Whac-a-Moon

This March, planetary viewing is all about Jupiter. Saturn rises a little before midnight, so you can catch it if you stay up late, but even if you wait until 4am, it transits at less than 40 degrees.

And the rest of the planets are hiding in daylight skies where they’ll be tough to find.

But Jupiter is perfectly placed for evening viewing this month.

It transits around nightfall, high in the sky (73 degrees), and stays high for most of the evening. Its turbulent atmosphere has lots to look at this year – plenty of detail around and even inside the Great Red Spot, some dark junior red spots nearby. White ovals!

Festoons! Everything a Joviphile might want.

Well, nearly everything – although there are plenty of double transits of the Galilean moons and their shadows this month, they’re all in the predawn or daylight hours. But there are plenty of single moon and shadow transits to watch – plus another sort of moon event that you might not have seen.

A few months ago, I got email from a novice Jupiter observer calling my attention to an interesting phenomenon of Jupiter’s moons that I hadn’t seen before. The person who mailed me wasn’t quite sure what he had seen, but he knew it was unusual, and after some further discussion we pinned it down.

He was observing Jupiter at 11/11/12 at 00.25 UT (which would have been mid-afternoon here in San Jose). Three of the moons were visible, with only Ganymede missing. Then Ganymede appeared: near Jupiter’s limb, but not right on it. As he watched over the next few minutes, Ganymede seemed to be moving backward – in toward Jupiter rather than away from it. Eventually it disappeared behind the planet.

It turned out that what he was seeing was the end of an eclipse.

Jupiter was still a few months away from opposition, so the shadow thrown by the planet streamed off to one side as seen from our inner-planet vantage point on Earth. At 0:26 UT on that evening, long before he started observing, Ganymede, still far away from Jupiter’s limb, had entered Jupiter’s shadow and disappeared into eclipse. It took over two hours for Ganymede to cross Jupiter’s shadow; but at 2:36, when it left the shadow, it hadn’t yet disappeared behind the planet. So it became visible again. It wasn’t until 2:50 that Ganymede finally disappeared behind Jupiter.

So it was an interesting effect – bright Ganymede appearing out of nowhere, moving in toward Jupiter then disappearing again fourteen minutes later. It was something I’d never seen, or thought to look for.

It’s sort of like playing Whac-a-mole – the moon appears only briefly, so you’ve got to hit it with your telescope at just the right time if you want to catch it before it disappears again.

A lot of programs don’t show this eclipse effect – including, I’m sad to say, my own Jupiter’s moons web page. (I intend to remedy that.) The open source program Stellarium shows the effect; on the web, Sky and Telescope’s Jupiter’s Moons page shows it, and even prints out a table of times of various moon events, including eclipses. I’ve used these tables to predict a few upcoming “Whac-a-moon” events.

They’re not all that uncommon – but only when the sun angle is just right. During late February and early March this year, I found several events for Ganymede and Europa (though, sadly, many of them were during our daytime). By mid-March, the angles have changed so that Europa doesn’t leave Jupiter’s shadow until after it’s disappeared behind the planet’s limb; but Ganymede is farther out, so we can see Ganymede appearances all the way through March and for months after.

The most interesting view, it seems to me, is right on the boundary when the moon only appears for a short time before disappearing again.

And with that in mind, the best upcoming event happens on Sunday night, March 10.

Reporting on that one got a little tricky – because that’s the day we switch to Daylight Savings time. I have to confess that I got a little twisted up trying to compare results between programs that use UTC and programs that use local time – especially when the time zone converter I was using to check my math told me “That time doesn’t exist!”

Darnit, if we’d all just use UTC all the time, astronomy calculations would be a lot easier! Anyway, here’s the scoop.

On Sunday night, March 10, at 7:40 pm PDT, Europa peeks out from behind Jupiter’s northeast limb. The sky will still be bright – the sun sets at 7:12 that night – but Jupiter will be 66 degrees up and well away from the sun, so it shouldn’t give you too much trouble.

Once Europa pops out, keep a close eye on it – because if Sky & Tel’s calculations are right, it will disappear again just four minutes later, at 7:44, into eclipse in Jupiter’s shadow. It will remain invisible for almost three hours, finally reappearing out of nowhere, well off Jupiter’s limb, at around 10:24 pm.

If you’re reading this article early, in February, there’s a slightly longer Europa Whac-a-moon event on Wednesday, Feb 20. Europa appears from behind Jupiter’s limb at 11pm, then enters the eclipse shadow at 11:08. If you’re staying up late, Europa will exit the eclipse at 1:46am.

As for Ganymede, this month’s best Whac-a-mede happens on March 26, where the moon appears from behind the limb at 7:12 – which unfortunately is about 15 minutes before our local sunset – then disappears into the eclipse shadow at 9:36, not to emerge again until twelve minutes past midnight. The Ganymede whacking interval will gradually get shorter over the next few months, so I’ll keep you posted on any likely dates in future columns..

Meanwhile, go out and whack some moons!