Western astrology is the system of astrology most popular in Western countries. Western astrology was founded by Ptolemy's Tetrabiblos in the 2nd century AD, and forms a continuation of Hellenistic astrology and ultimately Babylonian astrology. Western astrology is largely horoscopic, that is, it is largely based on the construction of a horoscope for an exact moment, 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.
During the Dark Ages in Europe knowledge of astrology was lost along with much other learning. It was revived and extended by Arab and Persian scholars and reimported into Europe in the Middle Ages. In medieval Europe, important political and military decisions were often made in consultation with astrologers. Nowadays, believers in astrology use it primarily for making personal decisions or attaining information about an individual through natal astrology. Today, astrology has become distinct from astronomy and mainstream scientists in general dismiss astrology as a form of pseudoscience.
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 through which the sun passes throughout the year, those constellations that are "Enlightened by the mind". 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. The Western Zodiac is drawn based on the Earth's relationship to fixed, designated positions in the sky, and the Earth's seasons.
The Sidereal Zodiac is drawn based on the Earth's position in relation to the constellations and follows their movements in the sky. 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. Thus, the tropical zodiac corresponds with the position of the earth in relation to fixed positions the sky (Western Astrology), while the sidereal zodiac is drawn based on the position in relation to the constellations (sidereal zodiac).
The 12 Signs
In modern Western astrology the signs of the zodiac are believed to represent twelve basic personality types or characteristic modes of expression. The twelve signs are divided into four elements fire, earth, air and water. Fire and air signs are considered masculine, while water and earth signs are considered feminine. The twelve signs are also divided into three qualities, cardinal, mutable and fixed.
A 13th Zodiac Sign?
According to NASA, in 2016, we have it all wrong. Read all about it here. In the meantime, I'm sticking to the old system.