Philosophy 167: Class 5 - Part 8 - Dialogue Concerning the Two Chief World Systems: Aesthetic Arguments for Copernicanism.

Smith, George E. (George Edwin), 1938-
2014-09-30

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Synopsis: Describes the virulent debate between proponents of the Ptolemaic and Copernican systems.

Subjects
Astronomy--Philosophy.
Astronomy--History.
Philosophy and science.
Celestial mechanics.
Galilei, Galileo, 1564-1642. Saggiatore.
Genre
Curricula.
Streaming video.
Permanent URL
http://hdl.handle.net/10427/012812
Original publication
ID: tufts:gc.phil167.68
To Cite: DCA Citation Guide
Usage: Detailed Rights
view transcript only

This is The Assayer. It's said to be a reply to Lothario Sarsi. The actual person it's a reply to is named Father Grassi SJ, he was a young Jesuit astronomer who wrote a very nice book attacking Copernicanism very carefully, and part of the attack is focusing on comets.
So it started with a relatively innocent publication by Father Grassi under a pseudonym. He was a priest. He shouldn't be involved in things like this too publicly. So under a pseudonym and Galileo replies under a pseudonym that he's wrong about the comet of 1618. Then Father Grassi pulls out his guns and starts firing big time at which time Galileo comes back and writes The Assayer which is almost as virulent a polemic as the two chief world systems, okay?
Now what's Galileo defending in this? It's usually said The Assayer is one of the first books in the philosophy of science because it describes scientific method page after page after page. Yeah, in defense of what? That comets are not celestial phenomenon at all, they're sublunary phenomenon in our atmosphere, with just a virulent attack on anybody who would say otherwise.
Including a long argument that Tycho didn't know how to do parallax at all and so his measurements were a waste of time. To which Kepler published a reply showing absolutely to the contrary. So Galileo can be fiercely, fiercely, vehemently wrong about something and be no less outspoken. Now there's a side story to this.
Young Father Grassi decided not to continue pursuing astronomy, having been attacked so virulently, etc. So instead he moved into the Vatican. And he was a major voice during the deliberations of what to do yes, what to do about Galileo, this has to be speculation, how much influence he had.
But he was a leading astronomer at the time within the Vatican. And so it would've been very natural to turn down. Yes these things come home to roost. But I put this up here trying to drive home to you, this of course is the English, I'll pass the whole series of books around.
A whole series of exchange. This is an English translation, of course, including Kepler's reply. But it shows you that Galileo could be absolutely totally wrong about something and no less vehement. He did not hold back, okay. Now for the dialogue itself. I'm not gonna give you that in English.
It's the, principle thing to notice is it's the two world systems. Ptolemaic and Copernican, with no mention of The Tychonic system, is never by name mentioned anywhere in the book. And that's intellectually a little dishonest because anybody opposed to Copernican astronomy at that point who is in astronomy itself was supporting Tychonism.
Okay. The defenders of Ptolemaic astronomy were not astronomers, any more. I'm sure there were plenty among philosophers and things like this. We don't trust these things, etc., cuz, they were gonna be very slow to give it up, particularly since it had Aristotle's imprimatur on it. At any rate, let me just describe the book.
It consists of course dialog between three people, Salvioti represents Galileo's voice himself. Simpitrio is a defender of scholasticism and not in this book, not a very bright one, okay? A knee-jerk totally dogmatic defender of scholasticism. And Segreto is a learned gentleman from Venice. There actually was such a person, so Galileo was honoring a friend by singling out this Segreto.
And he was the intermediary asking questions and he was the person Salviati was trying to convince. Day one is a systematic attack on scholastic Aristotelian philosophy as a philosophy of the natural world and it's great fun to read, it's wonderful polemics. Day two is a defense. It's a reply to all of the Ptolomeic and standard arguments against the possibility of the Earth being in motion, and includes a lot of claims about motion, including claimed experiments that Galileo had done, that you'll see next week he had probably not done.
But nevertheless it's a portion that does have a lot of anticipation of Galileo on motion qualitatively, not carefully the way we'll be reading the next two weeks. Day three then is a systematic defense of the claim, really it's the claim that the five planets go around the sun, and that the stars are very, very far away.
And the end of day three has a long defense of magnetism as presented by Gilbert. Okay, so it's the strongest defense of Gilbert I know of. And then day four which is the punch line of the whole thing, and the reason Galileo had planned to write it for years.
