Western astrology is the system of astrology most popular in Western countries. Western astrology originated in Mesopotamia during the 2nd millennium BC, from where it spread to much of the world. After spreading to Egypt, where further developments occurred, it then arrived into Greek or Hellenistic culture, where the Greek astronomer and astrologer Ptolemy in his work Tetrabiblos laid the foundations of the Western tradition. Western astrology is largely horoscopic, that is, it is largely based on the construction of a horoscope for an exact moment in time, such as a person's birth, in which various cosmic bodies are said to have an influence. In modern Western sun sign astrology, only the location of the Sun is considered.
The zodiac is the belt or band of constellations through which the Sun, Moon, and planets move on their journey across the sky. Astrologers noted these constellations and so attached a particular significance to them. Over time they developed the system of twelve signs of the zodiac, based on twelve of the constellations they considered to be particularly important. Most western astrologers use the tropical zodiac beginning with the sign of Aries at the Northern hemisphere Vernal Equinox always on or around March 21 of each year. Due to a phenomenon called precession of the equinoxes (where the Earth's axis slowly rotates like a spinning top in a 25,700 year cycle), there is a slow shift in the correspondence between Earth's seasons (and calendar) and the constellations of the zodiac. For this reason some Western astrologers use the Sidereal zodiac which still uses the actual star positions.
