Astrology is a type of divination that consists in interpreting the influence of planets and stars on earthly affairs in order to predict or affect the destinies of individuals, groups, or nations. At times regarded as a science, astrology has exerted an extensive or a peripheral influence in many civilizations, both ancient and modern. Astrology has also been defined as a pseudoscience and considered to be diametrically opposed to the theories and findings of modern science.

Astrology originated in Mesopotamia, perhaps in the 3rd millennium BC, but attained its full development in the Western world much later, within the orbit of Greek civilization of the Hellenistic period. It spread to India in its older Mesopotamian form. Islamic culture absorbed it as part of the Greek heritage; and in the Middle Ages, when western Europe was strongly affected by Islamic science, European astrology also felt the influence of the Orient.

The Egyptians also contributed, though less directly, to the rise of astrology. They constructed a calendar, containing 12 months of 30 days each with five days added at the end of the year, that was subsequently taken over by the Greeks as a standard of reference for astronomical observations. In order that the starry sky might serve them as a clock, the Egyptians selected a succession of 36 bright stars whose risings were separated from each other by intervals of 10 days. Each of these stars, called decans by Latin writers, was conceived of as a spirit with power over the period of time for which it served; they later entered the zodiac as subdivisions of its 12 signs.

In pre-Imperial China, the belief in an intelligible cosmic order, comprehended aspects of which would permit inferences on correlated uncomprehended aspects, found expression in correlation charts that juxtaposed natural phenomena with the activities and the fate of man. The transition from this belief to a truly astrological belief in the direct influence of the stars on human affairs was slow, and numerous systems of observation and strains of lore developed. When Western astronomy and astrology became known in China through Arabic influences in Mongol times, their data were also integrated into the Chinese astrological corpus. In the later centuries of Imperial China it was universal practice to have a horoscope cast for each newborn child and at all decisive junctures in life.

Once established in the Classical world, the astrological conception of causation invaded all the sciences, particularly medicine and its allied disciplines. The Stoics, espousing the doctrine of a universal "sympathy" linking the microcosm of man with the macrocosm of nature, found in astrology a virtual map of such a universe.

Greek astrology was slow to be absorbed by the Romans, who had their own native methods of divination, but by the time of Augustus, the art had resumed its original role as a royal prerogative. Attempts to stem its influence on the populace met repeatedly with failure.

Throughout pagan antiquity the words astronomy and astrology had been synonymous; in the first Christian centuries the modern distinction between astronomy, the science of stars, and astrology, the art of divination by the stars, began to appear. As against the omnipotence of the stars, Christianity taught the omnipotence of their Creator. To the determinism of astrology Christianity opposed the freedom of the will. But within these limits the astrological worldview was accepted. To reject it would have been to reject the whole heritage of classical culture, which had assumed an astrological complexion. Even at the centre of Christian history, Persian magi were reported to have followed a celestial omen to the scene of the Nativity.

Although various Christian councils condemned astrology, the belief in the worldview it implies was not seriously shaken. In the late European Middle Ages, a number of universities, among them Paris, Padua, Bologna, and Florence, had chairs of astrology. The revival of ancient studies by the humanists only encouraged this interest, which persisted into the Renaissance and even into the Reformation.

It was the Copernican revolution of the 16th century that dealt the geocentric worldview of astrology its shattering blow. As a popular pastime or superstition, however, astrology continued into modern times to engage the attention of millions of people, this interest being catered to in the 20th century by articles in the daily press, by special almanacs, and by astrology manuals.

Excerpt from the Encyclopedia Britannica without permission.