Jupiter’s rapidly moving moons constantly surprise us with their dance around the giant planet. There will be two spectacular shadow plays this week.
Jupiter’s moons are very small, even in a large telescope, but their shadows are slightly larger, and can often be seen crossing Jupiter’s face with a good amateur telescope. I’ve seen the shadows with telescopes as small as 90mm aperture, but a telescope with 6-inch or larger aperture will show them much more clearly. Steady atmospheric seeing is also essential.
If you live on the eastern seaboard, look for Jupiter just after sunset on Wednesday, May 20 around 8:10 p.m. EDT. The first thing you will notice is that only two of Jupiter’s usual four moons are visible. That’s because two of the Moons, Io and Callisto, are in front of Jupiter’s disk, and are said to be “in transit.” You probably won’t see Io, because its color and brightness blend in so well with the cloud tops behind it. You may be able to see Callisto because its dark surface stands out against Jupiter’s bright clouds. I usually see it as a tiny greyish spot. Look more closely, and you’ll see two small dark shadows on Jupiter’s face. One of these is Io’s shadow, but the other is not the shadow of Callisto. Instead it is the shadow of Ganymede, off to Jupiter’s right. That’s because of the angle at which the sun is illuminating the tableau.
Take another look later in the evening, around 9:55 p.m., and you’ll see that Ganymede’s shadow has left the disk and that Io’s shadow is about to be hidden behind Callisto. This will be the first time I have ever seen a moon’s shadow eclipsed by another moon.
Also keep a lookout for the Great Red Spot, though it is not nearly as “great” nor as “red” as it once was. It’s more usually seen as a light notch in the North edge of Jupiter’s South Equatorial Belt. At moments of steady seeing, its salmon pink color may appear briefly.
A week later on May 27, the situation nearly repeats itself, but is about two hours later, making it more easily seen across the whole of North America. Ganymede’s shadow starts across Jupiter’s face at 8:58 p.m. EDT. Io’s shadow follows at 10:01 p.m., and both shadows are present until Io’s shadow leaves at 12:18 a.m., followed by Ganymede’s at 12:34 a.m.
Notice that at around 11:48 p.m., Io’s faster moving shadow actually passes Ganymede’s, and the two shadows merge.
Once again, the Great Red Spot should be in evidence.
If you live west of the Eastern time zone, be sure to subtract the appropriate corrections from the times given above: 1 hour for CDT, 2 hours for MDT, and 3 hours for PDT.
If you'd like to follow along with NASA's New Horizons Mission to Pluto and the Kuiper Belt, please download our FREE Pluto Safari app. It is available for iOS and Android mobile devices. Simulate the July 14, 2015 flyby of Pluto, get regular mission news updates, and learn the history of Pluto.
Simulation Curriculum is the leader in space science curriculum solutions and the makers of Starry Night, SkySafari and Pluto Safari. Follow the mission to Pluto with us on Twitter @SkySafariAstro, Facebook and Instagram.