If you’re up and around on the morning of December 2, 2021, watch for the waning crescent moon very near the star Zubenelgenubi, the constellation Libra’s alpha star. Very pretty early morning scene!
Throughout the month, the moon passes near planets and stars, creating interesting scenes in the sky.
The moon starts December 2021 as a waning crescent, positioned in the early morning sky low in the southeast. December 2 finds it floating very near an interesting, moderately-bright star with a strange sounding name: Zubenelgenubi. The name rhymes with Obi-Wan Kenobi of the Star Wars franchise.
Many see Zubenelgenubi as a single star. People with clear dark skies and good eyesight can see two separate points of light in this star. One of the two is dimmer than the other. Binoculars easily reveal the dual nature of Zubenelgenubi. Using spectroscopy, astronomers have found that both stars are themselves double, making at least four stars in this system!
We in the Northern Hemisphere usually consider Zubenelgenubi as a summer star. Summer is when you’ll see this star – and its brother star Zubeneschamali in the constellation Libra the Scales – in the evening.
Every year in December, though, Zubenelgenubi returns from being behind the sun from our earthly vantage point. It’s just now beginning another cycle of visibility in Earth’s sky… a pretty sight near the waning moon on December 2!
Zubenelgenubi is nearly always shown with its brother star in Libra, Zubeneschamali. Here’s the view on June evenings, when you’ll notice them between 2 brighter stars: Antares in the constellation Scorpius and Spica in the constellation Virgo. This chart faces south around 10 p.m. in mid- to late June. Libra is a faint, diamond-shaped pattern of stars. Map created with Stellarium by AstroBob. Used with permission.
Bottom line: Watch for the waning crescent moon near Libra’s bright star Zubenelgenubi on the morning of December 2, 2021.
“I can sometimes see the moon in the daytime” was a cosmic revelation that John Jardine Goss first discovered through personal observations at age 6. It shook his young concept of the universe and launched his interest in astronomy and stargazing, a fascination he still holds today. John is past president of the Astronomical League, the largest U.S. federation of astronomical societies, with over 20,000 members. He's earned the title of Master Observer and has authored the celestial observing guides Exploring the Starry Realm and Carpe Lunam. John also writes a monthly stargazing column, Roanoke Skies, for the Roanoke Times, and a bimonthly column, Skywatch, for Blue Ridge Country magazine. He has contributed to Sky and Telescope magazine, the IDA Nightscape, the Astronomical League’s Reflector magazine, and the RASC Observer’s Handbook.
Deborah Byrd created the EarthSky radio series in 1991 and founded EarthSky.org in 1994. Prior to that, she had worked for the University of Texas McDonald Observatory since 1976, and created and produced their Star Date radio series. Today, she serves as Editor-in-Chief of this website. She has won a galaxy of awards from the broadcasting and science communities, including having an asteroid named 3505 Byrd in her honor. In 2020, she won the Education Prize from the American Astronomical Society, the largest organization of professional astronomers in North America. A science communicator and educator since 1976, Byrd believes in science as a force for good in the world and a vital tool for the 21st century. "Being an EarthSky editor is like hosting a big global party for cool nature-lovers," she says.
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