Special Io eclipse

by Akkana Peck

Akkana Peck wrote:

As far as future possible Io/shadow events this pass: from typing in dates to my applet, it looks like there's one tomorrow night [ ... ]

That was last night. The applet predicted that the transit would begin around 9:20pm PDT (that's 0420UT). A slow waiter almost caused us to be late getting back for dinner, but we burst into the back yard (where the FS128 and C8 had been cooling) at about 9:22 to find Jupiter just barely visible through the low, fast-moving clouds, and Io still hanging off the planet's disk. The transit began about five minutes later, and occasional holes in the clouds allowed us to watch the progress of the event.

When Io's shadow first appeared, it was just a speck hanging off the trailing, northern edge of the moon's disk. The clouds prevented using high enough magnifications to see a crescent shape, but it was clear that the shadow was very small, nowhere near the size of Io's disk. As the moon and shadow moved in away from the limb, the shadow's size grew, until finally it grew to a full found circle, the same size as the disk of Io itself and tangent to Io's disk.

Shortly after that, I lost Io against the brightness of Jupiter, and was only able to see the shadow. The clouds thickened, and we decided to take down rather than continue to chase holes waiting for egress.

This was almost exactly the reverse of the pre-opposition transit we observed on the 19th; in both cases, we saw Io occulting its shadow when both were near the limb, but the moon and the shadow farther apart when they were near Jupiter's meridian. I assume that on the other limb, the moon and shadow would have been even farther apart, but I didn't get to see that end in either transit.