He thinks he has a proof that the earth is moving and therefore has a proof against Tychonism. And it comes from the tides, I'll get to that in just a moment. He had had this idea way back and this was the punch line. So he doesn't have to mention Tychonism, all he has to do and you'll see it just a moment.
So the five planets go around the sun, which is more natural, for the Earth to be going with them or the sun to be going around? That's what he does in day three, in the section I asked you to read really carefully. Then in day four he response.
All right, look at a few things. This is his model, in the book, of the Copernican system, it shows the four satellites of Jupiter, and it shows the Moon going around the Earth. And you see perfect circles, you see them spaced out very nice and clean, no epicycles to replace the equine, no complications that are actually present in Copernican astronomy.
A complete extreme simplification of Copernican astronomy. Most people who read this book, to this day, do not know that Copernicus actually had epicycles on every one of those orbits and had to have them in order to match Ptolemy. That's part of the dishonesty of this book. Okay. It's portraying something that nobody that just was completely unrealistic.
There's nothing like this that he;s showing. His defense and occurs very late in day four, when asked about the actual motions, we cannot yet determine, surely the law, the Italian word is governy, not lagile. I'll come back to that. The law of revolution and the structure of the orbit of each planet, the study ordinarily called planetary theory, witness to this fact is Mars, which has caused modern astronomers so much distress.
Numerous theories have also been applied to the moon itself, since the time when Copernicus first greatly altered Ptolemy's theory. Now look, this is a man sitting there with all of Keppler's books, given to him complimentary by Keppler. And his view is the state of mathematical astronomy was such that its total dispute what the actual orbits were.
Now you're gonna see in this second half it was not a dispute among astronomers anymore. It was ellipses by the time this book was published. Now that's unfair. The book was drafted before the conclusion was reached from the transit of Mercury. But very soon after this the state was very different.
But in effect what Salviati is doing is ducking the issue by saying you don't have to put all the bells and whistles in there. Cuz we don't really know what the trajectories are. That's something that mathematical astronomy hasn't reached yet. Now, how could he say that? There are various possibilities flat dishonesty is one possibility, another is he didn't know enough mathematical astronomy to appreciate.
There's no sign he never did significant mathematical astronomy, okay or even had the skills to do it. Okay? That means training, it doesn't mean he couldn't have learned, obviously. You know you suit yourself as to what the explanation is, it's just not clear. I'm gonna go on in order to save myself time in the second half.
This is the main argument, it runs eight pages, I've simply pulled out the key. So this is the part I asked you to read in detail. To Mercury and Venus, you, Simpitrio have attributed a circular motion around the sun, without embracing the earth. Around the same sun, you have caused the three outer planets, Mars, Jupiter, and Saturn, to move, embracing the Earth within their circles.
What's the point? They don't show faces. So it's they're going around both the sun and the Earth. They're circumnavigating both. Next the moon can not move in any way except around the Earth and without embracing the sun. It now remains to a portion three things among the sun, Earth, and stellar sphere.
The state of rest, which appears to belong to the Earth, the annual motion through the zodiac, which appears to belong to the sun, and the diurnal movement, which appears to belong to the stellar sphere. With all the rest of the universe sharing it accept no Earth. And since it is true that all planetary orbs, I mean Mercury, etc., move around the sun as a center, it seems most reasonable for the state of rest to belong to the sun rather than the earth just as it does for the center of any moveable sphere to remain fixed rather than some other point remote from the center.
Next as to the Earth which is placed in the midst of moving objects, I mean between Venue and Mars. One of which makes its revolution in nine months and the other in two years. A motion requiring one year may be attributed to it much more elegantly than a state of rest, leaving the latter to the sun.
And such being the case, it necessarily follows, that the diurnal motion tube, belongs to the Earth. For if the sun stood still and the Earth did not revolve upon itself, but merely had the annual movement, our year would consist of no more than one day and one night.
That is six months of day and six months of night. Okay. That's the basic argument in day three. It's supplemented by a lot of observations of stars and how far away they have to be and things like that. But I take it that's not really an empirical argument, it's essentially an aesthetic argument for eliminating Tychonism.
We're granting the five planets go around the sun, but we're not given any direct evidence, other than my italics, as you see, it being most reasonable.