter. On 20 November; if the skies are clear, you should be able to see Callisto just clip the southern part of Jupiter’s disc, starting with a transit ingress (TrI) at 20:38 and ending with a transit egress (TrE) at 21:13.

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