September 2, 2009: Double shadow transit on Jupiter and more

Steve Gottlieb

Around 10:30 when I started observing in my driving in Albany there was a rare situation (only a few times a century) where all 4 Galilean moons were either transiting, in shadow, or being occulted -- so the result was no moons were visible off the disc of Jupiter! Callisto was invisible as it was being eclipsed by Jupiter's shadow, while Io was occulted (behind) by Jupiter. Finally, Europa and Ganymede were transiting Jupiter.

At midnight, Europa had finished transiting and was visible off the west side and Io had reemerged from Jupiter's shadow and was visible to the east of Jupiter. Both Ganymede and its shadow were both easily visible on Jupiter and Europa's shadow was clearly seen on the west side of Jupiter.

Ganymede's shadow was relatively large (only a couple of arc seconds), very dark and crisply defined. Ganymede's 1.8" disc was slightly smaller, lighter color with a less defined edge. Europa's shadow was close west of Ganymede's disc and appeared as a very small, dark spot. So, you could easily view three tiny discs on Jupiter simultaneously.

It's possible (though fairly rare) to see a triple shadow transit. The last one occurred on March 27, 2004 and the next one is October 12, 2013. The 2004 event included Callisto, Ganymede and Io. All three shadows were easily visible for an entire half hour starting around 11:30 as well as the disc of Callisto! Seeing was fairly good that night and not only were the shadows of the discs cleanly resolved, they could be individually distinguished in size. Europa was being occulted by Jupiter, so during the event there was an odd situation of only one moon off of Jupiter.

Steve