At the beginning of my post on the rise and fall of astrology I said that a full explanation would run to a full-length book or more and I attempted a short but informative sketch of history. Unfortunately the comments show that in at least one point I was too brief or too confusing and that I had completely ignored another aspect. This is an attempt to plug the holes in the dyke before the flood of ignorance brings the whole structure to collapse.
The first point is by and away most important as it concerns what I see as the main reason for the fall from grace of astrology in the second half of the 17th century. From the comments it is clear that Rebekah Higgitt is not clear what I was trying to say and if Becky who is a historian of science with the necessary background knowledge is not sure what hope do mere mortals have?
The whole subject hinges on a radical change of metaphysics that evolved over the second half of the 16th century and the whole of the 17th and also underlay that which is known as the scientific revolution. The metaphysics that mediaeval Europe inherited from the Greeks and which was renewed in the Renaissance was largely based on the philosophies of Aristotle and to a lesser extent his teacher Plato. Although teacher and pupil differed in many aspects of their thought they were united in their concept of the universe. Their universe was split into two spheres the celestial or heavenly and the terrestrial or earthly. These two spheres were fundamentally different the earthly sphere was constructed from four elements, earth, water, fire and air, was corrupt and subject to change and knew two forms of motion, natural straight towards the earth’s centre and all other motion which was per definition violent. The heavenly sphere was constructed of a fifth perfect element, the quintessence, was harmonious and unchanging and knew only one form of natural, perfect motion, circular. There are more details but they needn’t bother us here. These spheres were separated by the moons orbit and n’e’r the twain shall meet. However the perfect celestial sphere was reflected in the imperfect terrestrial sphere, which was so to speak a badly made model of its heavenly twin.
This situation led to the Renaissance philosophy of micro-cosmos/macro-cosmos or as above so below and the concept of celestial influence. This is the belief that the events in the upper world or celestial sphere influence the events in the lower world or terrestrial sphere and provided the underpinning for the belief in astrology and the acceptance of astrology as a legitimate system of knowledge. During the 16th and 17th centuries the developments in natural philosophy slowly dismantled this system of metaphysics and in doing so dissolved the concept of the spheres. The whole universe was constructed of the same substance(s) and motion was the same above and below the moon no longer natural to its sphere but the result of forces (whatever they may be). The final move in this process was Newton’s theory of gravity, which is a universal theory, which means it applies not only on the earth but on and between all the planets and even on comets the jewel in the crown of Newton’s theory.
With the collapse of the difference between the spheres the whole of the micro-cosmos/macro-cosmos philosophy goes out the window and with it the underpinning for astrology. Interestingly this change was at least partial guided by the adoption of two other Greek philosophies those of the Stoics and the Atomists whose metaphysics had never included the difference between the two spheres.
The second point turned up in a discussion in the comments on to what extent the failing empirical confirmation for astrology influenced its decline as an academic discipline. On the whole I don’t think it did. Astrology experienced its high point in European history during the Renaissance and its leading practitioners where largely serious scientist who made significant contributions to the evolution of the natural sciences in the Early modern Period, people like, Regiomontanus and his teacher Peuerbach, Cardano, Phillip Apian, Rheticus, Tycho, Galileo and Kepler. All of them were painfully aware of the empirical deficiencies of astrology and all of them undertook projects to try and remove those deficiencies. These projects were actually the main driving force behind the astronomical revolution and the creation of modern meteorology amongst other things. All of their efforts were in vain and they failed totally in their attempts to establish astrology as an empirical science, however I know of no single case of one of them giving up his belief in astrology. First with a change in the underlying metaphysics and a new generation did astrology loose its academic status.