Origins of the Term
Zodiac comes from the Greek word Zodiakos Kyrklos, or "circle of little animals". The Greeks pilfered the concept from the Babylonians who had decided that the Sun passed through 12 phases or signs whose representations could be sketched out in the constellations. The Greeks finished appropriating the concept around the Sixth Century B.C. They then came up with their own explanations for how these figures ended up in the sky.
Greek Mythology and the Zodiac
Aries: Latin for "the Ram", Krios in Greek. Krios was the friendly Ram who carried Phrixis and Helle across the sea away from their evil step-mother (step-father's don't seem to get a bad rap anywhere). Whilst Helle fell off half way across, Phrixis made it and showed his 'thanks' by killing and skinning the ram. He hung its skin in the Grove of Ares where the fleece turned gold. It was this Golden Fleece which inspired Jason and the Argonauts. Zeus placed the Ram in the sky after it was found by Jason and dedicated to him.
Taurus: from the Greek Tauros, "the Bull". There are two versions of this tale. In the first, Taurus was Zeus himself. Zeus disguised himself as a bull so that he could seduce the mythical Phoenician Princess Europa and father the Minotaur. Whilst this myth is even more implausible than most Greek myths, it is at least more interesting than the second version of the tale. In the second version, Tauros was the bull who, at Zeus' request, carried Europa across the Sea from Phoenicia to Crete.
Gemini: from the Latin translation of the Greek Didymoi, "the twins". Castor and Pollux (sometimes called Polydeuces), were the sons of Leda. They were half brothers. Apparently, Leda was pregnant to her husband when she was seduced by Zeus. This resulted in two fertilised eggs, each carrying twins. To her husband, the King of Sparta, she had Castor and Clytaemnestra. To Zeus, she has Pollux and Helen (later Helen of Sparta, then Helen of Troy). Castor and Pollux were two of the Argonauts and Zeus placed them in the heavens as a reward. Despite their confused parentage, they are often referred to as Dioscuri (dios kouri), or "sons of Zeus".
Cancer: from the Greek for "the Crab", Karkinos which in Latin is Cancer. One of Hera's pet monsters, whom Hercules crushed in his second labour. Thus, cancer was originally referred to as "the Asses' Crib". In many ways, this star sign hasn't got the most fortunate etymology or the most glowing myth behind it.
Leo: Latin for "the Lion", from the Greek Leon. The Nemean Lion was another beast that Hercules disposed of. But like Cancer, why did Zeus elevate this beast to the heavens? Some tales state that the Nemean Lion was born of the chimera, or of Typhon and Echidna. However, the Lion'splace in the Heavens is more easily explained by the tales that say that the Lion's parents were Zeus and Selene.
Virgo: "The virgin" in Latin, from the Greek Parthenos. This sign could represent numerous divine virgins. The most popular theory however is that this virgin is Astraea, who along with her mother Themis, was justice personified. She was the last of the immortals to stay on Eart during the Golden Age and ascended to the heavens when the pursuit of earthly justice became futile. The scales he used to weigh justice with went with her and became the constellation Libra. Another, less popular, version of the myth is that the constellation is Persephone. The only real support for this is that the constellation os brightest in the spring months - which is the time when Persephone is free from the Underworld. Ultimately, there were just too many mythical virgins for anyone to be completely sure who Virgo is meant to be.
Libra: Latin for "Balance" or "the Scales", from the Greek word Zyygos. Originally, the sign was known as "the claws" and were attached to Scorpio. They were later turned into the scales and transferred to Virgo. It is the youngest of the signs and also the only one which does not represent a living creature.
Scorpio: Latin for "the Scorpion", or Skorpios in Greek. Another star sign with multiple variations on its myth. In the first, the legendary Greek hunter - Orion - and the Goddess Artemis held a competition to test their humting prowess. This came as a result of Orion's boast that he could single-handedly kill every animal on Earth. Artemis used her deadly Scorpion to prove him wrong. As thanks, Artemis (or Zeus in some legends) turned the scorpion into a constellation. Orion also became a constellation as a consolation prize. In another version, Apollo orders the Scorpion to kill Orion as a result of Artemis' growing afftection for the Hunter. Artemis hangs her lost lover's image in the night sky. The third variation is that Gaia ordered the Scorpion to kill Orion as a result of his hunting. Being the Ancient Goddess who personified Earth and everything natural on it - she may have had a problem with Orion's over-hunting. Interestingly enough, Orion and Scorpio are on opposite sides of the night sky (possibly in order to avoid further conflict) so when Scorpio rises, Orion scurries away from his attacker.
Sagittarius: "the Archer" in Latin, from the Greek Toxotes. Sagittarius was always represented as a centaur with a bow. It is possible, that Sagittarius represents a particular centaur. The most likely candidate is Chiron, tutor of Achilles, Jason and Asclepius
Capricorn: "Capricornus" is a Latin compound of "capra" (she-goat) and "cornus" (horn), which comes from the original Greek term Aigokeros (goat-horned). The goat was originally known as Amalthea, and secretely fed the infant Zeus as he was growing up in Crete. This was after Zeus' mother Rhea had saved him from being devoured by his father Cronos. Amalthea's horn became the original "cornucopia", and as thanks - Zeus turned his goat-nanny into a constellation.
Aquarius: Aquarius is the Latin translation of the Greek Hydrochoos (the water-pourer). One version of the myth states that Aquarius was Deucalion, the only survivor of a Great prehistoric flood. He was placed in the sky after his death as a reward for restoring the human race. Another version states that Aquarius caused they great flood by pouring from his unlimited pot of water. Others say that Aquarius was actually Ganymede, Zeus' cup bearer who Zeus had a bit of a thing for.
Pisces: Again, Pisces is the Latin translation of the Greek Ichtyes (the fishes). Very little is known about this sign, which is a bit fishy if you ask me. But one of the most popular versions is that the fishes were Eros and Aphrodite who mythically turned themselves into fish in order to escape the Great flood.