Lib · Librae
Libra Constellation
IAU constellation mapLibLibra (Lib) is one of the 88 constellations recognised by the International Astronomical Union. It covers 538 square degrees of the equatorial sky in quadrant SQ3. Its brightest star is Zubenelgenubi. The Latin genitive of the name is Librae, used to form star names within the constellation. In Greek mythology, Libra is associated with greek tradition. The IAU standardised all constellation boundaries in 1930.
Quick facts
- IAU name
- Libra
- Abbreviation
- Lib
- Latin genitive
- Librae
- Hemisphere
- equatorial
- Area
- 538 sq deg
- Brightest star
- Zubenelgenubi
- Quadrant
- SQ3
- Family
- Zodiac
- Mythology origin
- Greek
- Discovery era
- Ptolemy's 48 (Almagest, c. 150 CE)
Mythology
Libra is the only zodiacal constellation representing an inanimate object rather than a living creature. Its two main stars, Zubenelgenubi and Zubeneschamali, bear Arabic names meaning 'the southern claw' and 'the northern claw', reflecting the earlier Babylonian and Greek tradition of treating these stars as the claws of the adjacent Scorpion. The Greeks did not originally recognise a Scales constellation here; the region was described as the claws of Scorpius (Scorpii Chelae). It was the Romans who clearly separated the figure as Libra (the Balance), linking it to the scales of justice held by the adjacent Virgo-Dike figure. This Roman association had political overtones: Julius Caesar was said to have been born under Libra, and the scales were connected to the ideal of Roman justice and equity. The autumn equinox, when day and night are equal, was historically positioned in Libra, reinforcing the symbolism of balance, though precession has now moved the equinox into Virgo. Zubenelgenubi (Alpha Librae) is one of the nearest double star systems visible to the naked eye, with the two components separated by 5,570 astronomical units.
Sources: Aratus Phaenomena 89-95; Cicero De Natura Deorum 2.43; Hyginus Astronomica 2.26.
Overview
Libra is one of the 88 constellations formally recognised by the International Astronomical Union (IAU). It occupies 538 square degrees of sky in the equatorial sky region, crossing both hemispheres, best visible at latitudes between +45 and -90 degrees in June. The constellation belongs to the Zodiac family of constellations. Its Latin genitive is Librae, the form used when naming stars within the constellation — for example, the brightest star Zubenelgenubi may appear in catalogues as Alpha Librae or similar. The IAU standardised the boundaries of all 88 constellations in 1930 under the direction of Eugène Delporte, using straight lines of right ascension and declination to eliminate the ambiguities of earlier variable-boundary systems.
Notable stars
The brightest star in Libra is Zubenelgenubi. Individual stars within Libra are conventionally named using Bayer designation — Greek letters followed by the Latin genitive of the constellation name (Librae) — introduced by Johann Bayer in his 1603 star atlas Uranometria. Flamsteed numbers (numeric designators followed by Libra) were added by John Flamsteed in his 1712 catalogue. Variable stars, double stars, and deep-sky objects within the Libra boundary are catalogued by the IAU with the abbreviation Lib.
History and mythology
The figure of Libra appears in the classical astronomical tradition. Ptolemy included this constellation in the Almagest (c. 150 CE), the definitive ancient catalogue of 48 constellations, which formed the foundation for Islamic, medieval European, and Renaissance astronomy. The constellation's figure and mythology were transmitted through works such as Aratus's Phaenomena, Eratosthenes's Catasterismi, and Hyginus's Astronomica. See the Mythology section above for the full narrative.
Sources & further reading (2)
- official-iau — accessed 2026-05-06
- encyclopedia — accessed 2026-05-06
Frequently asked questions
What does the name Libra mean?
Libra is the Latin name used by the IAU to designate this constellation. The name comes from classical Latin and Greek astronomical tradition. In star catalogues, stars within Libra are designated using the genitive form of the name (e.g. Alpha Libra or similar Latin genitive forms) — a naming convention introduced by Johann Bayer in his 1603 atlas Uranometria and still in use today.
How large is Libra compared to other constellations?
Libra covers 538 square degrees of sky. The full sphere of the sky contains 41,253 square degrees, so Libra occupies about 1.3% of the total sky. For comparison, the largest constellation is Hydra at 1303 square degrees; the smallest is Crux at 68 square degrees.
When and where is Libra visible?
Libra straddles the celestial equator and is visible from both hemispheres, though part of it may be low on the horizon depending on the observer's latitude. The brightest star, Zubenelgenubi, serves as the main visual anchor for locating the constellation. As with all constellations, the best viewing conditions are a dark sky away from artificial light pollution, with the constellation high enough above the horizon to minimise atmospheric absorption.
