1911 Encyclopædia Britannica/Libra
LIBRA (“The Balance”), in astronomy, the 7th sign of the zodiac (q.v.), denoted by the symbol ♎, resembling a pair of scales, probably in allusion to the fact that when the sun enters this part of the ecliptic, at the autumnal equinox, the days and nights are equal. It is also a constellation, not mentioned by Eudoxus or Aratus, but by Manetho (3rd century B.C.) and Geminus (1st century B.C.), and included by Ptolemy in his 48 asterisms; Ptolemy catalogued 17 stars, Tycho Brahe 10, and Hevelius 20. δ Librae is an Algol (q.v.) variable, the range of magnitude being 5.0 to 6.2, and the period 2 days 7 hrs. 51 min.; and the cluster M. 5 Librae is a faint globular cluster of which only about one star in eleven is variable.