Watch A Double Shadow Transit On Jupiter Tonight

Io and Europa cast their shadows on Jupiter in this photo made on March 22, 2016. You can see the two moons, too. Io is the orange one and Europa is just above it. Credit: Damian Peach

Jupiter is front and center at nightfall May evenings. The planet is the first “star” to show in the sky at dusk, when it appears in the southeastern sky a fist above fainter Spica in the constellation Virgo. Tonight (May 11), if you have a modest-sized telescope, say a 4 to 6 incher, you can see two of Jupiter’s moons cast their shadows on the planet at the same time.

This diagram shows Jupiter tonight at 9:04 p.m. CDT (10:04 p.m. EDT). For a brief few minutes, the shadows of both Io and Europa (black dots) will transit the planet. The red-dotted Io is the moon itself. Europa will be easy to see “off-planet” to the left. This view shows south up, as seen in a typical telescope. Created with Meridian

When a satellite casts a shadow on its mother planet, astronomers call it a shadow transit because the moon’s shadow transits or moves from one side of the disk to the other. Jupiter’s moons take time to orbit the planet, so transits usually last several hours. The shadows look like inky black dots or pinpricks against the brighter clouds that circulate about the planet. Ganymede, the largest of Jupiter’s four brightest moons, throws the biggest shadow. Europa, the smallest, casts the tiniest.

Single shadow transits are fairly common, but two happening simultaneously not so much. Three is rare. Io and Europa will cast their shadows on either side of Jupiter tonight for only a few minutes for skywatchers in the eastern half of the Americas and western Europe. For the Americas, this happens during evening twilight and early nightfall; for Europe in the early morning hours of May 12.

The sun will still be up and Jupiter too low for a good view from western North America.

This map shows the layout of the satellite around Jupiter this evening (May 11) through a small telescope. South is up. Created with Stellarium

Europa’s shadow makes its first appearance at 6:40 p.m. CDT at the eastern edge of the planet and departs the disk at 9:07 p.m. Io begins its trek at 9 p.m.  and exits Jupiter at 11:11 p.m. That means that the two black dots will be visible simultaneously for only about 7 minutes — from 9:00 to 9:07 p.m. !

Will it be clear? Will the seeing be good? While both shadows are visible at a magnification of around 70x, higher power will make them easier to see provided the air is steady. Use the highest magnification that provides the sharpest view.

While observers in the western states will miss the double event, you’ll at least get to see Io’s shadow soloing across the great planet. Below is a table summarizing the events.

Clear skies!

6:40 p.m. Shadow of Europa comes into view
9:00 p.m. Shadow of Io
9:07 p.m. Shadow of Europa exits
11:11 p.m. Shadow of  Io exits

4 Responses

    1. astrobob

      That’s too bad, David. I’m sorry you got hit with clouds. I was just able to get out in time from a presentation to catch both transits at the last possible moment. There will be more!

  1. Hi Bob, made a concerted effort last night for the double shadow transit, which started at around 1245pm ADT. Clouds moved in, of course, shortly after 1250pm. Had a very nice view of Jupiter and its Moons,and storm clouds, but no shadows could be seen. It was Io and Europa, so they are the smaller moons, so that might have made a difference.

    Thank you for your advice in how to tease them out, but I have to conclude that these shadow transits are difficult to see in the eyepiece.

    Did get a nice look at Comet Johnson though telescope, and did see the most shooting stars since last summer though. Was nice to have some calm weather as well.

    1. astrobob

      Hi David,
      Funny thing — I happened to be out looking through a friend’s 4-inch refractor. We were able to see Europa’s shadow at 9:30-10 p.m. CDT. At 12:45 ADT, only Europa’s shadow would have been visible. It is the tiniest. Was the atmospheric seeing poor? That would make Europa’s shadow very difficult to see. Don’t give up yet! Check out the next transit of Ganymede. I’m certain you’ll see it.

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