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December 19: Visit Triangulum
Jupiter’s innermost Galilean moon, Io, crosses the gas giant along with its shadow this evening in an event visible across the U.S.
Jupiter rises around 6:30 P.M. local time, climbing above the eastern horizon in the constellation Gemini. The giant planet is approaching opposition, now shining at magnitude –2.6. It’s the brightest light in the Twins. It should be easy to center the planet in your telescope, where its 46”-wide disk will show off details such as alternating light and dark cloud bands. The Great Red Spot will make an appearance late in the evening as well, rotating into view a few hours before midnight EST.
Early in the evening, all four Galilean moons sit east of the planet. In order from closest to farthest, they are: Io, Europa, Ganymede, and Callisto. Io approaches the planet over the next few hours; shortly before it reaches the eastern limb, its shadow appears on the cloud tops at 10:35 p.m. EST. Io begins its transit 30 minutes later.
The two are visible together in front of Jupiter’s disk until shortly before midnight CST (now early on the 21st for those on Eastern time), when Io’s shadow slips off the western limb around 11:50 P.M. CST. The Great Red Spot should now be visible on the disk as well.
Io again follows suit about 30 minutes later, ending its transit around 12:20 CST (now early on the 21st in CST as well).
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%)
*Times for sunrise, sunset, moonrise, and moonset are given in local time from 40° N 90° W. The Moon’s illumination is given at 10 P.M. local time from the same location.
