The Europa Eclipse -- results (Shallow Thoughts)

The Europa Eclipse -- results

I wrote last week about an upcoming eclipse of Europa by Jupiter's shadow. One of the interesting things I'd found was how much the predicted times of Europa's appearance from behind Jupiter, and subsequent disappearance into Jupiter's shadow, varied depending on which program you were using. I had just recently managed to get my own Javascript Jupiter page showing eclipse events, and its times didn't agree with any of the other programs either. So I was burning with curiosity to know who was right.

The predicted times were:

	Europa appears	Europa disappears
XEphem	7:43	7:59
S&T Jupiter's Moons	7:40	7:44
my Javascript Jupiter	7:45	7:52
Stellarium	6:49	7:05

I was out of town on March 10. I brought along a travel scope, an Orion 80mm f/6 Orion Express. Not the perfect planetary scope, but certainly enough to see Europa. (The Galilean moons are even visible in binoculars, as long as you mount the binoculars on a tripod or otherwise hold them steady.)

I synchronized my watch and had the telescope set up by 7:35. Sure enough, there was no Europa there. But at 7:38 on the dot, I saw the first hint of Europa peeking out. No question about it. I watched, and timed, and by 7:41 the whole disk of Europa was visible and I could start to think I could see blackness between it and Jupiter. I'd been to a school star party a few days earlier and hadn't cleaned my eyepieces afterward -- oops! -- so the view was a little foggy and it was hard to tell for sure exactly when Europa's disk cleared Jupiter.

In fact, no matter which eyepiece I used, the fogginess seemed to get worse and worse. I had a hard time seeing Europa at all. Finally I realized that I was looking through a tree branch, and moved the scope. But by the time I got it moved again, Europa had gotten even harder to see. That was when I realized that it had been going into eclipse practically the whole time I was watching it. It was already significantly dimmed by 7:43, very dim indeed by 7:48 and gone -- in the 80mm -- by 7:49:20, though I suspect it still would have been visible in a larger scope with clean eyepieces.

So that's why the times in different programs varied so much! Galilean moons aren't point sources: you can't predict a single time for a moon disappearance, appearance or eclipse. Do you want to predict the beginning of the event, the end of the event or the time at the moon's center point? And that goes double for eclipses, where the moon is gradually sliding into the shadow of Jupiter's atmosphere. I found that it took over seven minutes the moon to go from full brightness to fully eclipsed. So what part of that do you predict?

All in all, a very interesting observing session. I'm looking forward to observing more of these eclipses, doing more timings, and tuning my program to give better predictions. (I notice my program was significantly late on both the appearance and the eclipse. I'll work on that. Better to err on the early side, and not miss anything!)

While I was adding eclipses to my Jupiter program, I also added longer-range predictions, so it would be easier to find out when these events will happen. Once that was implemented, I looked for upcoming Whac-a-Moon events. I found one on Mar 26, when Ganymede appears at 7:29pm PDT (add 7 hours for GMT). Europa and its shadow are transiting Jupiter's disk, too, so there's plenty to look at. Ganymede then enters eclipse at 9:40pm PDT. A long time between the events, I know, but it's easy enough to leave a scope set up in the backyard and go out to check it now and then.

These times are from my Javascript Jupiter program and may be a few minutes late. Always be ready at least five minutes early in case the predictions are off, no matter which program you use. Don't say I didn't warn you.

I found no events in April visible at night in California (for other time zones, you can generate predictions on my Javascript Jupiter page). But May 8 has a decent one: Ganymede appears at 9:44pm PDT, then disappears into eclipse at 9:46. Based on what I saw tonight with Europa, that means the moon should start to fade almost immediately after it has emerged from behind Jupiter, maybe even before it has fully emerged. Ganymede's larger size may also mean the fade-out will take longer. Stay tuned. Jupiter will be very low by then, only 7 degrees above the horizon.

Not many events to observe -- this is a bit rarer than I'd thought. Of course, there are lots of moons disappearing into eclipse and appearing from out of it every night, so watching that long gradual appearance or disappearance isn't difficult; the only rare part is when they appear briefly between Jupiter and Jupiter's shadow. That is relatively rare, and I'm glad I had a chance to catch it.