Triple shadow transits, where three moons cross the face of Jupiter, happen only once or twice a decade. Catch the show tonight.
If you have a telescope, you'll want to take a look at Jupiter tonight (October 12th) between 4:32 and 5:37 Universal Time. (That's 12:32 to 1:37 a.m. Saturday morning, Eastern Daylight Time.) You'll see a rare occurrence: three moons will be casting their inky black shadows on the gas giant’s cloud tops.
The show begins just after 3:05 UT with Callisto’s elongated shadow entering the Jovian disk near the planet’s south polar region, followed by Europa’s shadow just below the South Equatorial Belt at 3:25. Io’s shadow joins the party at 4:32.
Observers in Europe and Africa are best placed to witness the entire event with Jupiter high in the sky. Observers along the East Coasts of North and South America will be able to catch the tail end of the transit with Jupiter low in the East. This event should not be missed — triple shadow transits are only visible once or twice a decade. Fortunately because the shadows are so dark, they should be easily visible even with the planet low on the horizon.