Wednesday, May 18, 2011

Galileo's Conversion

I discuss a fascinating new paper by Owen Gingerich and Albert van Helden at Forbes.


TheOFloinn said...

It's the connection between "there are moons that go around Jupiter" and therefore "the earth moves with a dual motion around the sun" that is unclear.

At best it shows that not every heavenly body lies on a circle centered on the earth. But the Ptolemaic system might be false without the Copernican system being true. After all, the Tychonic system was much favored by astronomers between the demise of the Ptolemaic (done in by the phases of Venus) and the triumph of the Keplerian. The Tychonic and Ursine models gave predictions as accurate as the Copernican (which had, remember, about 20 epicycles still because it required circular orbits and was based on corrupted data.)

John Farrell said...

True, although it's easier said in hindsight. I'd be interested in doing some follow-up on precisely which contemporaries were in fact moving to the Tychonic system. I'll ask Owen about this.

TheOFloinn said...

There is useful stuff here:

John Farrell said...

I discussed this briefly with Owen Gingerich (without getting into Kuhn).

He writes:

"Galileo probably thought the Tychonian system was too gerry-rigged to be worth the time of day; also he would have realized that the Tychonic system was a favorite of the Jesuits, and he might have chosen to thumb his nose at them by giving them the silent treatment (rather than stirring up a hornet's nest). The Tychonic System gave better results than the Prutenic Tables (Copernican) for the sun and moon because Tycho had worked very hard to get better parameters. But he had done nothing to support planetary calculations, and Ursus had nothing at all. It was Kepler's genius to take the Tychonic data and turn them into tables hugely more accurate than anything that had gone before. The Copernican system as we know it is actually the Keplerian system. Kepler chided Galileo for ignoring Tycho's system, but he himself realized that it made no sense physically."

TheOFloinn said...

That last bit is important. In a certain sense, the idea that astronomical mathematics ought to "make sense physically" was the real scientific revolution. Astronomy was a branch of mathematics, not of physics, and few regarded the the various computational contrivances as physically real. The only criterion was the accuracy of the predictions: the positions of the stars, the occurrence of eclipses, the spring equinox, etc. Hence, the official job title for an astronomer was "mathematicus."

The physics of Aristotle and the astronomy of Ptolemy were mutually contradictory. Moderns just sort of live with the contradiction between relativity and quantum theory, but the ancient resolution was to regard the physics as physics and the astronomy as mathematics. Two different realms of thought.

Imho, the revolutionary consequence of the telescope was that Europeans began to regard the heavens as a physical place in which one could discover new facts. The weird thing was that no such shift in POV occurred in China (when Jesuits brought the telescope there) or in dar-al-Islam when they obtained telescopes, too. Toby Huff has a fascinating new book out that covers this: Intellectual Curiosity and the Scientific Revolution.

It is almost impossible for a modern to look at the Tychonic/Ursine system(s) without an impression of a Rube Goldberg machine. A devotee of the Copernican system would surely have had the same reaction. "It's ugly." To which the empiricists might reply: "Sure, but it works." In fact, the outer planets really do go around the earth, and the sun and the inner planets can be regarded as doing so with only a shift in reference frame! And relativity tells us that no acceleration reference frame is privileged. LOL. If you don't regard astronomical calculations as being necessarily physically real, and focus only on the accuracy of the predictions....