Jupiter's moondance on a Saturday night

by Jamie Dillon

Liam was off with the Scouts on a snow trip to Truckee, so Jo and I were in the backyard watching the Galilean moons do their dance all thru Saturday night. It was wonderful. When I started peeking, after 7 pm, Callisto had already left the disk, but its shadow was getting ready to transit. From then on, we did catch the main events. Can't happen very often like this, with the plane of Jupiter's big moons being along our line of sight, and all 4 of them crossing the line between Jupiter and Earth.

It was not long after midnight, ca 0020, that Callisto's moon was finally exiting, but meanwhile sure enough Ganymede and its moon were there in front of Jupiter's disk, along with Io and its moon. As advertised, 3 shadows and 2 moons all at once. Europa had meanwhile ducked behind.

What was most interesting to me was the varying lag times between moon and shadow. Seen in realtime visual, it was dramatic. Callisto's moon followed it by some 5 hours. Ganymede's shadow came along some 2.25 hours the big moon. And Io's moon was only a half hour back. It happened that Io was right in the middle of the SEB, so with its shadow for a marker, it was possible to keep visual track of Io's transit. I'm used to seeing Io disappear onto the disk. There was a big pair of white ovals in the SEB just ahead of Io's shadow as well.

Hot action Jupiter. Also novel, for an observer who does spend a lot of time on hilltops, was packing up and being home without driving. Shoot, it was ten till 1, Ganymede was due to exit the disk at 0105, and I was talking myself into staying up another 15 minutes. Brushed the teeth, wandered back into the yard, and there was that huge moon heading off the disk, with the vastness of space to be seen again between it and its planet. Serious planet-fu.

Posted on sf-bay-tac Mar 30, 2004 13:31:59 PT
Converted by report.pm 1.1 Jul 09, 2004 23:34:39 PT