Steve’s Virtual Planet

Io’s Shadow Dance

The last few weeks have seen Jupiter climb higher in the sky out of the East North East in the constellation of Gemini. So, even with colder weather moving in, it’s still very easy to get a good look at the Great Planet before your hot chocolate gets cold! And if you plan ahead, you can also have better chance to catch a passing of a moon shadow from one of its Galilean moons.

Io’s shadow is a good one to look for, visible fairly often, in fact every few days. However, it is not always at a convenient time for your time zone. But, nevertheless, it happens often enough to think ahead and do some planning. You can start with an internet search on “Jupiter Moon Shadows”. There will likely be a number of articles, especially about the triple shadow event this past October. But also look for some of the tools for planning these events like the Sky & Telescope Jupiter’s Moon utility. One version is for Java and it will run in most web browsers but they also have an iPhone app. There are various other tools as well. My favorite is the Moons/GRS tab on Sky View Cafe.

The evening of December 12th is a chance to see one of these events. And if you setup your telescope by 9:30 or so, you will be able to see the beginning of the event play out. The initial passing of Io’s shadow begins right about 10:00PM EST, followed by the ingress of Io itself about 10:36PM. The shadow will be very easy to see if you have a steady view of Jupiter. Magnification is less of a factor than clarity, so choose an eyepiece to give you a crisp view. At first, you might even be looking right at it and not even know it. But as you keep watching, you will realize that the tiny spot is very dark and distinct. Indeed, check your planning chart; it is right where it’s supposed to be, a shadow from another world!

You can easily follow the shadow well into the evening until egress a little after midnight. But the real challenge is trying to see Io itself as it crosses the face of Jupiter. Well, to be honest, you are not likely to see it because of it’s size and mostly because of Jupiter’s brightness. But you never know! You might get a lucky alignment where the moon is over the darker southern equatorial belt and there may be enough contrast to make it out. Just look a little bit behind the path of the shadow.

Dress warmly and enjoy the show!