As I wrote in a recent post, I intend to write posts for each of the four days in the Dialogue Comparing the Ptolemaic and Copernican Systems1, the book that got Galileo in trouble with Rome. This, the final post about the Dialogue, is about Day Four. Here are the previous posts:
Day One: Galileo Dismantles Aristotle's Separation of Earth from the Heavens.
Day Two, Part 1: Galileo Attacks Aristotle’s Followers.
Day Two, Part 2: Galileo Insists the Earth is Spinning on its Axis.
Day Two, Part 3: Galileo on the Acceleration of Falling Bodies.
Day Two, Part 4: Galileo Channels Plato.
Day Three, Part 1: Galileo Calls Out Chiaramonti’s Manipulation of Data.
Day Three, Part 2: Galileo Praises Aristarchus and Copernicus.
Day Three, Part 3: Why the Apparent Motion of the Sunspots Supports the Copernican Model.
Day Three, Part 4: Galileo Discusses the Size of the Universe and the Seasons.
Day Three, Part 5: Galileo Discusses Magnetism.
In Day Four, by far the shortest of the Dialogue, Galileo presents his theory of the tides. For him, the tides were, along with the apparent motion of the planets and the apparent motion of the sunspots, proof of the Copernican model. Unfortunately, as I outlined in my previous post Sarpi and Galileo Were Wrong about the Tides, this theory is erroneous, and did not withstand the test of time.
So what was Galileo’s theory? As always, using Salviati as his mouthpiece, Galileo outlined the basic assumptions:
Certain necessary assumptions have been made, these are that if the terrestrial globe were immovable, 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. [p.417]
The theory is based on an analogy between the earth and a ship carrying water, essentially considering the earth to be a giant, oddly shaped bathtub with water sloshing around from variations in the motions of the earth. The variations are three-fold:
The combination of the annual revolution of the earth around the sun and the diurnal rotation of the earth about its axis creates an absolute motion within the waters of the earth, different, at any given moment, for every part of the earth. This is the primary cause of the tides.
Since this contrariety exists in the motion of the parts of the terrestrial
surface when it is turning around its own center, it must happen that in
coupling the diurnal motion with the annual, there results an absolute
motion of the parts of the surface which is at one time very much accelerated
and at another retarded by the same amount. [p.427]
As the combined earth-moon system revolves around the sun, there will be moments when the moon is between the sun and the earth, and at other moments in opposition to the sun. As the moon revolves around the earth, there will be variations in the revolution of the earth around the sun, and these will provoke the monthly variations in the tides.
As the earth revolves around the sun, there will be slight variations in the distance of the earth from the sun, with this distance minimized during the equinoxes. [Note that Galileo is still working with Copernicus’s circular orbits, despite the fact that Kepler had already published in his 1609 Astronomia Nova that the orbit of Mars was elliptical.] These variations provoke the annual variations in the tides.
In each of these three points, it is understood that without variations in the earth’s revolution and rotation, there would be no tides:
What is more to be wondered at, once it had occurred to the minds of some to refer the cause of the tides to the motion of the earth (which showed unusual perspicacity on the part of these men), is that in seizing at this matter they should have caught on to nothing. But this was because they did not notice that a simple and uniform motion, such as the simple diurnal motion of the terrestrial globe for instance, does not suffice, and that an uneven motion is required, now accelerated and now retarded. For if the motion of the vessels were uniform, the contained waters would become habituated to it and would never make any mutations. [pp.461-462]
Nevertheless, despite the historical understanding, since antiquity, that the tides are somehow related to the moon, and despite having no empirical evidence to support his theory of the tides, Galileo calls out Kepler by name for supporting occult properties and puerilities. The paragraph following the one just quoted is the one I showed in my previous post on the tides:
Likewise it is completely idle to say (as is attributed to one of the ancient mathematicians) that the tides are caused by the conflict between the motion of the earth and the motion of the lunar sphere, not only because it is neither obvious nor has it been explained how this must follow, but because its glaring falsity is revealed by the rotation of the earth being not contrary to the motion of the moon, but in the same direction. Thus everything that has been previously conjectured by others seems to me completely invalid. But among all the great men who have philosophized about this remarkable effect, I am more than astonished at Kepler than at any other. Despite his open and acute mind, and though he has at his fingertips the motions attributed to the earth, he has nevertheless lent his ear and his assent to the moon's dominion over the waters, to occult properties, and to such puerilities. [p.462, my emphasis]
In my opinion, Galileo went wrong on two separate points.
First, in the entire discussion of Day Four, Salviati seems to have forgotten everything that he told Simplicio and Sagredo on Day One, namely that as the earth rotates upon itself, all of the parts of the earth rotate along with it. And this should include not just the earth and the air, as discussed in Day One, but also the waters of the deep.
Second, as I wrote in my post Paolo Sarpi on Local Motion, Galileo accepts from Sarpi, and ultimately from Aristotle, that there is only local motion. So, after criticizing Aristotle and his followers for three days, ultimately Galileo accepts that there is no notion of action-at-a-distance, calling it occult and puerile. And this cannot be understood as an oversight on his part, as we can see in this exchange between Simplicio and Salviati:
Simp: If no reasons more agreeable to natural phenomena were presented to me, I should pass on unhesitatingly to the belief that the tide is a supernatural effect, and accordingly miraculous and inscrutable to the human mind — as are so many others which depend directly upon the omnipotent hand of God.
Salv: You argue very prudently, and also in agreement with Aristotle's doctrine; at the beginning of his Mechanics, as you know, he ascribes to miracles all things whose causes are hidden. [p.421]
This aversion to action-at-a-distance will recur many times as science, especially physics, develops over the successive centuries. For example, Albert Einstein, criticizing quantum mechanics in a 1947 letter to Max Born, complained of spukhafte Fernwirkungen (spooky actions-at-a-distance). I will return many times to this subject.
À suivre…
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Galileo Galilei. Dialogue concerning the two chief world systems — Ptolemaic and Copernican. Translated by Stillman Drake, foreword by Albert Einstein. University of California Press, 2nd ed., 1967.
Thanks for another interesting point! So Galileo was not right about everything. And he uses the same pompous rhetoric when he is wrong.
I've always thought that "local motion" depends on your definition of local. If you allow for interactions between protons and electrons, that's action at a very small distance. Interaction between the sun and the earth is action at a larger distance. But why would one require more explanation than the other?