Jimmy Westlake: Libra – the misfit of the Zodiac
April 29, 2013
In the course of one year, the sun makes a 360-degree circuit of the celestial sphere, passing in front of 12 different constellations in the background. These are the 12 constellations of the zodiac.
Zodiac is a word that literally translates to "the circle of animals." It 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 Bearer and Pisces the Fish.
Take another look at that list of constellations in the "circle of animals." Notice anything peculiar?
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 huge double constellation of Scorpius the Scorpion and Chelae (chay-lee), the Scorpion's Claws. The Romans ultimately 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 of his own in the heavens. They decided to amputate Chelae, the Scorpion's Claws, to form this new constellation. At that time, 2,000 years ago, 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, to honor the justice and wisdom of Julius Caesar.
The names of Libra's two brightest stars still hearken back to a time when Chelae, the Scorpion's Claws, occupied this space. Their Arabic 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, the Southern Claw. Zubenelgenubi is a1 close naked-eye double star, but the binoculars or telescope will reveal both stars clearly. Lying at a distance of 77 light-years, the two stars of Zubenelgenubi require more than 200,000 years to orbit each other one time.
The Northern Claw, Zubeneschamali, is more than twice as far from us as Zubenelgenubi.
You can spot the zodiacal misfit constellation of Libra at about 10 p.m. in early May, high in the southeastern sky. This spring and summer, the bright planet Saturn will guide you to Libra. Look for the two "zuben" stars of Libra just below, or east, of Saturn.
Jimmy Westlake teaches astronomy and physics at Colorado Mountain College's Alpine Campus. Check out Westlake's astrophotography website at http://www.jwestlake.com.
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