SkyWatching: Shadow Play on Jupiter

Jupiters rapidly moving moons constantly surprise us with their dance around the giant planet. There will be two spectacular shadow plays this week.

Jupiters moons are very small, even in a large telescope, but their shadows are slightly larger, and can often be seen crossing Jupiters face with a good amateur telescope. Ive 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 Jupiters usual four moons are visible. Thats because two of the Moons, Io and Callisto, are in front of Jupiters disk, and are said to be in transit. You probably wont 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 Jupiters bright clouds. I usually see it as a tiny greyish spot. Look more closely, and youll see two small dark shadows on Jupiters face. One of these is Ios shadow, but the other is not the shadow of Callisto. Instead it is the shadow of Ganymede, off to Jupiters right. Thats because of the angle at which the sun is illuminating the tableau. 

On Wednesday night, May 20, the shadows of Jupiters moons Ganymede and Io will cross Jupiters face. This shows the shadows at 8:10 p.m. EDT, just after Ios shadow has started across, and just before Ganymedes shadow leaves.  Credit: Starry Night software.

Take another look later in the evening, around 9:55 p.m., and youll see that Ganymedes shadow has left the disk and that Ios shadow is about to be hidden behind Callisto. This will be the first time I have ever seen a moons shadow eclipsed by another moon. 

Nearly two hours later at 9:55 p.m., Ganymedes shadow has left, and Ios shadow is about to be eclipsed by the moon Callisto. The Great Red Spot is well placed close to Jupiter’s central meridian. Credit: Starry Night software.

Also keep a lookout for the Great Red Spot, though it is not nearly as great nor as red as it once was. Its more usually seen as a light notch in the North edge of Jupiters 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. Ganymedes shadow starts across Jupiters face at 8:58 p.m. EDT. Ios shadow follows at 10:01 p.m., and both shadows are present until Ios shadow leaves at 12:18 a.m., followed by Ganymedes at 12:34 a.m. 

Exactly a week later, on Wednesday, May 27 at 10:05 p.m., the pattern repeats, except that the shadows are closer together and Callisto is no longer in front of Jupiter. Credit: Starry Night software.

Notice that at around 11:48 p.m., Ios faster moving shadow actually passes Ganymedes, and the two shadows merge.

At 11:48 p.m., Ios faster moving shadow catches up with Ganymedes, and the two shadows merge. Again, the Great Red Spot is well placed. Credit: Starry Night software.

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.

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