Here’s another astounding solar system image. The big guy in the picture is obviously Jupiter. But see that misshapen thing to its upper right? That’s Ganymede, its biggest Moon – in fact, Ganymede is slightly bigger than Mercury.
Ganymede, like any other body that big, is a sphere. So why does it look weird? Because, in that image, it was slipping into Jupiter’s shadow! If you had been standing on the moon’s surface at that time, you would see Jupiter in silhouette, blocking the Sun in your sky. It’s a solar eclipse, writ large. Very large: from Ganymede’s viewpoint Jupiter would be nearly 8 degrees across– 15 times larger than the Moon appears in our own sky! If I outstretched my arm, my palm would just cover Jupiter.
Looking at the whole series, I am reminded again that the planets in our sky are worlds like the Earth, places that exist, and not just points of light at night. They have moons, eclipses, weather, some even have seasons … and most amazing of all to me is that little disk of Ganymede. I grew up looking at that moon – and its three siblings Io, Europa, and Callisto– in my telescope, and they were never more than dots. It wasn’t until the Pioneer and Voyager probes that they became worlds in their own right. And now technology has advanced so much that even someone on Earth can spot their faces. You can even see some detail on Ganymede! Fantastic!
I’ll remind readers again that pictures like this, and many more, can be found on the Bad Astronomy and Universe Today bulletin board, too.’