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January 12: Io’s post-opposition transit
Point your telescope at bright Jupiter in Gemini to catch a transit of the large moon Ganymede overnight tonight. You’ll want to be ready to go with your telescope trained on the gas giant around midnight in the Eastern time zone. At that time, Ganymede is approaching Jupiter’s southeastern limb, while Io is moving farther away from the planet on the eastern side. Europa lies far east of Jupiter, while Callisto is alone far to the planet’s west.
Ganymede reaches the limb and begins to transit at 12:34 A.M. EST (January 14th in EST only). Due to Ganymede’s larger orbit, its shadow takes longer than Io’s to appear, slipping onto the cloud tops at 12:58 A.M. EST.
Everyone has plenty of time to watch the event as the pair crosses Jupiter for nearly three hours. By 4 A.M. EST (now the 14th across the U.S.), Ganymede’s transit has just ended while its shadow remains visible on the cloud tops, still approaching the limb. The shadow transit finally ends around 4:20 A.M. EST.
Sunrise: 7:21 A.M.
Sunset: 4:57 P.M.
Moonrise: 3:10 A.M.
Moonset: 12:32 P.M.
Moon Phase: Waning crescent (19%)
*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.
