On the night of September 2-3, 2009, a remarkable celestial event will take place. From 4:43 to 6:29 Universal Time on the 3rd (which is 12:43 to 2:29 a.m. EDT on the 3rd, or 9:43 to 11:29 p.m PDT on the 2nd), a casual look at Jupiter through a telescope will show no moons at all. It's quite common for one of the four Galilean moons be hidden, and it's not rare to see only two moons. But only a few times in a century do all four moons hide simultaneously behind or in front of Jupiter.

Tony Flanders

As the diagram at right shows, Callisto and Io will be either behind Jupiter or eclipsed by Jupiter's shadow, rendering them completely invisible to any telescope. But Europe and Ganymede will be in front of the planet, where their disks should (at least in theory) be visible at high magnification if the atmosphere is very steady. And for most of that time, one or both of the moons' shadows will also fall on Jupiter. The moons themselves are hard to see because they're similar in color and brightness to Jupiter. But the shadows are pitch black, so they stand out relatively well.

To enjoy the event to the fullest, you should try to watch the moons' disappearances and reappearances. Most of the 11-hour sequence should be visible from the American East Coast, and much of it is visible across all of the Americas and in Europe.

Most experienced Jupiter observers are used to reading the "phenomena of Jupiter's satellites" in compressed tabular form. You can also get it spelled out by our online Javascript Utility, as shown below. The two sets of times are slightly different due to calculational complexities, but the discrepancy is nearly irrelevant in practice. Jupiter's moons have substantial disks, so they don't wink off and on as stars do when a planet passes in front of them. Instead, they fade out and back in over a period of several minutes. Subtract four hours from UT to get EDT, five hours for CDT, six for MDT, and seven for PDT.

Wednesday, September 2, 2009
23:28 UT, Callisto enters occultation behind Jupiter.

Thursday, September 3, 2009
03:44 UT, Io enters occultation behind Jupiter.
04:00 UT, Europa begins transit of Jupiter.
04:46 UT, Ganymede begins transit of Jupiter.
04:56 UT, Europa's shadow begins to cross Jupiter.
06:32 UT, Io exits eclipse by Jupiter's shadow.
06:46 UT, Ganymede's shadow begins to cross Jupiter.
06:50 UT, Europa ends transit of Jupiter.
07:50 UT, Europa's shadow leaves Jupiter's disk.
08:24 UT, Ganymede ends transit of Jupiter.
08:44 UT, Callisto exits eclipse by Jupiter's shadow.
10:24 UT, Ganymede's shadow leaves Jupiter's disk.


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