In its modern form astrology asserts that the positions of the solar planets at the time an individual is born are somehow correlated with his or her personality, activities, preferences, and even major life events (accidents, marriages, divorces, etc.). There is no general agreement among astrologers as to how or why this can be. Nor is there agreement as to precisely which planetary positions lead to which specific traits or experiences. It is almost certain that no two astrologers will "cast" an individual's horoscope with precisely the same result. The predictions that do result are often so vague that verification is impossible, anyway.
Astrology is best understood by learning how it began. Like most urban, agricultural peoples, the Babylonians had a pantheon of many gods. They also had a well- developed science of observational astronomy, which served the highly utilitarian purpose of providing a calendar, times to plant and to harvest, times of religious festivals, etc. In this observational scheme each planet was important, and the priests whose task it was to make the observations named the planets for the gods in their pantheon -- Marduk, Isthar, Nergal, etc. By about 1000 B.C. there was an extensive Babylonian literature of "planetary omens." Since Nergal (Mars) was the god of war, a summer in which Nergal shone down brightly from the sky was a good time to wage war (or a time in which risk of war was great). Since Ishtar (Venus) was the goddess of love, a spring night in which Ishtar shone high in the West after sunset was a good time to make love.