Io Transit - In this image of Jupiter, you can see the shadow of one of the planet's moons, Io, being cast onto the gas giant. Io is one of the Galilean Moons discovered by Galileo with a telescope around the year 1610. The Moons are Io, Europa, Ganymede, and Callisto. Since then, many more moons have been discovered around Jupiter, but the Galilean Moons are the biggest and easiest to spot with a telescope. You can see Io's shadow as a small black dot on the upper, thicker cloud band. Transits of Jupiter's moons such as this one are not that rare, and you can probably see one if you have a telescope and a good sky calendar.