Watch Jupiter’s Polka-dot Parade Tonight

Jupiter's moon Io casts a black shadow on the planet's fast-moving clouds in this picture taken by the Hubble Space Telescope. The moon's diameter is 2,274 miles or about 100 miles larger than our own moon. Credit: NASA/ESA

With Jupiter so bright and close this fall, my hope is that everyone with a telescope will drop by the planet regularly. Over the months, we’ve looked in on Jove’s clouds belts and the nightly dances of its four brightest moons Io, Europa, Ganymede and Callisto. Today I’d like to introduce you to another facet of Jovian observing – shadow transits and eclipses. These happenings, very striking to the eye, are often the highlight of a night spent with the planet. They demonstrate in dramatic fashion how Jupiter is truly a solar system in miniature.

A shadow transit is the transit or passage of one of the moon’s shadow across Jupiter’s bright clouds. Though a small telescope, the shadow looks like a perfectly circular inky black spot crossing the planet’s face in about 2 – 2 1/2 hours time. Because Jupiter’s innermost bright moon Io orbits quickest, its shadow takes about 25 minutes less time to cross than Europa’s, the next moon out.

Ganymede's shadow covers a part of Jupiter's south polar region in a photo taken on October 3. The Great Red Spot is visible in the equatorial belt below the shadow. Ganymede itself is seen off to the right. Credit: Damian Peach

The pale moons themselves also transit the planet at or around the time as their shadows, but they’re much harder to see against the cloud tops. And since the diameter of each moon is different, the size of the shadow each casts is different, too. Here are the moons’ diameters from smallest to biggest:

* Europa – 1,939 miles (3122 km)
* Io – 2,274 miles (3660 km)
* Callisto – 2,995 miles (4821 km)
* Ganymede – 3,279 miles (5262 km)

Just by looking at the numbers, you might guess Ganymede’s shadow is considerably larger than Europa’s, and you’d be right. It’s is a prominent mole compared to Europa’s “pinprick” appearance and far and away the easiest to see in almost any telescope. You can see it yourself as soon as tonight, when Ganymede’s shadow transits the planet between 7:50 and 9:45 p.m. Central time.  More transit times through the end of the month are described in the table at the end of this blog.

When you gaze at a moon’s shadow, consider that whatever is under it sees an eclipse of the sun. Since Jupiter is currently 5 times farther from the sun than Earth, our star is only a fifth the size we see it from here. Given the moons’ diameters and distances from the planet, they easily block the sun completely from the perspective of someone in an imaginary hot air balloon watching an eclipse from Jupiter’s blustery cloud tops.

This is how several upcoming shadow transits and one eclipse reappearance will look through the telescope. Ganymede's shadow lies just south of the Great Red Spot tonight. The south direction is up and east to the right. At the end of an eclipse, a moon departs the shadow and returns to view. Created with Chris Marriott's SkyMap

Because the Earth and Jupiter are currently oriented such that Jupiter’s equator and the orbits of its four big moons are not exactly edge-on to the Sun and Earth, each moon and its shadow takes a slightly different path across the planet. The shadows of the innermost moons Io and Europa touch the broad South Equatorial Belt while Ganymede’s crosses the south polar region. What’s really fun is seeing a moon shining brightly in space next to Jupiter at the same time its ‘shadow dot’ moves across the clouds. In your mind’s eye, you can picture the sunlight shining off to one side casting the shadow. Captivating!

When Earth and Jupiter are aligned again in 2015, the shadows will move across the planet’s equatorial region. Not only that, the alignment will be so precise, the moons will eclipse each other!

Eclipses of the moons by Jupiter are also a blast to watch. Just like Earth, Jupiter casts a shadow behind it into outer space. Before an eclipse, a moon is covered (occulted) by the planet’s limb before moving into the shadow behind the planet. As you might suspect, you can’t see it when it’s in eclipse, but it’s fascinating to watch reappear after eclipse.

Because the moons are disks and not star-like points, they don’t appear instantaneously but take a few minutes to gradually return to their original brightness as they exit the shadow. Indeed, as a moon emerges from eclipse, it seems to materialize out of empty space! If you plan to watch one, be sure to start a few minutes before the times shown below. That way you’ll better appreciate the dramatic effect of a reappearance.

The table lists dates and times (Central Standard) for shadow transits and eclipses easily visible during evening hours across the mainland U.S., Canada and South America through month’s end. For Eastern time, add an hour, Mountain time subtract an hour and for Pacific, subtract two hours. I don’t mean to leave out the rest of the world, so for observers everywhere, click HERE for a complete listing of moon events, including occultations, through March 2012. I hope you’ll have clear skies to see at least one of these beauties.

Ganymede shadow transits
* Nov. 14 (tonight) from 7:50 – 9:45 p.m. CST
* Nov. 21-22 from 11:53 p.m. – 1:47 a.m.

Io and Europa shadow transits
* Nov. 18 Europa from 7:32 – 9:59 p.m.
* Nov. 19 Io from 9:34 – 11:44 p.m.
* Nov. 25-26 Europa from 10:08 p.m. – 12:34 a.m.
* Nov. 26-27 Io from 11:30 p.m. – 1:39 a.m.
* Nov. 28 Io from 5:58 – 8:08 p.m.

Reappearance from eclipse
* Nov. 20 Io at 9:02 p.m.
* Nov. 27 Europa at 6:56 p.m. and Io at 10:57 p.m.

8 Responses

  1. BobN

    Off topic, but did you see the photos of the Soyuz launch today? I cannot believe they lifted off in a heavy snowstorm – hope no cold O-rings were involved. But to take a chance like that for such an important mission – if it had failed, the ISS would have had to be abandoned for who knows how long. That’s the Russians though, a stiff shot of vodka, and hit the launch button!

  2. I took a walk before moonrise and enjoyed Jupiter’s brilliance high overhead. Now a French-vanilla three-quarter moon is rising over the lake, its shimmering causeway stretching to the east shore. Lovely night. By the way, your mention of hot air ballooning reminded me of a description I heard of that sport on a TV documentary. An aficionado told us that “to understand what it’s like to go hot air ballooning, stand in a cold shower wearing a wool sweater, roll up hundred dollar bills, and shove them down the drain as fast as you can.” Stargazing is cheaper.

  3. RC

    Bob,
    Jupiter is so much bigger than it’s moons (compared to earth and our moon). Do Jupiter’s moons cast a shadow on the planet every time they revolve around it? Or is it like our moon, where there there are not eclipses every month?

    1. astrobob

      Hi RC,
      It depends on your perspective. From Earth, Io, Europa and Ganymede do, but not always Callisto. It orbits farther out, and unless Jupiter, Earth and the sun are nearly edge-on to one another, its shadow misses the planet despite its very slight orbital inclination of 0.28 degrees. If Earth were always aligned with Jupiter’s equator (as we are every 6 years) all four moons would always cast a shadow on the planet. And you’re right, since Jupiter’s bigger, it really helps. If not for its size, you can easily see how the shadow of Ganymede would have missed the planet this evening from our perspective.

  4. David

    Hey Bob! HAVE YOU SEEN THAT VALCANO ERUPTING IN AFRICA!? That is a thing of beauty! It been going for 9 days strong and its amazing!

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