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Astronomy and Astrology


Most medieval thinkers assumed that the motions of the heavenly bodies affect the course of nature in the sub lunar realm. Until the twelfth century, Latin knowledge of the heavens came largely from the late Roman works mentioned earlier, all of which were not only nonmathematical but often inconsistent or at least highly fanciful. When Arab astronomy began to be known in the Europe, bringing with it knowledge of Greek mathematical astronomy, the situation changed dramatically. Diversity of day and night, the seasons, the weather, growth of plants and animals, and so forth are explained, first, by the obliquity of the ecliptic or apparent path of the sun, moon and the planets relative to the apparent rotation of the sphere of the fixed stars and then by the individual motions of the sun, moon and planets through the zodiac. Insofar as these supposedly important causal circumstances could be reliably known, physicians, attempted to take account of them in explaining human illnesses and in determining the appropriate timing of medical procedures. Alchemy also assumed the effect of heavenly emanations on the development of metals. Such theories are les evident in later university works, perhaps because they were thought to call freedom of the will dangerously into question, but even so eminent a thinker as Albert the Great took the principle of celestial causation quite seriously.

How was one to think of the science built on this view of the heavens? In Islam, Avicenna had initiated a tendency to categorize astrology as natural philosophy and astronomy as mathematics, a move that raised significant questions about relations of the two disciplines to one another. Astrology became the discipline that addressed the physics of the heavens, as well as applying this physics of heavenly influences on earth. Astronomy, built mathematical models to track the positions of the planets but it often built those models unconstrained by considerations of physical plausibility.
From the time of Plato, most natural philosophers were agreed that a spherical heaven surrounds a spherical earth, although they differed about the details. Aristotle has posited a set of such spheres, each with its own uniform motion but each also carried with the movement of the spheres surrounding it, meant to account for the observed positions of the planets through the year. From the time of Heptarchs and Apollonius, many mathematical astronomers lost hope of accurately ‘saving the phenomena’ of planetary motions using models containing only concentric uniformly rotating spheres. They therefore proposed models in which spheres rotated around centers that were not the center of the cosmos or even changed their rates of rotation. This led to division of labor over the centuries between natural philosophers seeking physically realistic theories of the heavens and mathematical astronomers proposing theories that accurately predicted planetary positions.

This break mathematical astronomy and natural philosophy led to many methodological or epistemological discussions, as well as to many efforts, over the years, to reintegrate the science of the heavens. Did mathematical astronomy take its premises from natural philosophy at least in assuming that all heavenly motions are circular because they are the motions of spheres made up of either for which such motion in natural? Or could mathematical astronomy be an autonomous science that did no need to look to natural philosophy, but could simply build mathematical theories to fit observations? Astronomers had known since the time of Heptarchs that the same planetary motions can be accounted for equally well in different ways: a model with an eccentric may give the same predictions as a model with a deferent and epicycle. In mathematics proper such as arithmetic or geometry, the foundations are axioms that are better known to the mathematicians that the theorems proved on the basis of these foundations. But in astronomy that is not necessarily that case.

It is worth pondering that the whole Aristotelian natural philosophy was built on the observation or ‘empirical fact’ that the heavens rotate once a day, carrying around the stars and planets. It was by inference from this universally observed ‘fact’ that Aristotle and Aristotelians, following a reasonable and empirical scientific method, concluded that the heavens must be composed of a fifth element, either, moved in eternal rotation by immaterial unmoved movers. The ongoing existence of systems of mathematical astronomy, in which it was apparent that the process of reasoning from observations to higher level general theory could not guarantee that the higher level theory was uniquely true, even if its predictions were accurate, no doubt served to inject a degree of ongoing caution into natural philosophy’s epistemological claims.

posted by Nikhil @ 7:07 PM