Rare Treat: Catch Callisto’s Shadow On Jupiter’s Cloud Tops Tonight

Callisto’s shadow will follow along Jupiter’s south-south temperate belt from 9:09 p.m. to 12:54 a.m. tonight Feb. 22-23, 2014. A smaller telescope magnifying around 60x or higher will show it. Credit: Meridian software

If you have a 4-inch or larger telescope, tonight you’ll have the opportunity to see a rare shadow transit of the Jupiter’s bright moon Callisto. All four of Jupiter’s easily visible Galilean satellites, so-called because they were first seen by Galileo in the early 1600s, routinely pass in front of the planet and cast shadows on Jupiter’s cloud tops.

Europa is the smallest of the four with a diameter of 3,122 miles and casts the smallest shadow. Io’s shadow is larger and easier to spot, while Ganymede and Callisto – the largest of the quartet – cast the biggest shadows.

Jupiter’s four bright moons and their orbits. Io orbits closest. Each casts a shadow into space, which, when intercepted by Jupiter, appears as a black dot on the planet’s clouds. Credit: Ethan Siegel

Because Io and Europa orbit closest to Jupiter they make more frequent transits across the planet. Ganymede, being further from Jupiter, lines up less often.

Currently we see about five shadow castings per month for Ganymede with only about two for more distant Callisto. When you consider that some Callisto transits occur during daylight hours when Jupiter is unobservable, you can see how infrequent they truly are.

The last time Callisto “dotted” Jupiter was Feb. 6 between 3 a.m. and sunrise. Given the early hour, I suspect very few skywatchers across the Americas witnessed that event. Tonight’s transit occurs during convenient viewing hours starting at 9:09 p.m. CST (7:09 p.m. Pacific, 10:09 p.m. Eastern) and wrapping up about 3 1/2 hours later at 12:54 a.m. CST Feb. 23.

Shadow transits, particularly of Callisto, are leisurely affairs. That’s because it’s the most distant of the four moons and orbits slowest. Io, the closest, puts on a show lasting a little more than half as long or about 2 hours 10 minutes.

Callisto’s next shadow transit occurs on March 12 between 4:11-8:03 p.m. CST. With sunset at around 7 p.m on that date, the transit ends only an hour after sunset in the central U.S. After that we’ll have to wait until April for the next one.

I hope you’re skies are clear tonight so you can check it out.

8 Responses

  1. Tim Childers

    Thanks for posting that there would be a transit tonight. Got a good view of Jupiter and the 4 moons with my 4 inch Newtonian and a 4mm eyepiece. Even though I could make out two dark bands on Jupiter I could not see the shadow. Normally I cannot even see the second band, just the equatorial band so I thought it was very clear tonight from the Baltimore suburbs. Saw three larger moons on the east side and one on the western side. I checked a couple of times between 9:30 Eastern Time and 11 PM but still not enough magnification to make out the small shadow. Hope I see on the web some images tomorrow of tonight’s transit.

      1. Tim Childers

        I think I saw it a few times when the image sharpened up briefly but it is at the limit of my little scope and eyepiece. 😉

        1. astrobob

          Hi again Tim,
          I just came in from a blustery cold night. Luckily the seeing improved and the shadow was very obvious when the air steadied.

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