The Copernican Revolution from the Inside
score: 16 (8 votes) ·
I feel like you're leaving out some arguments against the Ptolemaic model. As I understand it, Galileo wrote his dialogue at the suggestion of the pope who wanted to have a nice pro and cons list. The fact that the pope was even considering heliocentrism tells me that there must have been big problems with the geocentric view. Why would the head of a very conservative organisation (even if he was more on the open-minded end of the spectrum) entertain a new theory if the old theory is perfectly fine? And indeed Wikipedia tells me that the Ptolemaic model could not explain the observed phases of Venus and the motion of sunspots.
The motion of sunspots brings me to another topic. I think this paragraph is a bit misleading:
Moreover, Galileo’s observations of sunspots and moon craters weren’t unproblematic. In both cases there is evidence to indicate that he was fooled by optical illusions. And though he was also right about the existence of moons orbiting Jupiter, which contradicted the uniqueness of the earth as the only planet with a moon, what he actually observed rather seems to have been Saturn’s rings (Ladyman, 2001) .
The sunspots were at least known since 300 BC. And I can't imagine how you can mistake Saturn's rings for Jupiter's moons. I think what your source is saying is that he mistook Saturn's rings for moons of Saturn which is an entirely understandable mistake.
So, the Ptolemaic model definitely had problems and if I learned anything about humans it's that those problems were probably being ignored for too long. Wikipedia also tells me that at the time of Galileo the Tychonic model was actually quite popular because it solved so many problems of the Ptolemaic model. So, the question is, was it irrational of Galileo to prefer the Copernican model over the Tychonic model (given the data that he had)?
I wouldn't say so. Galileo rightly saw the Tychonic model as a weak compromise that didn't dare to go all the way. Sure, the parallalax was a problem but you can Defy the Data if you have a strong prior. If we steelman Galileo just a bit then his accomplishment was realizing that it's quite possible that the Earth is moving (you ordinarily woulnd't notice the difference) and thus, you should prefer a simple theory with a moving Earth over a more complicated theory with a stationary Earth.
A modern day example of sticking to the prior in the face of contrary evidence is this article by Bryan Caplan.