Don’t Miss Tonight’s Rare Triple Jupiter Moon Transit

All eyes will be on Jupiter tonight during the triple shadow transit. The map shows the sky facing east from the Chicago, Ill. region around midnight. Created with Stellarium

Jupiter and his four bright moons are one of the first things a beginning amateur astronomer looks at through a telescope. Watching the moons make new and surprising arrangements as they circle about the planet night after night makes for never ending observing enjoyment.

Jupiter’s moon Europa (at left) casts an inky dot on Jupiter on Sept. 24, 2013. Credit: John Chumack

Sometimes Jupiter’s shadow eclipses a moon, other times a moon passes in front of the planet and casts an inky black shadow on its cloud tops. Moons also orbit in front of Jupiter, though these events are more difficult to see unless you happen to catch the satellite right at the planet’s edge.

Tonight three of those moons – Io, Europa and Callisto – simultaneously cast their shadows on Jupiter’s cloud tops for just over an hour between 11:32 a.m. and 12:37 a.m. CDT (4:32-5:37 UT). This rare event last happened on March 28, 2004. They’re called shadow transits and single ones are fairly common. You can find online tables listing transits and satellite eclipses or use a free program like Meridian that gives you a little picture along with time information.

The March 28, 2004 triple transit. Shadows from left: Ganymede, Io and Callisto. You can also see the disks of Io (white dot) and Ganymede (blue dot) in this photo taken in infrared light by the Hubble Space Telescope. Credit: NASA/ESA

Seeing two shadows cross the planet at once is very uncommon, but three’s as rare as a blue rose. When averaged out, they happen only once or twice a decade. That’s why you should go out tonight with your telescope for a look.

If bad weather intervenes, the next transits won’t happen until June 3, 2014 (not visible in the Americas) and Jan. 24, 2015. After those we cool our heels until October 2025.

Here’s the breakdown. The triple transit will be visible across the eastern half of the U.S., Europe and western Africa. East Coasters will have the best view in the U.S. with Jupiter some 20-25 degrees high in the northeastern sky around 1 a.m. local time. Things get more challenging in the Midwest where Jupiter climbs to only 5-10 degrees. By the time the planet rises in the mountain states, only Io and Europa’s shadows remain. If you live in the Pacific Time Zone and farther west, you’ll have to wait until 2015.

Jupiter polka-dotted with shadows cast by its moons Io, Europa and Callisto around midnight CDT Oct. 12. Watch for the Great Red Spot to come into view during the transit. Created with Claude Duplessis’ Meridian software

Let’s look at how each piece of the event will play out. Times below are CDT:

* Callisto’s shadow enters the disk – 10:12 p.m. Oct. 11
* Europa – 10:24 p.m.
* Io – 11:32 a.m.
** TRIPLE TRANSIT from 11:32 – 12:37 a.m.
* Callisto departs – 12:37 a.m.
* Europa departs – 1:01 a.m.
* Io departs – 1:44 a.m.

European amateurs have the best view with Jupiter high in the southern sky before dawn. In the eastern U.S., Jupiter’s up some 20-25 degrees in the eastern sky around mid-transit time. That should be high enough to escape the worst of the low-lying atmospheric turbulence that tends to blur planet images when using higher magnifications. The Midwest will see a Jupiter only 5-10 degrees high. Let’s hope the air is calm and clear!

6 Responses

  1. Troy

    I’m hoping Damien Peach gets a good shot of it. I don’t think it is worth it for me to pack up my scope on the off chance I’ll be able to see it at a measly 5 degree altitude. Bummer.

    1. astrobob

      Too bad about its elevation. Were it clear tonight I’d still give it a try. Weather here in Duluth is poor.

  2. Troy

    P.S. You didn’t mention that a quadruple shadow transit is impossible so this is as good as it gets. I was able to catch the Callisto transit (also pretty rare itself) about 17 days ago. At least I got that.

    1. astrobob

      Hi Troy,
      Yes, you’re right. Quadruples are impossible because of the 4:2:1 resonance of Io-Europa-Ganymede paired with Callisto’s period. I left it out but did include it in a more extensive article I did earlier this week for Universe Today.

  3. Edward M. Boll

    Singles are the most common, then doubles and rarest of all triples. This is the analogy that I thought of. Single births are common among people, with rarely twins and very rarely triplets. I did meet a set of triplets once that lived in our area but I have never seen quadruplets to my knowledge.

    1. G W

      Not a particularly good analogy. Quad moon shadows are not possible. Quad children are. I personally know some quints!

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