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by jjk166
273 days ago
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No it didn't. Epicycles were from the get go nothing but an attempt to fit a mathematical function to observed data to predict future positions of planets. It's a geometric method of curve fitting which is a weaker form of the fourier series, and the system was developed by greek mathematicians trying to improve upon Babylonian computations that didn't even have a geometric model. There is a reason that the moon, the only thing in the cosmos that does in fact orbit the earth, has the most complicated series of epicycles to describe its motion. Ptolemy rejects Aristotle's cosmology which relied on perfect spherical motion. Ptolemy really did believe that the planets moved according to his model (ie it wasn't just a pure computational tool) but he was very clear that his model was based purely on mathematics. Not only did he not give a reason for why the cosmos should take this form, he openly speculates that the answer is unknowable, and works under the assumption "maybe they can move wherever they want and they just like moving this way." Further, cycles were not added over time [1]. On day one there were 31 cycles and circles, and these were exactly the same ones being used at the time of Copernicus. You also don't need many epicycles to accurately produce a path identical to keplerian orbits. Completely arbitrary orbits can be described with finite epicycles. [2] Indeed the problem was that Ptolemy didn't fit the data by adding more epicycles, but instead through the Equant, which moved the positions of the centers of the epicycles, which meant adding more epicycles would not make it more accurate. The story of ever more epicycles being added to a bloated old theory that was streamlined by heliocentrism is a modern myth. [1] https://diagonalargument.com/2025/05/20/from-kepler-to-ptole... [2] https://web.math.princeton.edu/~eprywes/F22FRS/hanson_epicyc... |
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That’s a count of the total need to describe the motion of multiple celestial bodies.
I’m referring to the number of cycles needed to describe the motion of a single celestial body. There wasn’t enough data at high enough precision to need 17 cycles to describe the motion of a single celestial body until much later. At the time lesser precision was more common, but that someone really did go to such an extreme to create the best fit.
> Completely arbitrary orbits can be described with finite epicycles.
The number of points isn’t fixed with continuous observations. Your best fit for past data keeps needing new cycles over time unless you’re working backwards from a much better model. Even then you run into issues with earthquakes changing the length of the day etc. The basic assumptions they where working from don’t actually hold up.
Also, I’m reasonably sure you couldn’t actually write out an infinite decimal representation of the irrational number e using a finite number of epicycles. Not something I’ve really considered deeply, but it seems like an obvious counter example.