Philosophy 167: Class 5 - Part 9 - Dialogue Concerning the Two Chief World Systems: Galileo's Theory of the Tides.

Smith, George E. (George Edwin), 1938-


  • Synopsis: Describes Galileo's theory of tides and how it implies that the Earth rotates.

    Opening line: "Then we get to day four and this is again the summary of the argument."

    Duration: 9:14 minutes.

    Segment: Class 5, Part 9.
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Then we get to day four and this is again the summary of the argument. I'll give you the motion in just a moment. This we have already examined at length by showing that all terrestrial events from which it is ordinarily held that the Earth stands still and the Sun and fixed stars are moving, would necessarily appear the same to us if the Earth moved and the other stood still.
Heard that? All the events appear the same. Among all sublunary things, it is only in the element of water as something which is very vast, and is not joined and linked with the terrestrial globe as are all its solid parts, but is rather because of its fluidity, free and separate and a law onto itself.
I meant to tell you by the way, the translation of law back there where I showed you the correct Italian, Stillman Drake has the unfortunate fault that whenever he has the option of translating Galileo to make him look more like a 20th century science, rather than less, he always makes him look like more.
The word law did not become used in the way he used it on that prior slide til after Descartes. And you saw, it wasn't the Italian word law, it was the Italian word governing. So I, even here, I'm invoking this just to tell you, you can't entirely trust this Stillman Drake translation.
I'll tell you next week why he does that, he has reason to. After having many times examined for myself the effects and events, partly seen and partly heard from other people, which are observed in the movements of the water. After moreover having read and listened to the great follies, which many people have put forth as causes of these events.
A few pages later, he cites Kepler for his purile thinking of saying the moon causes the tides. That's the reference to the follies of the many people, that's one of many. I have arrived at two conclusions which are not likely to be drawn and granted. Certain necessary assumptions having been made, that's a great qualifier.
These are that if the terrestrial globe were removable, the ebb and flow of the oceans could not occur naturally. And that when we confer upon the globe the movements just assigned to it, the seas are necessarily subjected to an ebb and flow, agreeing in all respects with what is to be observed in them.
Sagredo. The proposition is crucial, both in itself and in what follows as a consequence. Therefore, I shall be so much the more attentive in listening to its explanation and verification. And now in questions of natural science like this one at hand, a knowledge of effects is what leads to an investigation, and discovery of the causes.
Without this, ours would be a blind journey, etc. What's his theory of the tides? The Earth rotates, the Earth moves around the sun. That means at any moment, the actual, quote, absolute speed of the surface on the Earth, on one side of the Earth, is faster than on the other side.
On one side it's subtracting from the translational motion, the rotation is. On the other side it is adding. That causes the seas to locally slosh. Let me keep, I'm going to describe the whole thing. Why are there two high tides a day? Because it sets up a resonance.
And it's sloshing back and forth in just the right timing to do two a day. Why is there a monthly effect? Because the Earth and the Moon are actually like the little device you put on to tune clocks. They're moving around their common center of gravity, which is true, but that produces a second effect that gives you a monthly consequence.
And I forget off-hand why there's an annual effect. But it's all sitting there. And the point being made, this is the only possible natural explanation of the tides. Nothing else will do. The others do such things as have the moon acting from a distance, that's crazy, etc.
No, I'm serious, that's the view that's put in there.
I assigned it to read. You can read it for yourself. Kepler, Newton, Descartes, and Galileo all proposed theories of the tide. Says no, only one of them is much related to the one today. And even Newton's is really quite substantially off. Newton ignores the rotation of the Earth.
That's a very big deal, as a matter of fact. But it's not the rotation versus the annual motion is the rotation. I'll just do this for a moment, teach you a little physics. It's called Coriolis Forces, Newton discovered them but his name is not attached to them because he didn't publish about it.
Think of the surface of the sea and what speed it's moving. Think of the depth, the lowest point in the sea and what speed it's moving circumferentially. Those are different speeds. The consequence of viscosity is to set up vortices. And those produce currents in the ocean and have a huge effect on tides.
You should know about them. That's what dominates our weather. The same effect occurs, high altitude versus low altitude, and is why the weather is so difficult to handle, etc., it's a fundamental point. You put that together with the driving force of the Sun and the Moon, and the friction of the bottom of the seas, and you get a theory of the tides done properly for the first time in the 1770s by Laplace.
Long way away. We'll see it at the end of this course, but it's a long way away. At any rate, somebody I got to know, his name is Paolo Palmieri, wrote a dissertation defending Galileo's Theory of the Tides. It was done in England. He's obviously Italian. He's now a professor at University of Pittsburgh, HPS, and has been for quite some time.
That's history, philosophy, and science. He had an interesting background. He was an engineer, he was an engineer for Ferrari and went back to get a PhD in history of science when he was near 40. What was his specialty? The effect on racing cars of the sloshing of fuel in fuel tanks.
So, he actually was in a position to evaluate. And the dissertation, as I think you can see the number of pages, it's 150 pages. I've given you three pages. You can download it on JSTOR. You can see the table of contents here. The one quote, this is a quote from Galileo, but he's gonna defend it.
The reciprocations of movement, what I just described as resonance, just mentioned are made and repeated with greater and less frequency, that is, shorter and longer times, according to the various lengths of the vessels containing the water. In the shorter space, the reciprocations are more frequent and they are rarer in the longer, just as in the above example.
In the plumbobs, I'm not gonna read this whole thing. So this is a claim. He's done experiments to show this sloshing occurs and the frequency depends on the length of the sloshing, and the next page is showing you Paolo's reproductions of such experiments to show that he could, indeed, have carried them out and observed exactly what he said he observed.
That there is such a sloshing and it does have a definite frequency that depends on the width and the length. So, Palmieri has made the point. It's not that he's making the point that this is a correct theory of the tides. What he's arguing is Galileo was not crazy here at all.
He had something that he knew about and he tried to extend it sensibly to the phenomenon of the tides. Questions on that? If not, I'm running late, and I wanna save time. So I'm gonna move through the trial very quickly now. I mean, read the book for yourself.
It's a wonderful book to read. Just listen to me. It is not a major contribution to science. It is a huge contribution to the climate in which science was done. And the Protestant world made sure of that, for the next 300 years, actually, it's a little more than 300 years.
In 1992, the church decided, well actually in the 1880s, Leo V decided, that what Galileo said in the letter to the Grand Duchess, became the official church's position on the relation between scripture and science, almost word-for-word. And then in 1992, Galileo was formally acquitted by the Catholic Church.
Didn't do him much good. But the decision was made, and it was wrong. And there's a very nice book by Finocchiaro called Retrying Galileo, it looks at all of the attempts to reconsider him from 1633 to the present.