Libra (constellation)

Libra ( /ˈliːbrə/) is a constellation of the zodiac. Its name is Latin for weighing scales, and its symbol is (Unicode ♎). It is fairly faint, with no first magnitude stars, and lies between Virgo to the west and Scorpius to the east.

Notable features

The brightest stars in Libra form a quadrangle:

• α Librae, Zubenelgenubi ("southern claw"), a visual binary;
• β Librae, Zubeneschamali ("northern claw");
• γ Librae, Zubenelakrab ("scorpion's claw");
• σ Librae, Brachium, an eclipsing variable.

σ Librae was formerly known as γ Scorpii despite being well inside the boundaries of Libra. It was not redesignated as σ Librae until 1851 (by Benjamin A. Gould).

Planetary system

Libra is home to the star Gliese 581, which has a planetary system consisting of at least 6 planets, including Gliese 581 g, the best candidate planet for human habitability and extra-solar life yet discovered,^[1] Gliese 581 c, the first Earth-like extrasolar planet to be found within its parent star's habitable zone, Gliese 581 d, discovered in 2007 to be another Earth-like planet, and Gliese 581 e, the smallest mass exoplanet orbiting a normal star,^[2] all of which are of significance for establishing the likelihood of life outside of the Solar System.^[3] The family of candidate habitable planets was extended in late September 2010 to include Gliese 581 g, a tidally locked planet in the middle of the habitable zone, and currently the most likely discovered exoplanet to be capable of supporting life.^[4]

History and mythology

Libra is the only zodiac sign that does not symbolize a living creature. It was known in Babylonian astronomy as MUL Zibanu ("the scales"), or alternatively as the Claws of the Scorpion. The scales were held sacred to the sun god Shamash, who was also the patron of truth and justice.^[5] Since these times, Libra has been associated with law, fairness and civility. In Arabic zubānā means "scorpion's claws", and likely similarly in other Semitic languages: this resemblance of words may be why the Scorpion's claws became the Scales.

In Roman mythology, Libra is considered to depict the scales held by Astraea (identified as Virgo), the goddess of justice.

Astrology

As of 2002^[update], the Sun appears in the constellation Libra from October 31 to November 22. In tropical astrology, the Sun is considered to be in the sign Libra from September 23 to October 22, and in sidereal astrology, from October 16 to November 15.

Visualizations

Traditionally, α and β Librae are considered to represent the scales' balance beam, and γ and σ are the weighing pans.

H.A. Rey has suggested a way to connect the stars more fully to graphically show a balance. Beta Librae represents the top of the balance, Gamma Librae and Alpha Librae represent the balance beam. Upsilon Librae and Tau Librae represent the left plate of the balance, while Sigma Librae represents the right plate. All these stars are of the third magnitude.

Namesakes

USS Libra (AKA-12) was a United States navy ship named after the constellation.

Citations

References

• H. A. Rey, The Stars — A New Way To See Them. Enlarged World-Wide Edition. Houghton Mifflin, Boston, 1997. ISBN 0-395-24830-2.
• Ian Ridpath and Wil Tirion (2007). Stars and Planets Guide, Collins, London. ISBN 978-0007251209. Princeton University Press, Princeton. ISBN 978-0691135564.

External links

• The Deep Photographic Guide to the Constellations: Libra
• Libra constellation
• Star Tales – Libra

Coordinates: 15^h 00^m 00^s, −15° 00′ 00″