Galileo Insists the Earth is Spinning on its Axis

Galileo's Dialogue, Day 2, Part 2

As I wrote in a recent post, I intend to write a post 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 is the second post about Day Two, the longest day in the Dialogue. 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 examines whether the Earth is at rest, focussing specifically on diurnal motion, i.e., whether the Earth spins around its axis once every 24 hours.

The discussion begins with a quick review of Day One by Sagredo, who concludes that it is more likely for “the earth to enjoy the same perfection as other integral bodies of the universe,” as opposed to the view that “the substance of the heavenly bodies … to be a quintessence most different from our generable, corruptible, alterable bodies; in short, to be a movable and a moving body no less than the moon, Jupiter, Venus, or any other planet.” To which Salviati responds that “I did not conclude this, just as I am not deciding upon any other controversial proposition. My intention was only to adduce those arguments and replies, as much on one side as on the other … and then to leave the decision to the judgment of others.” [pp.105-106]

This review is followed by the discussion on Aristotle’s followers, presented in the previous post. Salviati concludes that the problem is not so much Aristotle himself, but the cowardice of his followers:

Is it possible for you to doubt that if Aristotle should see the new discoveries in the sky he would change his opinions and correct his books and embrace the most sensible doctrines, casting away from himself those people so weak-minded as to be induced to go on abjectly maintaining everything he had ever said? […] It is the followers of Aristotle who have crowned him with authority, not he who has usurped or appropriated it to himself. And since it is handier to conceal oneself under the cloak of another than to show one's face in open court, they dare not in their timidity get a single step away from him, and rather than put any alterations into the heavens of Aristotle, they want to deny out of hand those that they see in nature's heaven. [pp.110-111]

To which Simplicio replies:

But if Aristotle is to be abandoned, whom shall we have for a guide in philosophy? Suppose you name some author. [p.112]

Salviati’s response is clear:

So put forward the arguments and demonstrations, Simplicio — either yours or Aristotle's — but not just texts and bare authorities, because our discourses must relate to the sensible world and not to one on paper. [p.113]

And then begins the real discussion of the day:

we should follow this up by examining that other proposition which holds it to be probable that the earth is fixed and utterly immovable as to its entire globe, and see what chance there is of making it movable, and with what motion. [p.113]

The problem that Salviati is dealing with is that of diurnal motion, which has been known since ancient times by observing the night sky. On a clear night, one can see the stars apparently rotating on an axis whose center is the North Pole in the northern hemisphere, in an east-to-west direction. And on successive clear nights, the same stars appear in the same places, hence their name “the fixed stars,” always apparently rotating in an east-to-west direction. The only exceptions are the planets, five of which were known at the time (Saturn, Jupiter, Mars, Venus and Mercury), which seem to move in a west-to-east direction from night to night, the Sun and the Moon.

So the question that has vexed philosophers and scientists is the nature of this diurnal motion. Are the heavens, including all of the stars and planets, spinning around the earth, once every day, or is the earth spinning on its axis? The Aristotelian/Ptolemaic system sided with the heavens moving, while the Copernican system had the earth spinning on its axis.

What Salviati argues is that it is more probable that the “fixed stars” are in fact fixed, and that it is the earth that is spinning on its axis with a 24-hour period in a west-to-east motion. He does this by putting forward the following seven points:

First, Salviati argues that it is far more probable that the earth is spinning on its axis than to have the entire rest of the universe rotating around the earth:

Now if we think of the velocity of motion required to make a complete rotation in a single day and night, I cannot persuade myself that anyone could be found who would think it the more reаsonable and credible thing that it was the celestial sphere which did the turning, and the terrestrial globe which remained fixed. [p.115]

Second, if it were the heavens that were rotating around the earth, then there would be a great contradiction between the east-to-west apparent motion of the stars and the west-to-east apparent motion of the planets:

If this great motion is attributed to the heavens, it has to be made in the opposite direction from the specific motion of all the planetary orbs, of which each one incontrovertibly has its own motion from west to east, this being very gentle and moderate, and must then be made to rush the other way; that is, from east to west, with this very rapid diurnal motion. Whereas by making the earth itself move, the contrariety of motions is removed, and the single motion from west to east accommodates all the observations and satisfies them completely. [p.117]

Third, if it were the heavens that were rotating around the earth, then there would be this highly improbable situation of the period of the planetary orbits increasing as does their distance from the earth, while the much further away fixed stars would orbit around the earth in 24 hours:

But if the earth is desired to remain motionless, it is necessary, after passing from the brief period of the moon to the other consecutively larger ones, and ultimately to that of Mars in two years, and the greater one of Jupiter in twelve, and from this to the still larger one of Saturn, whose period is thirty years — it is necessary, I say, to pass on beyond to another incomparably larger sphere, and make this one finish an entire revolution in twenty-four hours. […] But by giving mobility to the earth, order becomes very well observed among the periods…. [p.119]

Fourth, if it were the heavens that were rotating around the earth, then the speed of motion of the stars would depend on whether they were close to the earth’s polar axis or the earth’s equator.

