The transit of Ganymede’s shadow 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 outside and explore the beauty and power of Jupiter. The fifth planet will not only reach opposition on December 7, but will also be at perigee (the closest point to Earth) on December 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 most exciting events of the Galilean moons: transits, occultations, eclipses and shadow transits!
Transits occur when one of the moons moves in front of Jupiter’s face. Because the moons have orbital periods of just a few days, transits occur hundreds of times per year. At the opposition on December 7, you can catch Io starting to transit at 4:30 EST. Or, if you’re not an early riser, you can watch Europa transit 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 watch the start and end of its transit, as it moves across the disk (entrance) and slides off it (exit). Transits generally last 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 the opposition, you can watch Io’s white dot fade from view on December 8 at 1:49 a.m. EST and re-emerge at 4:01 a.m. EST. The Great Red Spot is not visible during occultation, but while you wait you can explore Jupiter’s multicolored cloud bands.
Eclipses are similar to occultations, except that instead of simply passing behind the planet, a moon can enter or exit Jupiter’s shadow. At opposition, Jupiter’s shadow is directly behind it, but later in the year you can observe one of the moons suddenly reappear some distance from the planet’s disk. On February 1 at 12:51 a.m. EST, for example, you can watch Io emerge from Jupiter’s shadow almost half a planet away!
Shadow transits are my favorite to observe. These occur when one of the moons crosses Jupiter’s face; a umbra transit specifically refers 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, the moon’s shadow may be visible on the disk well before or after the moon’s transit, allowing both the moon and the shadow to be easily seen at the same time.
In the early hours of December 30 in the United States, 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 am EST towards the bottom of the planet. This is a good event for the western US and gets more difficult further east. East Coast observers may see Jupiter set just before Ganymede’s shadow appears, or the planet becomes muddy as it approaches the horizon. But west coast observers should be able to see both shadows until the transit of Io’s shadow completes at 6:54 a.m. EST.
To spot these Jupiter phenomena, a telescope is your best bet. Although 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 offer more satisfying views.
If you’re interested in recording your observing efforts, the Astronomical League offers a program called Galileo’s TOES, in which you observe each of these four phenomena (transits, occultations, eclipses, and umbral transits) for each of the four Galilean moons and record the entry and exit times. More information about this program can be found at www.astroleague.org/galileos-toes. To see forecasts for event times and their animations, check https://shallowsky.com/jupiter.
Good observation, by Jupiter!
