Why is the ninth month called September?
late 14c., from Old French zodiaque, from Latin zodiacus "zodiac," from Greek zodiakos (kyklos) "zodiac (circle)," literally "circle of little animals," from zodiaion, diminutive of zoion "animal" (see zoo).
Libra is not an animal, but it was not a zodiac constellation to the Greeks, who reckoned 11 but counted Scorpio and its claws (including what is now Libra) as a "double constellation." Libra was figured back in by the Romans. In Old English the zodiac was twelf tacna "the twelve signs," and in Middle English also Our Ladye's Waye and the Girdle of the Sky.
A band of the celestial sphere extending about eight degrees north and south of the ecliptic, representing the portion of the sky within which the paths of the Sun, the Moon, and the planets are found. In astrology, the zodiac is divided into 12 equal segments, each of which is named after a constellation through which the ecliptic passes in that region of the sky. The traditional beginning point of constellations is Aries, followed in calendrical order by Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Sagittarius, Capricorn, Aquarius, and Pisces. See also equinox.
Note: The twelve constellations, or signs, of the zodiac are important in astrology.