The Night Sky on December 20: Io and Its Shadow Dance Across Jupiter
Prepare to be captivated! This Saturday, December 20, marks a spectacular celestial event as Io, one of Jupiter's Galilean moons, and its shadow transit the magnificent gas giant. But here's where it gets intriguing—this phenomenon is not just a simple sighting; it’s a chance to observe intricate details of both Io and Jupiter that are usually obscured from view.
This event, taking place overnight into December 21, will be visible throughout the United States. As Jupiter approaches opposition, it shines brightly at an impressive magnitude of –2.6 in the constellation Gemini, displaying a disk size of 46 arcseconds. This clarity allows stargazers to see the planet's cloud bands and even the famous Great Red Spot with ease.
The transit begins at 10:35 p.m. EST when Io's shadow first appears on the turbulent atmosphere of Jupiter. Approximately half an hour later, Io itself will start to cross in front of the planet. The entire spectacle comes to a close in the early hours of December 21, with Io's shadow leaving Jupiter’s western edge around 11:50 p.m. CST, closely followed by Io completing its own transit about half an hour later.
If you're looking for something to enjoy in the night sky this week, don’t miss our comprehensive "Sky This Week" column, which outlines more exciting astronomical events (https://www.astronomy.com/observing/the-sky-this-week-from-december-19-to-26-2025/).
On December 19, observers can also journey to the Triangulum Galaxy (https://www.astronomy.com/observing/the-sky-today-friday-december-19-2025/), but the highlight remains Io’s crossing.
As Jupiter rises around 6:30 p.m. local time in Gemini, it becomes the brightest object within the Twins. It should be relatively straightforward to locate Jupiter through your telescope, where you’ll see its 46-inch-wide disk revealing numerous fascinating features, including shifting light and dark cloud bands. The Great Red Spot is expected to rotate into visibility just before midnight EST, adding to the allure of the viewing experience.
Earlier in the evening, all four Galilean moons—Io, Europa, Ganymede, and Callisto—will be positioned east of Jupiter. From closest to farthest, they are arranged as follows: Io, Europa, Ganymede, and Callisto. As time progresses, Io will move closer to the planet, and moments before reaching Jupiter's eastern limb, its shadow will cast down onto the cloud tops at 10:35 p.m. EST, initiating its transit approximately 30 minutes later.
The captivating interplay of Io and its shadow will continue until just before midnight CST (which is early December 21 for Eastern Time observers) when Io's shadow departs off the western limb of Jupiter around 11:50 p.m. CST. By now, viewers should also be able to spot the Great Red Spot on the planet's surface.
Roughly 30 minutes after the shadow exits, Io will conclude its transit around 12:20 a.m. CST, marking the end of this extraordinary celestial display.
For reference, here are some relevant times for December 20:
- Sunrise: 7:18 A.M.
- Sunset: 4:37 P.M.
- Moonrise: 8:10 A.M.
- Moonset: 4:57 P.M.
- Moon Phase: Waxing Crescent (1%)
*All times for sunrise, sunset, moonrise, and moonset are presented in local time for a location at 40° N, 90° W. Furthermore, the Moon's illumination percentage is noted at 10 P.M. local time from the same coordinates.
