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SYFY WIRE Bad Astronomy

Hubble Watches a Panoply of Moons Dance Around Jupiter

By Phil Plait

On Jan. 24, 2015, three of Jupiter’s largest moons crossed directly in front of Jupiter’s face at the same time. This is called a triple transit, and it’s a relatively rare event. It was observed from many locations, but arguably the Hubble Space Telescope had the best view of all:

Wow! You can see yellowish volcanic Io in the upper right, Europa on the lower left, and Callisto to its upper right. You can see two moon shadows, too: Europa’s to its upper right, and Callisto’s way over on the upper right on Jupiter.

It took me a second to wrap my head around the geometry: Why is Europa’s shadow close to the moon, but Callisto’s is so far off? That’s because Europa orbits Jupiter much closer in than Callisto, so the shadow it casts doesn’t fall as far away. Io casts a shadow too, but it’s off to the right, and misses Jupiter, so we don’t see it.

Jupiter itself is magnificent. The dark belts and light zones cross the planet, huge stripes of atmospheric gases circulating around the planet in opposite directions. And the amazing white and ruddy storms dotting it here and there add even more detail to the swooping and festooned clouds. It’s breathtaking.

Here’s an animation of the event that kinda blew my mind:

(Click that to see it full-res because wow.) You can see Jupiter rotating (it takes less than 10 hours to spin once, so things move pretty fast) as well as the moons’ and shadows’ motions. Watch it loop a few times and things get clearer; Europa’s rapid versus Callisto’s more languid orbital speeds, for example.

And it gets better. Jupiter has dozens of moons, most of them small. Two of the dinkier ones are lumpy rocks named Amalthea (about 250 kilometers along its long axis) and Thebe (100 or so km across). You can see both in the Hubble images! Here’s an animation of them:

How frakking awesome is that? Amalthea was discovered in 1892, the first moon of Jupiter to be discovered since Galileo spotted the big four. Thebe wasn’t seen until 1979, when Voyager 1 flew past the planet. And here we are, getting them for free as a sort-of value-added bonus on the other transits. It’s a pentuple transit! Incredible.

Right now, Jupiter is approaching opposition—when it’s opposite in the sky from the Sun. That means it’s as close to Earth as it gets for the year, so now’s the time to observe it! It rises at sunset, and is high in the east when it gets good and dark out. Even binoculars will show you the four big Galilean moons (after all, Galileo’s telescope wasn’t all that different in size to modern binocs) and a small telescope will start to reveal details on the planet.

Don’t have a ‘scope? Then seek out a local astronomy club. I just bet there will be plenty of star parties over the next few weeks with Jupiter stealing the show.

You really don’t want to miss it. Jupiter is an astonishing sight through a telescope, and there’s nothing like knowing that the photons hitting your eyes were reflecting off of alien worlds just 40 minutes previous.

I get chills thinking about it. Maybe we can’t touch these worlds directly, but the Universe has other ways of connecting us to them. Take advantage of it.

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