| The Southern Equatorial Belt "went missing" during the very close opposition of 2010, giving me an excellent opportunity to capture shadow transits. The animation on the left shows the shadow of Io, the nearest Galilean moon to Jupiter, following the Great Red Spot (GRS) across the face of Jupiter - the GRS is particularly well seen in the last frame. It can be seen that the shadow is moving faster than the GRS - it clearly catches it up. It's very obvious if you click on the animation, to speed it up by a factor of 2. Although Jupiter rotates in only a little under 10hrs, Io orbits so quickly (1day 181/2hrs) that, when seen from the Earth, its motion is twice that of an object on the planet. This speed difference is well seen in the composite image below, showing each frame of the animation - the overall time from first to last is about 25mins. |
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| And just in case you thought I'm making this up, here's a wider angle shot showing that there really were only three Galileans visible that night. Callisto, Europa and Ganymede are lined up to the left of Jupiter, but that was it! | |
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| The next animation shows not only the shadow of Europa, the second-nearest Galilean to Jupiter, (to the far right of the planet's disc) but also the moon itself (just to the right of Jupiter). It is clear that the moon and the dark spot move at the same speed, showing that the spot is indeed a shadow and not a feature on the planet's surface. The fact that Europa precedes its shadow tells us that the image was taken somewhat after opposition. By this time (about 3wks afterwards) the Earth had overtaken Jupiter on their anti-clockwise orbits [when viewed from a northern-hemisphere perspective] and so, as seen from the Earth, the Sun was shining on Jupiter slightly "from the right". The individual frames are again shown below - the overall time from first to last is about 15mins in this case. |  |
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 | And just to complete the set, here we have the shadow of Galilean number three, Ganymede, standing out boldly from the upper margin of the South Temperate Belt: Ganymede is the biggest moon of Jupiter (bigger than Mercury!) and so casts a large shadow. The moon itself is visible to the right of Jupiter, and Io can also be seen, to top right. I did not attempt an animation of Ganymede because, as it is further away from Jupiter than both Io and Europa, it moves quite slowly, taking about 3hrs to cross the planet's disc. Its greater orbital distance is also shown by the fact that the moon is further away from its shadow than is Europa. |
| I said "complete the set" just now, but of course we are lacking a transit of Callisto. However, as mentioned on the main Jupiter page, Callisto usually misses the disc of Jupiter completely - it can only perform a transit when the orbital plane of the Galileans is close to being aligned with that of the Earth. This happens at two places on Jupiter's orbit of 11.86 years i.e. once every 5.93 years as seen from the Sun, but to easily observe the phenomenon from the Earth it is best if Jupiter is also at opposition, which happens every 13 months. The two occasions coincide once every 5 or 6 years, and the alignment is often good enough (given that the disc of Jupiter is very much larger than that of Callisto) that transits will be possible at both these intervals. The last oppositions for which this was true happened in July 2020 & August 2021 - the next will be in January 2026 & February 2027. | |
| Having captured both Europa and Ganymede together with their shadow I thought I would do the same for Io. I consulted my prediction program for the next time this would happen and set up the telescope well in advance. I captured a set of images but Io did not cross the edge of Jupiter's disc when I thought it should. Perplexed, I looked at the program again only to find I had forgotten to add on the hour for BST!! Ah well, even Homer nods sometimes. I thus brought the telescope back inside, intending to try again later, but unfortunately everything misted up - the night was very cold. I heated the mirror-end of the telescope to dry it off but by the time the optics had stabilised after this thermal assault I could only grab a few usable shots - fortunately, enough to enable me to get a decent image after stacking and adjusting. This is shown at the right. Io has just appeared, with its shadow close to the edge of Jupiter's disc. Note that the distance between the moon and its shadow is quite small, reflecting the fact that Io is the closest Galilean to Jupiter. |  |
 | I thought I might as well process the earlier images though, and thus came up with another animation of Io on Jupiter's disc, shown here. Its movement is much less than in the previous animation, as the total time span is only about 7mins, but this time the interval between each frame is the same (it was not in the previous case). It is obvious from the animation that Jupiter itself is rotating as well as the shadow moving - the shift in position of features in the northern and southern "caps" is particularly clear. As before, the individual frames from the animation are shown below. |
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In the same way that the shadow of a Galilean can fall on Jupiter, giving the equivalent of a solar eclipse, the shadow of Jupiter can fall on a Galilean - this is the equivalent of a lunar eclipse. The analogy is quite close because, seen from space, the shadow of the moon would be only a small dark spot on the Earth's surface whereas the shadow of the Earth would completely engulf the moon [assuming the eclipse was total, of course]. In the case of the Jovian system, the shadows of the Galileans produce distinct spots on Jupiter (as we have seen above) while the shadow of Jupiter completely covers the Galileans. When in eclipse they will, therefore, receive no illumination from the Sun and so will (visually, at least!) disappear as seen from Earth. Eventually, their orbital motion will take them out of the shadow, making them visible again.
The image below shows us a case in point. Instead of the usual four Galileans, we have only two - Callisto and Ganymede. Io and Europa could be non-visible for one of three reasons - they could be in front of Jupiter, behind Jupiter or in eclipse - but I knew they were actually both in eclipse. More interestingly from a photographic point of view, I also knew they would emerge from Jupiter's shadow within a few minutes of each other. Click on the image to see what happened.
Abracadabra! Before your very eyes, Europa emerges into the light closely followed by Io. Note that they do not appear from behind the edge of Jupiter's disc, as they would if they were simply hidden behind it, but materialise some distance away - this proves they were actually in eclipse. Note also that the re-appearance is gradual, not sudden - this is because the moons are quite large objects and so take time to fully clear the shadow. The point of emergence is not the same for both as they orbit different distances from Jupiter and so encounter different parts of the shadow.
While it is clear that the eclipsed moons must move in order to leave the shadow, it is not easy to see how much each has actually moved. The animation below shows this more clearly - click or tap on it to see the movement between images taken about 25mins apart.
A further click/tap will show the actual distance each of the four moons has moved between the second and third frames. I have coloured Io green, Europa red, Ganymede blue and Callisto purple. It is clear that Io has moved the most, followed by Europa and then Callisto, as would be expected from their relative distances from Jupiter. Ganymede is the odd one out as it seems to have hardly moved at all. This is because when the images were taken it was on the part of its orbit where it was mainly going away from us and so it did not move much laterally - a small shift is visible in the three-frame animation. The other three moons were all mainly travelling across the line of sight and so we clearly see their motion.
