Jimmy Westlake: Libra, misfit of the Zodiac
May 21, 2010
Steamboat Springs — In the course of one year, the sun makes a 360-degree circuit of the sky, passing through 12 constellations that form a band around the sky called the Zodiac. Zodiac is a word that literally means "the circle of animals." The Zodiac contains the familiar constellations of Aries the Ram, Taurus the Bull, Gemini the Twins, Cancer the Crab, Leo the Lion, Virgo the Virgin, Libra the Scales, Scorpius the Scorpion, Sagittarius the Archer, Capricornus the Sea Goat, Aquarius the Water Carrier and Pisces the Fish.
Take another close look at that list of constellations in the "circle of animals." Notice anything odd? The "circle of animals" includes one non-animal — Libra the Scales. How did this inanimate object become a member of the exclusive Zodiac club?
To the ancient Greeks, there were only 11 constellations in the Zodiac, including the double constellation of Scorpius the Scorpion and Chelae the Scorpion's Claws. The Romans eventually adopted many of the Greek constellations, including the 11 in the Zodiac.
After the assassination of Julius Caesar in 44 B.C., the Romans wanted to honor Caesar with a constellation in the heavens. They decided to amputate Chelae, the Scorpion's Claws, to form this new constellation. At that time, the autumnal equinox occurred when the sun was seen in this part of the sky and the hours of daylight and darkness were balanced. The Romans created a new star pattern here and named it Libra the Scales, in honor of Julius Caesar.
The names of Libra's two brightest stars still hearken back to the time when Chelae, the Scorpion's claws, occupied this space. Their names are Zubeneschamali and Zubenelgenubi, the Northern Claw and Southern Claw of the Scorpion. If you own a telescope, or even a good pair of binoculars, try aiming it at Zubenelgenubi. Zubenelgenubi is a challenging naked-eye double star, but the binoculars or telescope will reveal both stars clearly. Lying at a distance of 77 light years from Earth, the two stars of Zubenelgenubi require more than 200,000 years to orbit one another. The northern claw star, Zubeneschamali, is more than twice as far away as Zubenelgenubi.
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You can spot the zodiacal misfit constellation of Libra at about 10 p.m. high in the southeastern sky about midway between the two bright stars Spica and Antares.
Jimmy Westlake teaches astronomy and physics at Colorado Mountain College's Alpine Campus. He is an avid astronomer whose photographs and articles have been published all across the world. Check out Jimmy's astrophotography website at http://www.jwestlake.com.