Zodiac hystory and symbolism
While Western astrology is essentially a product of Greco-Roman culture, some of its more basic concepts originated in Babylonia. Isolated references to celestial "signs" in Sumerian sources are insufficient to speak of a Sumerian zodiac. Specifically, the division of the ecliptic in twelve equal sectors is a Babylonian conceptual construction.
By the 4th century BC, Babylonians' astronomy and their system of celestial omens were influencing the Greek culture and, by late 2nd century BC, Egyptian astrology was also mixing in. This resulted, unlike the Mesopotamian tradition, in a strong focus on the birth chart of the individual and in the creation of horoscopic astrology, employing the use of the Ascendant (the rising degree of the ecliptic, at the time of birth), and of the twelve houses. Association of the astrological signs with Empedocles' four classical elements was another important development in the characterization of the twelve signs.
The body of astrological knowledge by the 2nd century AD is described in Ptolemy's Tetrabiblos, a work that was responsible for astrology's successful spread across Europe and the Middle East, and remained a reference for almost seventeen centuries as later traditions made few substantial changes to its core teachings.
Polarity and the four elements
Empedocles, a fifth century BCE Greek philosopher, identified Fire, Earth, Air, and Water as elements. He explained the nature of the universe as an interaction of two opposing principles called love and strife manipulating the four elements, and stated that these four elements were all equal, of the same age, that each rules its own province, and each possesses its own individual character. Different mixtures of these elements produced the different natures of things. Empedocles said that those who were born with near equal proportions of the four elements are more intelligent and have the most exact perceptions.
Each sign is associated with one of the classical elements, and these can also be grouped according to polarity: Fire and Air signs are considered positive or extrovert, masculine signs; while Water and Earth signs are considered negative or introvert, feminine signs.
The three modalities
Each of the four elements manifests in three modalities: Cardinal, Fixed and Mutable. As each modality comprehends four signs, these are also known as Quadruplicities. They are occasionally referred to as crosses because each modality forms a cross when drawn across the zodiac. Christian astrology relates the three qualities to the three aspects of God in the trinity.