If you’re a Jupiter watcher, the night of Wednesday 31 January is full of fireworks for you. Three of Jupiter’s Galilean moons are involved in occultations, transits and shadow transits this evening, all visible to observers in the northern hemisphere before Jupiter sets in the west.
As twilight fades around 17:15, Jupiter is high in the sky almost due south. You can’t miss it. All four major Galilean moons are visible in binoculars or small telescopes; in order from the east, you will see Callisto, Europa, then Jupiter itself, with Io and Ganymede on the opposite, western side.
The show begins at 17:26, as darkness approaches. Jupiter’s innermost satellite Io will disappear behind, or will be “occulted” by the western limb of the planet. The disappearance will take a few seconds, as the satellite slowly slips behind the planet and can be seen in binoculars or small telescopes. In all these images, north is up and east is to the left, as seen in large binoculars.
You have time now to eat dinner, before, at 19:02, the satellite Europa starts to move in front of the planet, “transiting” across the disc and moving from east to west. At the beginning of the transit, it may still be possible to see Europa for a few minutes before it is lost in the glare from Jupiter’s surface. This leaves Jupiter with only two of its major satellites, Ganymede and Callisto, visible.
Then, about two hours later at 20:57, Io will emerge from the shadow of Jupiter, on the opposite side of the planet from where it started. This reappearance takes several seconds as Jupiter’s shadow uncovered Io’s surface and you should see the satellite growing brighter as this happens.
Shortly afterwards at 21:24, Europa will emerge from its transit, now on the western side of the planet, while 18 minutes later, at 21:42, its shadow will start to cross Jupiter’s disc, from east to west. Whereas satellite transits are difficult to see due to the lack of contrast between the satellite and the planet, the inky-black shadow of Europa on Jupiter is very distinct and can be well seen in telescopes of 100 mm aperture or more.
As you watch these events, you will have noticed Ganymede edging ever closer to the planet’s western limb until at 22:37 it finally disappears behind Jupiter, concluding a busy night of satellite observing. Jupiter sets in the west around midnight local time. Look out also for the Great Red Spot as it crosses Jupiter’s disc during the evening.
Do please tell us if you were able to watch these phenomena and good luck!