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.
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.
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.
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!