Western astrology is the system of astrology most popular in Western countries. Western astrology is historically based on Ptolemy's Tetrabiblos (2nd century CE), which in turn was a continuation of Hellenistic and ultimately Babylonian traditions.
Western astrology is largely horoscopic, that is, it is a form of divination based on the construction of a horoscope for an exact moment, such as a person's birth as well as location (since time zones may or may not affect a person's birth chart), in which various cosmic bodies are said to have an influence. Astrology in western popular culture is often reduced to sun sign astrology, which considers only the individual's date of birth (i.e. the "position of the Sun" at that date).
Astrology is a pseudoscience and has consistently failed experimental and theoretical verification.
Astrology was widely considered a respectable academic and scientific field before the Enlightenment, but modern research has found no consistent empirical basis to it.
Core principles
A central principle of astrology is integration within the cosmos. The individual, Earth, and its environment are viewed as a single organism, all parts of which are correlated with each other. Cycles of change that are observed in the heavens are therefore reflective (not causative) of similar cycles of change observed on earth and within the individual. This relationship is expressed in the Hermetic maxim "as above, so below; as below, so above", which postulates symmetry between the individual as a microcosm and the celestial environment as a macrocosm.
As opposed to Sidereal astrology, Western astrology evaluates a person's birth based on the alignments of the stars and planets from the perspective on earth instead of in space.
At the heart of astrology is the metaphysical principle that mathematical relationships express qualities or 'tones' of energy which manifest in numbers, visual angles, shapes and sounds – all connected within a pattern of proportion. An early example is Ptolemy, who wrote influential texts on all these topics. Al-Kindi, in the 9th century, developed Ptolemy's ideas in De Aspectibus which explores many points of relevance to astrology and the use of planetary aspects.
The zodiac
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 in the sky (Western Astrology), while the sidereal zodiac is drawn based on the position in relation to the constellations (sidereal zodiac).
The twelve 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, also called modalities, Cardinal, fixed and mutable.
Note: these are only approximations and the exact date on which the sign of the sun changes varies from year to year. Zodiac sign for an individual depends on the placement of planets and the ascendant in that sign. If a person has nothing placed in a particular sign, that sign will play no active role in their personality. On the other hand, a person with, for example, both the sun and moon in Cancer, will strongly display the characteristics of that sign in their make up.