An Accidental Eclipse from 700 Million Kilometers Away

Astronomers keeping an eye on Jupiter's moon Io got a surprise on Dec. 16 when it looked like something had taken a bite out of it: What they saw was another of Jupiter's moons, Europa, passing directly in front of Io, what astronomers call a transit (or more generically a "mutual event"). Jupiter's moons orbit the giant planet directly above its equator, and a few times a year everything lines up just right so that, from Earth, we see the orbits edge-on. When that happens the moons can pass in ...