Astrology is not a science in the sense that physics or chemistry are sciences. In the natural sciences, we try to make logical correlations between our observations of physical phenomena. We try to find rational explanations called theories and often attempt to predict behavior of a system by applying a theory. Theories that do not give reliable explanations of a phenomenon or that can not accurately predict a phenomenon are thrown out.
Physical sciences presume that there is a cause and effect that can be explained by theories. As physical sciences mature, scientists are usually able to refine the therories into models that explain links that relate the effect to the cause.
Ancient man believed that the positions of stars might influence men's lives. Thus at one time astrology was treated like a science because astrologers looked for correlations between the stars and the events in men's lives. Astronomy is the study of heavenly bodies. At that time astronomy consisted in charting, calculating and predicting the locations of stars and planets. Thus the early Greeks who tried to make astrology into a science were good astronomers.
The early astronomers could see the sun, the moon, four planets, and twelve constalations they called the signs of the zodiac. Other stars and constellations were also visible, but they were ignored for the purpose of astrological investigations. Now consider the number of personality traits people display and the number of good and bad things that can happen to a person. Furthermore, some of these things that happen can be attributed to war, nutrition, accidents, education or any of a number of other factors that may not be influenced by the stars. Pity the early astrologers. They were attempting the Herculean task of correlating all of this information.
Modern scientists working in relatively simple fields like physics or chemistry plan their experiments so that they are trying to correlate as few variables as possible. Ideally, a scientist is trying to correlate two phenomenon. The early astrologers simply were attempting the impossible. Even with modern computers, we would be hard pressed to systematically correlate the positions of stars and planets with human behavior and the fortunes of individuals.
Of course, astrologers had to eat. Thus they had to produce something to encourage their patrons to keep supporting them financially. The astrology we know today is a collection of mythical explanations of human behavior as influenced by the positions of planets with respect to the signs of the zodiac. These myths were crafted to provide the information the patrons desired. While explanations are based on daily positions, the most important set of astronomical positions is where planets and stars were at the moment of a person's birth. Astrologers never could explain how the stars control our lives. They only could come up with fanciful explanations of what would happen in people's lives if the stars were in certain locations.
It is highly improbable that ancient astrologers could have reliably correlated enough information to have developed a reliable scientific data base. Even if a data base of observations were available, the myths and explanations the astrologers crafted have little chance of being reliable. Most astrology is simply tall tails that are carefully crafted so that almost any personality trait or almost any fortune of man can be explained by one or another of these vague explanations.
In my mind, the most convincing bit of information is that astronomers tell me that the positions of the signs of the zodiac have shifted since the days of the early Greeks. By some estimates, the constellations we identify with the signs of the zodiac are as much as 180 degrees out of phase with the charts used by astrologers. If by chance, astrology was a valid science in ancient Greece, it is no longer valid because of the shifts in the positions of the signs of the zodiac.
At one time astrologers may have been scientists, but modern astrologers are little more than story tellers.