Zodiac

Origin and etymology

"Zodiac" comes from the Greek zōdiakos kyklos ("circle of little animals"), because ten of the twelve zodiacal constellations represent animals (Aries the ram, Taurus the bull, Cancer the crab, Leo the lion, Scorpio the scorpion, Capricorn the goat-fish, Pisces the fish, etc.). The two exceptions are Gemini (human twins) and Libra (the scales, the only inanimate object).

The division into twelve sectors was systematised by the Babylonians in the first millennium BC, based on observation of the constellations through which the Sun's apparent path passed. The number 12 is not arbitrary: it corresponds approximately to the number of lunar months in a solar year (12 moons × 29.5 days ≈ 354 days, almost a year). The Greeks and then the Arabs adapted the system. Modern tropical Western astrology bases the signs on seasons (Aries always begins with the spring equinox), not on the actual constellations.

Tropical zodiac vs. sidereal

There are two main zodiacal systems that yield slightly different results. The tropical zodiac (Western) divides the year into twelve fixed sectors based on seasons: Aries always starts on 21 March (spring equinox), regardless of where the actual constellations are. It is the zodiac used in most popular horoscopes and in classical Western astrology.

The sidereal zodiac (used in Vedic Indian astrology and some Western schools) divides the sky into twelve sectors corresponding to the actual constellations. Due to precession of the equinoxes (slow movement of Earth's axis taking 26,000 years to complete a cycle), there is currently about 24° of difference between the two systems. If you are Aries in Western astrology, you are probably Pisces in sidereal.

How it is used

The zodiac is the basis of every astrological reading: solar sign, lunar sign, ascendant, planetary positions, houses — everything is expressed in terms of "such planet at such degree of such sign". For popular horoscopes only the solar sign is used (1/12 of the population shares sign). For finer readings, the complete natal chart is cast with all planets in their respective signs and houses.