Zodiac signs

The concept of the zodiac was used by the Babylonians before 2000 BC as a method of visualizing the passage of time. The zodiac worked as a symbolic calendar. It was divided into twelve parts as suggested by the appearance of 12 moons in a year. The signs are geometric divisions of the celestial sphere, each corresponding to one twelfth of a year. The word zodiac is derived from the Greek zodiakos

The signs of the zodiac, as enumerated by Egyptian astronomer, Ptolemy, in the 2nd-century AD, are the ones we know today. The same names are used for both signs in astrology and for constellations in astronomy, but it's important to make a distinction between signs and constellations. Signs are geometric sections, each 30° wide, corresponding with particular periods of time of the year, but which don't necessarily physically correspond with the constellations of the same name.

By the time of Ptolemy the zodiac was already at least two thousand years old. But together with its burgeoning astrological use the basic function and structure of the "calendar of the zodiac" remained. The sign of Aries marks the beginning of the year at the vernal equinox. The retreating crab in Cancer represents the retreat of the Sun from its farthest northern point at the time of the summer solstice. Leo, the symbol of fire, represents summer heat. The scales of Libra signify the balance between day and night at the autumnal equinox. The decline of the sun's power is represented in Scorpio by the scorpion, the symbol of darkness. The water-bearer, Aquarius, represents the rainy season which, in Egypt, meant the yearly flooding of the Nile. The fishes of Pisces, symbolize the return of life and the resumption of agriculture.

Each year the sun passes entirely around the zodiac and returns to the point from which it started--the vernal equinox--and each year it falls just a little short of making the complete circle of the heavens in the allotted period of time. As a result, it crosses the equator just a little behind the spot in the zodiacal sign where it crossed the previous year. Each sign of the zodiac consists of thirty degrees, and as the sun loses about one degree every seventy two years, it regresses through one entire constellation (or sign) in approximately 2,160 years, and through the entire zodiac in about 25,920 years. (Authorities disagree concerning these figures.) This retrograde motion is called the precession of the equinoxes. This means that in the course of about 25,920 years, which constitute one Great Solar or Platonic Year, each one of the twelve constellations occupies a position at the vernal equinox for nearly 2,160 years, then gives place to the previous sign.