Libra - The Scales

Libra is a zodiacal constellation lying between Virgo and Scorpio. It is north of Centaurus and south of the head of Serpens. It is best seen in the early summer in the Northern Hemisphere and in the autumn and winter in the Southern Hemisphere.

History and Mythology

Libra, the balance or scales, is the only zodiacal constellation that does not represent a living thing. Originally Libra was named Chelae, the Latin for "claw", and it was part of the larger adjacent constellation Scorpius. The Romans added Libra to the zodiac, in honor of Julius Caesar, and his reform of the Roman calendar.

Libra was said to be called the scales because in classical times, the autumnal equinox occurred when the Sun was in Libra, and the length of day and night were about equal. Because of the shifting of the sky caused by precession, the Sun is no longer in Libra on the autumnal equinox. It now appears in Libra in October.

The Greeks and Arabs referred to Libra as the claws of the Scorpion. The stars of Libra have also been seen as an altar on a mound connected with the Tower of Babel. Libra has been seen as the lighthouse of Alexandria, one of the Seven Wonders of the Ancient World, which appeared as a great lamp between the Scorpion's claws.

Notable Objects

Although Libra is a faint constellation, it contains several interesting stars. Alpha Librae or Zubenelgenubi, which means "southern claw" in Arabic, is a wide binocular double star consisting of a blue-white, 3rd magnitude star and a white, 5th magnitude companion.

Beta Librae or Zubeneschamali, Arabic for "northern claw", has a magnitude of 2.6, and it is one of the few bright stars with a distinct greenish color. Delta Librae or Zubenelakrab, Arabic for "scorpion's claw", is an Algol-type eclipsing binary system about 300 light years away. Its brighter blue-white star is eclipsed by a yellow giant companion, causing the system's combined magnitude to drop from 4.4 to 5.8 every 2.3 days.

Sigma Librae is a variable 3rd magnitude red giant star 290 light years away. It also has the designation Gamma Scorpii, and is one of the very few stars that belong to two constellations. Its traditional name Zubenhakrabi means "the scorpion's claw" in Arabic; however, since this star falls within the modern boundaries of Libra, that designation is no longer used.

NGC 5897 is a loosely scattered, faint globular cluster 45,000 light-years away. It is a difficult object in small instruments. NGC 5792 is a 12th magnitude spiral galaxy near the northwest boundary of Libra. It is seen nearly edge-on, with a bright nucleus and faint arms. It can be seen in a 10 inch or larger telescope.