Clouds and poor seeing plagued much of North America on the night of March 27–28, 2004. Nonetheless, many people stayed up to observe the remarkable triple-shadow transit on Jupiter, an event that happens roughly once every decade. From 8:00 to 8:19 Universal Time on the 28th, two moons and three shadows lay on Jupiter’s disk.
Seeing all three shadows at once proved to be quite difficult because Callisto’s shadow started to slide off Jupiter’s disk shortly after Ganymede’s shadow was fully in place. The forecast called for both shadows to lie on Jupiter simultaneously for 19 minutes, but that referred to the shadow’s centers. The shadows are fairly large, and it takes several minutes for them to traverse their own length. They also can be rather hard to see when they lie on the planet’s limb.
In compensation, Ganymede’s grey disk stood out unexpectedly well because it lay in front of the North Tropical Zone, which is particularly broad and bright right now. Io transited in front of the North Equatorial Belt and became nearly invisible shortly after it entered the disk.
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