Starship Asterisk*

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Animation of Jupiter Triple Moon Shadow Transit, January 23, 2015

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A rare celestial event will take place on Friday night, January 23, 2015, when the shadows of three of Jupiter’s four largest moons will fall upon Jupiter at the same time. The shadows of these three moons – Io, Europa, and Callisto – will cross Jupiter’s disk throughout the evening. The moons will also cross in front of the planet. Galileo discovered these moons, and one more, Ganymede, in 1610. Ganymede will also be visible on January 23, but unlike the others it will not cast a shadow on Jupiter. Three moon shadows will not appear simultaneously on Jupiter again, from Los Angeles, until 2032.

Callisto’s shadow is the first to appear on Jupiter, at 7:11 p.m. PST. Io’s shadow joins it at 8:35 p.m., PST. The triple-shadow transit begins when Europa’s shadow joins the other two at 10:27 p.m., PST, and concludes when Io’s shadow moves past Jupiter at 10:52 p.m., PST. Callisto’s and Europa’s shadows move off of Jupiter’s disk at 12:00 midnight PST, and 1:22 a.m. PST (January 24), respectively.

As if the triple-shadow transit weren’t enough, another rare Jupiter shadow event precedes it. Io will begin to cross in front of Jupiter’s disk at 8:54 p.m., PST, 19 minutes after its shadow. At 9:40 p.m., PST, swiftly-moving Io will be eclipsed as it runs into the shadow of slower-moving Callisto. This will darken Io, and make it suddenly conspicuous as a black dot against the bright clouds of Jupiter. Io’s shadow on the planet will merge with Callisto’s shadow for 19 minutes. Then, Io will again become visible at 10:12 p.m., PST, when it appears as a bright speck in front of Callisto’s giant shadow.

Griffith Observatory will provide a live video stream of the Jupiter triple-shadow transit. The webcast will begin at 8:30 p.m., and conclude at 11:00 p.m. ...

[c][imghover=http://www.spacetelescope.org/static/ar ... c1504b.jpg]http://www.spacetelescope.org/static/ar ... c1504c.jpg[/imghover]Three moons and their shadows parade across Jupiter
Comparison of beginning and end of sequence
Credit: NASA, ESA, Hubble Heritage (STScI/AURA)[/c]

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These new NASA/ESA Hubble Space Telescope images capture a rare occurrence as three of Jupiter’s largest moons parade across the giant gas planet’s banded face. Hubble took a string of images of the event which show the three satellites — Europa, Callisto and Io — in action.

There are four Galilean satellites — named after the 17th century scientist Galileo Galilei who discovered them. They complete orbits around Jupiter ranging from two to seventeen days in duration. The moons can commonly be seen transiting the face of Jupiter and casting shadows onto its layers of cloud. However, seeing three of them transiting the face of Jupiter at the same time is rare, occurring only once or twice a decade.

The image on the left shows the Hubble observation at the beginning of the event. On the left is the moon Callisto and on the right, Io. The shadows from Callisto, Io and Europa are strung out from left to right. Europa itself cannot be seen in the image.

The image on the right shows the end of the event, just over 40 minutes later. Europa has entered the frame at lower left with slower-moving Callisto above and to the right of it. Meanwhile Io — which orbits significantly closer to Jupiter and so moves much more quickly — is approaching the eastern limb of the planet. Whilst Callisto’s shadow seems hardly to have moved, Io’s has set over the planet’s eastern edge and Europa’s has risen further in the west. The event is also shown from start to finish in a video.

Missing from this sequence is the Galilean moon Ganymede which was outside Hubble’s field of view.

The moons of Jupiter have very distinctive colours. The smooth icy surface of Europa is yellow-white, the volcanic sulphur surface of Io is orange and the surface of Callisto, which is one of the oldest and most cratered surfaces known in the Solar System, is a brownish colour.

The images were taken with Hubble’s Wide Field Camera 3 in visible light on 23 January 2015. Whilst Hubble captures these moons in great clarity they can also be seen with a small telescope or even a decent pair of binoculars. Why not try it at home?

Click to view full size image

bystander wrote:I wonder if there are Hubble images of Callisto's eclipse of Io. It would be before the opening frame above. I think that would make an interesting video.