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.
Greek physicians Hippocrates and Galen, among others, noted similarity of crabs to some tumors with swollen veins. The Old English word was displaced by French-influenced doublet canker but was reintroduced in the modern medical sense c. 1600. In reference to the zodiac sign from late Old English; meaning "person born under the zodiac sign of Cancer" is from 1894. The sun being in Cancer at the summer solstice, the constellation had association in Latin writers with the south and with summer heat. Cancer stick "cigarette" is from 1959.
zodiacal constellation, late 14c., from Latin scorpio (poetic scorpius) "scorpion," also the zodiac constellation (see scorpion). The meaning "person born under or ruled by the sign of Scorpio" is recorded from 1941. Since 1922 the official (International Astronomical Union) scientific designation of the constellation has been Scorpius, leaving Scorpio to astrology.
1670s in astrology, of planets, "one-tenth part of the zodiac distant from one another;" 1882 in statistics; from French décile or Medieval Latin *decilis, from Latin decem "ten" (from PIE root *dekm- "ten") on the model of quintilis, sextilis.
ancient constellation, 12th sign of the zodiac, representing two fishes united by a cord attached to their tails, late Old English, from Latin pisces, plural of piscis "a fish" (from PIE root *pisk- "a fish"). Applied to persons born under this sign by 1924 (also Piscean).
1610s, originally in astrology and said to have been introduced by Kepler, "aspect of planets when they are 72 degrees from each other" (a fifth of the zodiac), from Latin quintus "the fifth" (from PIE root *penkwe- "five") + ending from quartile. Its use in statistics in reference to a division data points into five parts of more or less equal size dates to 1951.
early 14c., constellacioun, "position of a planet in the zodiac;" late 14c., "one of the recognized star patterns handed down from antiquity" (in the zodiac or not), from Old French constellacion "constellation, conjuncture (of planets)" and directly from Late Latin constellationem (nominative constellatio) "a collection of stars," especially as supposed to exert influence on human affairs," from constellatus "set with stars," from assimilated form of Latin com "with, together" (see con-) + past participle of stellare "to shine," from stella "star" (from PIE root *ster- (2) "star").
The oldest sense is astrological, of the position of planets ("stars") relative to the zodiac signs on a given day, usually the day of one's birth, as a determiner of one's character. "I folwed ay myn inclinacioun/By vertu of my constillacioun" (Chaucer, "Wife's Prologue," c. 1386). In modern use "a group of fixed stars to which a definite name has been given but does not form part of another named group (compare asterism). Figuratively, "any assemblage of a brilliant or distinguished character"(1630s).
The classical northern constellations probably were formed in prehistoric Mesopotamia; the Greeks likely picked them up c. 500 B.C.E., and Claudius Ptolemy (c. 90-c. 168) of Alexandria codified 48 of them, all still current, in his "Almagest" (2c.). The canonical list was expanded from 16c. as Europeans explored southern regions whose stars were invisible from Alexandria and as astronomers filled in the dimmer regions between the established figures, so that by the late 19c. as many as 109 constellations were shown on maps. The modern roster was set at 88 by the International Astronomical Union in 1922.
Old English ramm "male sheep," also "battering ram, instrument for crushing or driving by impact," and the zodiac sign; earlier rom "male sheep," a West Germanic word (cognates: Middle Low German, Middle Dutch, Dutch, Old High German ram), of unknown origin. Perhaps [Klein] connected with Old Norse rammr "strong," Old Church Slavonic ramenu "impetuous, violent."