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> the evidence for Riccioli’s system is weighing down the scale-pan, while Galileo’s less substantiated suggestion rises upward What exactly is the difference

> the evidence for Riccioli’s system is weighing down the scale-pan, while Galileo’s less substantiated suggestion rises upward What exactly is the difference between the two theories? It would be interesting to see them both in the context of the time.

rcxdude 2y ago
Not all that much: they both suffered from assuming that everything was perfect circles, so they needed nested epicycles to match observations. Galileo's model actually needed more than Riccoli's (one reason why it was not so popular). They both predicted the observed motions of planets equally well, the two main differences were the nature and position of the stars and whether the earth or sun were at the center, Those two are related: if the earth is at the center, then you can put the stars more or less wheverever you want in the model and things don't change. If the sun is at the center, you need to put the stars very far away because the observations of the day didn't show any parallax of the stars due to the earth's rotation around the sun. This was unpopular because both telescopic and naked eye observations of the stars appears to show them as a disk (about the same size as Saturn) instead of a point, which would mean stars would need to be even larger than the solar system to match observations. It was only centuries later that this was shown to be an optical illusion. The other objection was the Galilean model also requires the earth to rotate around its own axis, which was also an unpopular idea for a variety of reasons, chiefly it creates a bunch of questions which need a good understanding of ideas like mass, velocity, and momentum to answer, and it wasn't until Newton that all that was really put into a good framework. The most robust argument was related to coriolis effects: if the earth is rotating, you would expect that a dropped object would not fall straight down, but instead land slightly west of the point where you dropped it. People had tried this experiment and did not see the effect (mostly because it's really small). So Galileo more or less could not really produce an experiment which his model could explain and Riccioli's model could not, and for the areas where it differed he could only really shrug his shoulders and say 'maybe the effect is too small to measure'. (Not for lack of trying: apart from the dropping weights experiments done by others, Galileo also attempted to measure parallax of two stars that are close together in the sky. Unfortunately he picked a binary system where the stars are also close together in distance, and so he didn't see anything. He also had an argument based on the tide, which was apparently what originally convinced him of the idea. This argument was unfortunately obviously wrong in some important ways: he thought that the tides were due to the earth spinning and 'pulling along' the oceans, and his model worked out that the tides would rise and fall once a day, in contradiction to the experience of every sailor at the time)

rcxdude 2y ago

Not all that much: they both suffered from assuming that everything was perfect circles, so they needed nested epicycles to match observations. Galileo's model actually needed more than Riccoli's (one reason why it was not so popular). They both predicted the observed motions of planets equally well, the two main differences were the nature and position of the stars and whether the earth or sun were at the center, Those two are related: if the earth is at the center, then you can put the stars more or less wheverever you want in the model and things don't change. If the sun is at the center, you need to put the stars very far away because the observations of the day didn't show any parallax of the stars due to the earth's rotation around the sun. This was unpopular because both telescopic and naked eye observations of the stars appears to show them as a disk (about the same size as Saturn) instead of a point, which would mean stars would need to be even larger than the solar system to match observations. It was only centuries later that this was shown to be an optical illusion. The other objection was the Galilean model also requires the earth to rotate around its own axis, which was also an unpopular idea for a variety of reasons, chiefly it creates a bunch of questions which need a good understanding of ideas like mass, velocity, and momentum to answer, and it wasn't until Newton that all that was really put into a good framework. The most robust argument was related to coriolis effects: if the earth is rotating, you would expect that a dropped object would not fall straight down, but instead land slightly west of the point where you dropped it. People had tried this experiment and did not see the effect (mostly because it's really small). So Galileo more or less could not really produce an experiment which his model could explain and Riccioli's model could not, and for the areas where it differed he could only really shrug his shoulders and say 'maybe the effect is too small to measure'. (Not for lack of trying: apart from the dropping weights experiments done by others, Galileo also attempted to measure parallax of two stars that are close together in the sky. Unfortunately he picked a binary system where the stars are also close together in distance, and so he didn't see anything. He also had an argument based on the tide, which was apparently what originally convinced him of the idea. This argument was unfortunately obviously wrong in some important ways: he thought that the tides were due to the earth spinning and 'pulling along' the oceans, and his model worked out that the tides would rise and fall once a day, in contradiction to the experience of every sailor at the time)