This difficulty is the immense disparity between the motions of the stars, some of which would be moving very rapidly in vast circles, and others very slowly in little tiny circles, according as they are located farther from or closer to the poles. [p.119]

Fifth, if it were the heavens that were rotating around the earth, the periods of the starry orbits would vary over time:

[T]he same stars will keep changing their circles and their velocities, since those which two thousand years ago were on the celestial equator, and which consequently described great circles with their motion, are found in our time to be many degrees distant, and must be made slow in motion and reduced to moving in smaller circles. [p.119]

Sixth, the rotating heavens are supposed to be attached to a very unlikely sphere:

[T]he unlikelihood is increased … by the incomprehensibility of what is called the "solidity" of that very vast sphere in whose depths are firmly fixed so many stars which, without changing place in the least among themselves, come to be carried around so harmoniously with such a disparity of motions. [p.120]

Seventh, moving the rotating heavens would require incredible strength and power:

[I]f we attribute the diurnal rotation to the highest heaven, then this has to be made of such strength and power as to carry with it the innumerable host of fixed stars, all of them vast bodies and much larger than the earth, as well as to carry along the planetary orbs despite the fact that the two move naturally in opposite ways. Besides this, one must grant that the element of fire and the greatest part of the air are likewise hurried along, and that only the little body of the earth remains defiant and resistant to such power. Besides this, one must grant that the element of fire and the greater part of the air are likewise hurried along, and that only the little body of the earth remains defiant and resistant to such power. [p.120]

What ensues from Salviati’s seven points is a lively debate over a set of Aristotelian objections to terrestrial diurnal motion brought up by Simplicio, along with some potential objections, not thought of by Simplicio, that Salviati himself brought up. These objections include, were the earth to spin:

• the particles making up the earth would not be able to follow the earth;

• a weight dropped from a tower would not fall to the foot of a tower, but away from the latter;

• cannon balls shot west would go further than cannon balls shot east, while cannot balls shot north or south would end up falling, respectively, north-west or south-west;

• birds would be constantly fighting an easterly wind, and would be blown westwards.

All of these objections are dealt with in the same way by Salviati, by (very!) patiently explaining, and reëxplaining, that as the earth rotates, everything that is attached to it, including its own component particles, the rocks, towers, people, animals and plants on its surface, and the air around the earth, along with the birds flying therein, are rotating with the earth. Hence, from the point of view of visible motion on the earth, there is no difference whatsoever between an immobile earth and a rotating earth. In other words, with respect to the above examples, were the earth to spin:

• the particles making up the earth would rotate along with the earth;

• a weight dropped from a tower would fall to the foot of a tower;

• cannon balls shot north, east, south or west would all travel the same distance;

• birds would fly as if the air were immobile.

Salviati even uses a rhetorical trick to further the discussion. When Simplicio enumerates the arguments against terrestrial motion, he gives the example of someone standing on the top (the uppermost platform) of a mast on a ship sailing forward, and dropping a rock from the top: he argues that the rock would drop, not to the foot of the mast, but, rather, towards the stern (the rear) of the ship. Initially, Salviati accepts Simplicio’s explanation, first focussing on explaining how the earth and everything connected to it rotates with it. Only once he has done this does he return to the question of the rock being dropped from the top of a mast. At that point, he explains that, just as the rock being dropped from the top of the tower falls to the foot of the tower, so does the rock being dropped from the top of the mast fall to the foot of the mast.

