Two Shadows and the Red Spot Cross Jupiter

From time to time, observers with good quality telescopes can watch the small, round, black shadows of the Galilean moons traverse Jupiter’s disk. On Monday morning, December 23, sky-watchers located in the Americas can watch two of the shadows and the Great Red Spot crossing the southern hemisphere of Jupiter together for about an hour. At 2:48 a.m. Eastern Time (or 07:48 GMT), the red spot and the small shadow of Io will join the much larger shadow of Ganymede, which began its own crossing of the planet’s south polar zone 70 minutes earlier. Ganymede’s shadow will leave Jupiter at 3:48 a.m. EST (or 08:48 GMT), leaving Io’s shadow and the spot to continue on alone until 4:58 a.m. EST. Watch for Io itself to move off of Jupiter’s disk by 4:35 a.m. EST.