Glossary Z

Zodiac. The ring of 12 constellation signs that appear to encircle the Earth along the path of the ecliptic. Each sign occupies precisely 30° of longitude and takes its name from the constellation that it contains. A given zodiacal constellation may be larger or smaller in longitude than the corresponding zodiacal sign. The signs progress eastwards in order: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius, Pisces. The starting point is the position of the Sun at the spring equinox, the so called first point of Aries, which is defined as 0° ecliptic longitude.
   The name 'zodiac' is taken from the Greek word ζῴδιον which was used by Aristotle, Eudoxos and later astronomers to mean a sign of the zodiac, being derived ultimately from the word for an animal, because all the signs were originally animal or human (in Greek times, Libra was the Claws of the Scorpion). However, the zodiac was not a Greek invention, but originated in Mesopotamia around 4000 BCE, starting with the four founder constellations: Taurus, Leo, Scorpio and Aquarius. By about 400 BCE at the latest, the Mesopotamians had added eight other constellations and defined sectors of sky around them, each of 30° in longitudinal extent, in order to help them with their positional calculations.
   The zodiac was introduced into Greece by Eudoxos in the middle of the C4th BCE, but he used the middle of each sign as his reference point, that is 15° longitude relative to the start of the sign. The system was redefined by Hipparchos probably around 138 BCE to use 0° as the reference point.