The Galileo mystery.

[ollypenrice]

I have never understood this. In his Dialogue Concerning the Two Chief World Systems Galileo's mouthpiece, Salviati, ridicules the old Ptolemaic system by saying,  'To save appearances, Ptolemy introduces vast epicycles... all of which can be done away with by one very simple motion of the earth.' *   But they can't. Copernicus' De Revolutionibus is full of epicycles which, as we now know, were needed to replicate the elliptical orbits of the planets. His much earlier manifesto, the Commentariolus is indeed epicycle-free but he couldn't get it to hold up to close observational scrutiny.

Why does Galileo gloss over the epicycles in De Revolutionbus? His attention to detail in demolishing Ptolemy is meticulous. The observed size of Mars, the phases of venus, etc, are explored in detail. Not only is this mysterious, it's also ironic. Galileo's strident advocacy of De Revolutionibus got it suspended (not banned) by the church and yet Galileo seems not to have read it!

Very odd...

Olly

* The full passage is here.

SALV. The illnesses are in Ptolemy, and the cures for them in Copernicus. First of all, do not all philosophical schools hold it to be a great Impropriety for a body having a natural circular movement to move irregularly with respect to its own center and regularly around another point? Yet Ptolemy's structure is composed of such uneven movements, while in the Copernican system each movement is equable around its own center. With Ptolemy it is necessary to assign to the celestial bodies contrary movements, and make everything move from east to west and at the same time from west to east, whereas with Copernicus all celestial revolutions are in one direction, from west to east. And what are we to say of the apparent movement of a planet, so uneven that it not only goes fast at one time and slow at another, but sometimes stops entirely and even goes backward a long way after doing so? To save these appearances, Ptolemy introduces vast epicycles, adapting them one by one to each planet, with certain rules about incongruous motions--all of which can be done away with by one very simple motion of the earth. Do you not think it extremely absurd, Simplicio, that in Ptolemy's construction where all planets are assigned their own orbits, one above another, it should be necessary to say that Mars, placed above the sun's sphere, often falls so far that it breaks through the sun's orb, descends below this and gets closer to the earth than the body of the sun is, and then a little later soars immeasurably above it? Yet these and other anomalies are cured by a single and simple annual movement of the earth.

[ian_bird]

Son - you're on your own.

(Blazing Saddles in case you missed it.)

Cheers

Ian

[Putaendo Patrick]

Your point is very interesting, and I'm certainly not expert enough nor familiar enough with the theme to give a substantial answer.

Galileo was interested in the ebb and flow of tides but his original title was reduced to Dialogues by the Inquisition. Your title only ocured much later. So perhaps it is orientated more on an Earth-based argument?

Given that Galileo was under considerable pressure from the Inquisition, perhaps he felt it more prudent to concentrate on demolishing the standing of "old school" thinking to give space for new ideas rather than launching into Copernicus?

I have little doubt that Galileo knew he was publishing a controversial document, so my feeling is that he tempered it according to the political/religious limitations imposed in his day. First destroy Ptolomy, then there's space to construct an alternative?

[sharkmelley]

I think the main point being argued was whether or not the earth is in motion.  Once you accept that the earth together with the other planets are in motion around the sun (but in circular orbits) then the epicycles to fit the observations become  much smaller.   Later on, when you allow elliptical orbits, the epicycles disappear entirely.

Mark

[ollypenrice]

Your point is very interesting, and I'm certainly not expert enough nor familiar enough with the theme to give a substantial answer.

Galileo was interested in the ebb and flow of tides but his original title was reduced to Dialogues by the Inquisition. Your title only ocured much later. So perhaps it is orientated more on an Earth-based argument?

Given that Galileo was under considerable pressure from the Inquisition, perhaps he felt it more prudent to concentrate on demolishing the standing of "old school" thinking to give space for new ideas rather than launching into Copernicus?

I have little doubt that Galileo knew he was publishing a controversial document, so my feeling is that he tempered it according to the political/religious limitations imposed in his day. First destroy Ptolomy, then there's space to construct an alternative?

I think Galileo was being anything but prudent. He got himself into trouble by being highly provocative in the Dialogues. The best example would be his giving one of the Pope's suggestions to Simplicio, the simpleton amongs the Dialogue's characters, to expound. (The Pope had suggested that the Copernican model could be considered an excellent computational device rather than a description of reality. Giving that argument to Simplicio to propose can only be seen as 'asking for it!')

Copernicus' De Revoluionibus, which his seniors in the Catholic church had urged him to publish, had co-existed with the church for 73 years before Galileo's advocacy of it (or his remarkably sloppy version of it) got the theory into doctrinal difficulty. It's certainly worth asking whether the quarrel had more to do with Galileo personally than with heliocentricity. It's conflict with scripture was very slight and far more significant 'heresies' had been happily ignored.

Olly