Eclipses: the Long Goodbye

Jupiter is still visible in April, but it’s starting to get low in the western sky. If you set up right around sunset, you can take advantage of a handy phenomenon: the seeing is often unusually steady for a short time after sunset. (A similar effect happens at dawn.) The atmosphere gets more turbulent as the night cools, so you’re looking for that brief period when the air and the ground are in equilibrium, so there’s no heat flowing from one to the other – and therefore no heat waves to interrupt your view. An hour or so later, as the air cools faster than the ground, you’ll start seeing more heat-induced turbulence and the seeing will get a lot less steady than it was at sunset. That’s especially true if you have to look over asphalt streets or your neighbor’s roofs.

Last month I wrote about eclipses of Jupiter’s moons: in particular, an event on Mar 10 where Europa popped out from behind Jupiter, then disappeared into Jupiter’s shadow just a few minutes later.

I predicted, based on Sky & Telescope’s online Jupiter’s Moons page, that Europa would appear from behind Jupiter at 7:40, and would disappear into eclipse at 7:44.

After my column had gone to press, I found time to work on my own Jupiter’s moons code to add eclipse predictions. To check my work, I simulated the Europa event in several different programs. I was amazed how much the times varied. Here’s what they predicted. (Note that Stellarium evidently has a time zone bug.) [ See TABLE 1]

I was itching to observe the event myself and find out what times things really happened. Who was right? I was betting on XEphem, which is generally known for accuracy.

Unfortunately, I was out of town on March 10. I brought 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, if you mount them on a tripod or otherwise keep them steady.)

I synchronized my watch and had the telescope set up by 7:35, since the earliest prediction for Europa’s appearance was 7:40. 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 an exact 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? For eclipses, since it takes seven minutes or more for the moon to go from full brightness to fully eclipsed, what part of that do you predict?

So it was 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 predictions, so it would be easier to find out when these events will happen. Unfortunately, all of the whac-a-moon events for April take place during California daytime. But there’s one just before April begins, on Mar 26, when Ganymede appears at 19:29 (while Europa and its shadow are transiting Jupiter’s disk) and enters eclipse at 21:40.

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.

Note: 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.

May 8 is better: Ganymede appears at 21:44, then disappears into eclipse at 21:46. 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.

Of course, there’s plenty to see on Jupiter besides moons – its stormy atmosphere has been spectacular this year, though it’s a little past its prime now since it’s so low.

It’s a good thing Jupiter is so interesting, since there aren’t many other planets to look at this month. Saturn is well placed for late night viewers, though it’s not as high in the sky as we might like. The rings are tilted 18 degrees to us. Nearly all the other planets are too close to the sun for good viewing. Late in the month, you can barely catch Venus low in the evening sky. Pluto rises a bit after midnight, so an ambitious Plutocrat might be able to catch it in the predawn hours. All in all, I’m happy Jupiter is there!