Posted by Steve Holmes2 at 14:57 on 2010 Nov 16
I’m not sure how late 9yr-olds are allowed to stay up these days but if you get a clear night on Wednesday (17th) and can avoid hypothermia (!), your "apprentice" might be interested in the disposition of Jupiter’s Galileans that evening. From 21:15 until 23:06, there will only be one moon visible! (Callisto – a long way to the east). Io is either occulted or in eclipse, Europa is in transit (as is its shadow) and Ganymede is occulted. The start is defined by Ganymede disappearing behind Jupiter, the end by Europa appearing again (both at the western limb). Io comes out of eclipse at 23:37 and Ganymede re-appears at the eastern limb at 00:15. [All times UTC]While such "only one moon visible" events are not particularly rare they are definitely unusual so should be worth observing. It could also be the start of a discussion about how often such an event should occur. With randomly-chosen orbit periods such a line-up would be very rare but of course the periods of the inner three Galileans are locked together by gravitational resonance and so line-ups actually happen much more frequently.Clear skies!Steve