Transits, occultations, eclipses and shadow transits of the Jovian moons are perfect events for telescopic viewing.

Ganymede’s shadow transit on April 15, 2020 is captured here through an 11-inch Schmidt-Cassegrain. The moon’s shadow is directly above the Great Red Spot, while the moon itself is to the left. Credit: Molly Wakeling

It may be cold this month, but now is a great time to get out and explore the beauty and power of Jupiter. Not only will the fifth planet reach opposition on December 7th, it will also be at perigee (closest to Earth) on December 6th, putting on the biggest and brightest show of the year.

Known for its Great Red Spot, Jupiter also has four bright, fast-moving moons: giant Ganymede, cratered Callisto, volcanic Io and icy Europa. And there’s no better time to brave the cold and explore some of the Galilean moons’ most exciting events: transits, occultations, eclipses and shadow passages!

Transits occur when one of the moons moves across Jupiter’s surface. Because the moons have orbital periods of only a few days, transits occur hundreds of times per year. At opposition on Dec. 7, you can catch Io beginning transit at 4:30 a.m. EST. Or, if you’re not an early bird, you can see Europe transiting at 10:18 PM EST on December 5th. It can be difficult to make out the Moon once it passes in front of Jupiter, but you can see the beginning and end of its transit, as it moves into the disc (entry) and slips off it (exit). The transits generally last about two to three hours. I like to use the SkySafari virtual planetarium app to visualize when and where these events will occur.

Occultations occur when one of the moons goes behind the planet and disappears from view. Around opposition, you can see the white dot on Io disappear from view on December 8 at 01:49 EST and reappear at 4:01 AM EST. The Great Red Spot is not visible during the occultation, but you can explore Jupiter’s multicolored cloud bands while you wait.

Eclipses are similar to occultations, except that instead of just passing behind the planet, a moon can pass into or out of Jupiter’s shadow. At opposition, Jupiter’s shadow is directly behind it, but later in the year you may see one of the moons suddenly reappear some distance from the planet’s disk. On February 1 at 12:51 EST, for example, you can see Io emerging from Jupiter’s shadow nearly half a planet width away!

Shadow passages are my favorite to observe. These occur when one of the moons crosses Jupiter’s surface; a shadow transit refers specifically to the shadow cast by the Moon on the Jovian clouds. Near opposition, the shadow cast by the Moon will be close to, if not directly behind, the Moon itself. At other times of the year, a moon’s shadow may be visible on the disk long before or after the moon itself has passed, making both the moon and the shadow easily visible at the same time.

In the wee hours of December 30 in the US, both Io and Ganymede will have shadow transits at the same time, with Io’s shadow appearing at 4:42 am EST at about the same latitude as the Great Red Spot and Ganymede at 5:37 am EST towards the base of the planet. This is a good event for the western US and gets tougher further east. East Coast observers may see Jupiter set just before Ganymede’s shadow appears, or the planet become muddy as it nears the horizon. But West Coast observers should be able to see both shadows until Io’s shadow transit completes at 6:54 a.m. EST.

To discover these Jupiter phenomena, a telescope is your best option. While the moons are visible through 7×50 binoculars, if you want to capture these events in detail and enjoy the beauty of Jupiter’s colorful clouds, a telescope with at least a 6-inch aperture will provide more satisfying views.

If you are interested in recording your observing efforts, the Astronomical League offers a program called Galileo’s TOES, where you observe each of these four phenomena (transits, occultations, eclipses, and shadow transits) for each of the four Galilean moons and record entry and exit times. More about this program can be found at www.astroleague.org/galileos-toes. To see predictions for event times as well as animations of them, check out https://shallowsky.com/jupiter.

Happy observation, by Jove!