by Akkana Peck
The Io transit is happening now. Amazingly, we have clear skies here in San Jose, but alas, the seeing is rotten. Still, Io and its shadow are both clearly visible; I can't see the shadow's shape, but the shadow looks very much smaller than the disk of Io, which it usually doesn't, leading me to believe that Io is in fact occulting its own shadow now, and that if the seeing were better we'd see a crescent shadow. I know Bill said Starry Night said they would be tangent, but SN has been wrong before ...
We have the 6" and the 4.5" out now, and just set the C8 out to cool.
Anyone else looking and have steadier skies? What shape is the shadow?
To combat the seeing we set up the Tak FS128 with a binoviewer (barlowed) and 7.5mm eyepieces (about 275x).
The shape of the shadow is unclear. Most of the time it looks round, but every now and then the seeing settles and it looks like it might be crescent after all. We ... just ... couldn't ... tell.
The shadow being smaller than the moon was a limb effect; the difference in shadow size as the shadow moved from the limb to the meridian and on to the other limb was dramatic.
The other three Galileans were clustered close together -- the difference between them was dramatic. Anyone who doubts the ease of telling them apart would have been convinced tonight.
Quote of the evening, as Io had ended its transit but the shadow was still visible: "That's a great yin/yang effect, with the moon against the darkness and the black thing against the bright thing." Perhaps we're not always as articulate as we might be during an observing session ... but the view was lovely, anyway.
Slightly later, there was an even weirder effect: the shadow exiting Jupiter's limb looked like a tiny mouth Jupiter was opening to swallow its bright moon, like a huge mouth-brooding fish.