The zodiac was born as the first celestial coordinate system
Around 401 BC, under Persian rule, Babylonian astronomers carved the Sun's yearly path into 12 equal slices of 30 degrees each, modelled on idealised 30-day months. Long before horoscopes made it famous, that division gave them the first known system of celestial coordinates, a tool astronomers still use for tracking planets and asteroids.
The zodiac is a band stretching about 8 degrees either side of the ecliptic, the Sun's apparent annual route, and the Moon and bright planets all travel within it. Its name comes from Greek for little animals, reflecting how many of the signs are creatures or mythical hybrids. Some of the figures are older than the system itself: Babylon's MUL.APIN star catalogue, compiled around 1000 BC, drew on still earlier lists, and Bronze Age sources already knew Gemini as the Great Twins and Cancer as the Crayfish.
Modern calculations place the introduction between 409 and 398 BC. Babylonian astronomers anchored the signs to stars rather than seasons, starting Cancer at a star in Gemini and Aquarius at one in Capricornus. Their observing techniques were rudimentary, so they measured planets against a set of 'normal stars' near the ecliptic, and their diaries usually gave a planet's position by sign alone. What mattered most was the date a planet crossed from one sign into the next.
The idea spread to Greek astronomy by the 2nd century BC and helped shape the Hindu zodiac; Ptolemy described its construction in the Almagest in the 2nd century AD. Egypt had its own sky divisions, the 36 decans, which appear in the tomb of Seti I as early as the 14th century BC and later on the famous Dendera zodiacs.
Earth's slow wobble, the precession of the equinoxes, has shifted things since Babylonian times: the point of the March equinox has drifted out of Aries and into Pisces, so the Sun no longer sits in the constellations the old dates suggest. Astrologers nonetheless still start Aries at the March equinox. The narrower band holding the Moon's path, just 5 degrees either side, is sometimes called the zodiac of the Moon.
Source: Zodiac
