Rare Shadows on Jupiter
Anyone who looks at Jupiter through a telescope is almost guaranteed to see the Galilean satellites (Io, Europa, Ganymede and Callisto) circling the giant planet. On rare occasions, observers catch one of those moons casting its shadow on Jupiter's cloudtops. On Jan 24th, observers in North America and the Carribean saw not one, but three shadows. Efrain Morales Rivera sends this picture from Aquadilla, Puerto Rico:
"I was happy to be able to observe this rare event," says Rivera. A full-sized version of his image matches each shadow to a moon. One belongs to Io, one to Europa, and one to Callisto. If you think you count four shadows, that's because the solid body of Callisto is passing in front of Jupiter and its dark silhouette looks like a shadow, too.
The reason for this "triple shadow transit" has to do with Jupiter's seasons. Jupiter is about to have an equinox--that is, the sun is about to cross Jupiter's equatorial plane. This edge-on alignment with the sun encourages Jupiter's moons to cast their shadows on the planet below. For the record, Jupiter's equinox is on Feb. 5th. Observers of the giant planet should remain alert for shadows.