| Reports | TAC | Join mailing list |
by Akkana Peck
Last Tuesday night (10/19) there was an Io pass which I thought might be close enough for Io to occult its shadow, with both the moon and the shadow right next to the GRS (I meant to post a suggestion here that people watch it, but I was still catching up after a week's vacation). My Jupiter applet at http://www.shallowsky.com/jupiter.html showed a crescent shadow but isn't necessarily that accurate. The seeing wasn't very steady here in San Jose (CA), unfortunately, and Io wasn't visible when near the center of Jupiter's disk, but the shadow was clearly visible, and looked round, not crescent, though it was hard to tell for sure in the unsteady air.
We began our observations at about 9:15, when they were just past meridian (when Jupiter appeared above the tree in the front yard) and watched the transit as it progressed toward the limb, using a Takahashi FS128 and a C5, swapping the binoviewer between the two, and an 80mm f/7 refractor (even the 80f7 showed the event clearly, at 110x and 145x). As the shadow neared the limb, we noticed a dark patch at the northern leading edge of the GRS, and wondered whether that might be Io; but a few minutes later, a pale patch appeared between the GRS and the shadow which was clearly the moon's disk.
At first, Io's disk was tangent to the shadow's disk. But as they neared the limb, the moon's disk increasingly overlapped the shadow, until at the point where they were about to leave the disk, only a tiny crescent of the shadow remained, as a "drop shadow" highlighting the leading edge of Io's disk against its parent planet, giving the moon an exceptionally three-dimensional look.
Then the shadow disappeared off the limb, and the moon became a bright pimple on Jupiter's cheek, then popped out to go its separate way.
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 (around 10pm PDT), and another one early Thursday night (6pm PDT, before Jupiter rises here on the west coast, but people farther east might be able to catch it). Then on November 1, there's a Europa pass that's still pretty close -- my guess is that Europa won't actually be close enough to touch its shadow, but I'll plan on watching anyway.