Callisto-Io conjunction 9 March UT

by Morris Jones

Last night was a fair Jupiter observing night at the San Francisco Amateur's City Star Party. I was showing Jupiter in the early evening in the AP180, and we were treated to a nice shadow transit by Callisto.

Later in the evening Callisto and Io had a very close conjunction. The two were inseparable in most of the scopes. I was alerted to the event when another astronomer called over, "Why does Jupiter only have three moons now?"

I sat at the eyepiece to watch the mutual event at 270x. The Callisto- Io pair had a slight elongation. During moments of good seeing I could detect color differences between the two while they appeared to overlap and then separate.

Now here's the problem. I can't reconcile what I remember observing with what I would have expected.

I saw a bluish grey moon and an orangish moon. I assumed naturally that the bluish grey one was Callisto, and the orangish moon was Io. But when I replay the event today in SkyMap, the moon I remember as bluish grey appears to be Io, and the orangish one Callisto.

I looked up the albedo numbers for the two, and Callisto should rightly be darker than Io.

Of course it's perfectly reasonable that my memory is faulty. In the waning hour of a public star party I didn't pause to take notes and make a sketch.

That'll teach me. :)