Sky This Month: December 2021

Jupiter starts December in the eastern part of Capricornus, about 16.5° east of Saturn. It shines at magnitude –2.3, but quickly dims 0.1 magnitude and then crosses into Aquarius Dec. 14. The waxing Moon stands below Jupiter on Dec. 8, while Jupiter stands 10° west of the Moon on Dec. 9.

Jupiter’s 37″-diameter disk looks great through a telescope — your best views will be as twilight descends and for the first hour or so after dark. By the end of December, Jupiter dips below 20° around 7 P.M. local time and sets by 9 P.M.

Check the configuration of its four Galilean moons — their relative positions change nightly. On Dec. 1, an occultation and eclipse reappearance occur close together. First, Callisto reappears from behind Jupiter at 9:13 P.M. EST, followed by Europa exiting Jupiter’s shadow at 11:15 P.M. EST.

Watch Io disappear behind Jupiter Dec. 4 at 9:35 P.M. EST. The following night, both Io and its shadow travel across the face of Jupiter shortly after darkness falls. Io’s transit ends at 9:14 P.M. EST, followed by its shadow 75 minutes later. Two days later, Ganymede, Jupiter’s largest moon, transits on Dec. 7 beginning at 9:03 P.M. EST. The event continues for more than 3 hours.

Neptune is a binocular object shining at magnitude 7.8 most of the month and located in Aquarius the Water-bearer. It stands high in the southern sky Dec. 1 and remains visible all evening until it drops very low after 11 P.M. local time. The distant planet lies 3° northeast of the 4th-magnitude star Phi (ϕ) Aquarii on Dec. 1. That night, Neptune is at its stationary point; it barely moves all month. Neptune stands 4.5° north of a First Quarter Moon on Dec. 10.

At the ice giant’s huge distance of nearly 30 astronomical units (where 1 astronomical unit or AU is the average Earth-Sun distance) from us, its disk spans only 2″ through a telescope. Use high magnification on a steady night of seeing to reveal its bluish-green disk.

Uranus stands high in the sky against the backdrop of Aries the Ram every evening and sets in the early morning hours. It lies about 11° southeast of Hamal, the brightest star in Aries. Uranus shines at magnitude 5.7, an easy object for binoculars once you find the right field of view. The ice giant stands about 3° northeast of the gibbous Moon on Dec. 14.

December is a great time to view Uranus with a telescope, given its high altitude after dark. Uranus spans 4″ with a distinctive greenish-blue hue. At a distance of 1.7 billion miles (19 AU), it’s a wonder to behold.