It may be cold this month, but now is a great time to get outside and explore Jupiter’s beauty and might. The fifth planet will not only reach opposition on Dec. 7, it will also be at perigee (closest to Earth) on Dec. 6, putting on its biggest and brightest show of the year.
Known for its Great Red Spot, Jupiter also boasts 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 transits!
Transits occur when one of the moons moves across the face of Jupiter. As the moons have orbital periods of just a few days, transits occur hundreds of times per year. At opposition Dec. 7, you can catch Io beginning to transit at 4:30 a.m. EST. Or, if you aren’t an early bird, you can catch Europa transiting at 10:18 p.m. EST Dec. 5. It can be hard to distinguish the moon once it passes in front of Jupiter, but you can watch the beginning and end of its transit, as it moves onto the disk (ingress) and slides off of it (egress). Transits generally last around two to three hours. I like to use the SkySafari virtual planetarium app to visualize when and where these events will occur.
Occultations happen when one of the moons goes behind the planet and disappears from view. Around opposition, you can watch the white dot of Io vanish from sight Dec. 8 at 1:49 a.m. EST and reemerge at 4:01 a.m. EST. The Great Red Spot isn’t 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 can watch one of the moons suddenly reappear some distance away from the planet’s disk. On Feb. 1 at 12:51 a.m. EST, for example, you can witness Io emerging from Jupiter’s shadow almost a half-planet-width away!
Shadow transits are my favorite to observe. These occur when one of the moons crosses the face of Jupiter; a shadow transit refers specifically to the shadow that is cast by the moon onto the jovian clouds. Near opposition, the shadow the moon casts will be close to, if not directly behind, the moon itself. At other times of the year, a moon’s shadow can be visible on the disk well before or after the moon itself has transited, allowing both the moon and the shadow to be easily viewed at the same time.
In the wee hours of Dec. 30 in the U.S., both Io and Ganymede will have shadow transits at the same time, with Io’s shadow appearing at 4:42 a.m. EST at about the same latitude as the Great Red Spot, and Ganymede’s at 5:37 a.m. EST toward the bottom of the planet. This is a good event for the western U.S. and gets tougher farther 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 spot these Jupiter phenomena, a telescope is your best option. While the moons are visible through 7×50 binoculars, if you want to catch these events in detail and enjoy the beauty of Jupiter’s colorful clouds, a telescope with at least 6 inches of aperture will deliver 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 ingress and egress times. More on 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 observing, by Jove!
