Themis wrote:Well, I am not sure anybody today has the right to look down on Ptolemy's assertions and assumptions as being 'really poor'. They turned out to be wrong in the end, but that's not the same thing at all, and it took many centuries before it was possible to be reasonably sure that he was wrong. At the time, and on the basis of what was then known, he was an admirably clear thinker and a very able mathematician.
Maybe it would be more accurate to say it would be poor reasoning with what we know today.
Well, yes. If you have excellent physical and observational reasons to think that the sun is so much bigger that the planets that they are, in effect, in orbit about it, and if you understand the physics of gravitation and of the motion of planets in elliptical orbits you won't start, as Ptolemy did, by assuming that the earth is stationary at the centre of the cosmos. If Newton had known what we know now, he would not have constructed his dynamics the way he did, based on a steadily flowing universal time. If we knew what cosmologists will know in a couple of centuries, Stephen Hawking would have known that he was wrong about ... something,
But that knowledge is the fruit of many centuries of careful observation, and argument about what model or models of the cosmos makes best sense of observation and is consistent with current knowledge of how the universe works. Ptolemy's work on mathematical astronomy, commonly called the Almagest, is the most ancient surviving example of a careful and consistent attempt to unite two things:
1. A physical model of the cosmos, in his case based on a spherical earth that is not moving, and on circular motion of the heavenly bodies. These were not arbitrary or baseless assumptions in the context of his time. (Just try imagining that somebody starts asking you why we can't feel that the earth is spinning daily and moving through space at high speed: you won't find it an easy task to explain away their objections. Dammit, there are even people who still think the earth is flat - a notion that Ptolemy disposed of very carefully and thoroughly early on in his book.)
2. A large body of observations, some of them from ancient Babylon, and some from nearer his own day, to which Ptolemy applied some quite difficult trigonometry to derive such basic constants as the radii of the various orbital circles involved, and their speeds of rotation.
Ptolemy's theory was clearly structured and observation-based. It made predictions that were clear and hence subject to testing (a basic requirement for any piece of science). It was changed and developed by later astronomers for centuries (much of that work took place in the Arab-speaking world), and when Copernicus proposed his new model it used nearly all the theoretical structures that Ptolemy had built. We might go so far as to say no Ptolemy, no Copernicus. Hence no Galileo, no Newton, no Einstein and so on. (That's a simplification, of course, but there is some truth in it.)
Science is cumulative and critical. We go forward from one stepping stone to another, and Ptolemy's work was a large and firm stone that helped us advance in our journey to ... where?
And back to the OP: astrology was not a stupid idea once. Now, as you rightly say "it would be poor reasoning with what we know today".