As the discussion continues, Simplicio utters the following statement, showing the contempt that the Peripatetics have for people who do detailed experiments or calculations. This contempt continues to this day in the English-speaking world, in which engineers are looked down upon, unlike in many other countries:

Philosophers occupy themselves principally about universals. They find definitions and criteria, leaving to the mathematicians certain fragments and subtleties, which are then rather curiosities. Aristotle contented himself with defining excellently what motion in general is, and showing the main attributes of local motion; that is, that sometimes it is natural, sometimes forcible, sometimes it is simple, other times composite, on some occasions uniform and on others accelerated; and for the accelerated motions he was content to supply the causes of acceleration, leaving to mechanics or other low artisans the investigation of the ratios of such accelerations and other more detailed features. [pp.163-164, emphasis mine]

As the discussions continue, at some point Salviati exclaims in frustration:

Aristotle's error, and Ptolemy's, and Tycho's, and yours, and that of all the rest, is rooted in a fixed and inveterate impression that the earth stands still, this you cannot or do not know how to cast off, even when you wish to philosophize about what would follow from assuming that the earth moved. [p.171]

In other words, Salviati is stating that the Peripatetics are not discussing in good faith. Even when they are accepting for the sake of argument the possibility that the earth is spinning on its axis, they make so many false assumptions about the nature of that hypothetical motion that the discussion is pointless.

The discussion moves back to one of the specific cases mentioned above, namely a cannon ball being shot west or east. Salviati states:

And this seems to me an appropriate time to take notice of a certain generosity on the part of the Copernicans toward their adversaries when, with perhaps too much liberality, they concede as true and correct a number of experiments which their opponents have never really made. [p.180]

It turns out that given the size of the earth and the estimated surface velocity from its rotation, shooting a cannon ball west or east, if it did make a difference, as supposed the Peripatetics, would only make a difference of a few centimetres, when in fact the shooting accuracy of cannons of that time was measured in metres. In other words, there would be no way to effect a real experiment to determine which of the Aristotelian or Copernican hypotheses was correct. This leads Salviati to state:

But it must be seen that this variation due to the rest or motion of the earth, since it can only be very small, cannot but be submerged in the large ones which continually occur on account of accidents. And this is all said and granted for good measure to Simplicio merely as a warning of how carefully we must tread in conceding the truth of many experiments to those who have never performed them, but who boldly would produce such as are needed to serve their purposes. [pp.181-182]

And a bit later, Salviati savages Simplicio:

I see that you have hitherto been one of that herd who, in order to acquire a knowledge of natural effects, do not betake themselves to ships or crossbows or cannons, but retire into their studies and glance through an index and a table of contents to see whether Aristotle has said anything about them; and, being assured of the true sense of his text, consider that nothing else can be known. [p.185]

Once these different objections have been dealt with, Salviati brings up the most important objection, which dates back at least to Ptolemy, about the earth losing those things attached to it were it to rotate:

Now there remains the objection based upon the experience of seeing that the speed of whirling has a property of extruding and discarding material adhering to the revolving frame. For that reason it has appeared to many, including Ptolemy, that if the earth turned upon itself with great speed, rocks and animals would necessarily be thrown toward the stars, and buildings could not be attached to their foundations with cement so strong that they too would not suffer similar ruin. [p.188]

Here Salviati brings up a really interesting point:

Ptolemy argues either against those who considered the earth always movable or against those who thought it to be stable for a time and then to be set in motion. If against the former, he ought to have said: “The earth has not always moved, for there would never have been men nor animals nor edifices on earth, the terrestrial whirling having not permitted them to stay.” But since his reasoning is, “The earth does not move, because beasts and men and buildings placed on the earth would be precipitated from it,” he assumes the earth to have been once in that state which would have allowed beasts and men to stay and build them. From this the conclusion is drawn that the earth has been fixed at some time; that is, adapted to the stay of animals and the building of edifices. [p.190]

Salviati is having fun here. He is implying, tongue-in-cheek, that Ptolemy is arguing that Pythagoras set the earth spinning a while back, and that Ptolemy is afraid that the buildings that have been built, plus their builders, will all fly off into space if the earth isn’t stopped again!

After this fun, Salviati provides two geometrical proofs that imply that it is impossible for objects on the surface of the globe to fly off. Unfortunately, according to the notes in Drake’s translation of the Dialogue, there are flaws in the proofs. Nevertheless, Drake does make the following note:

Strauss remarks that this probably is the first scientific attempt to use the abcissae [x axis] and ordinates [y axis] of a single diagram explicitly for magnitudes of two different sorts (time and velocity). This fact alone would place the Dialogue in the highest rank of scientific importance. [p.478]

The discussion then moves on to examining two contemporary anti-Copernican texts. The first is the Disquisitiones mathematicae de controversiis ac novitatibus astronomicis (Mathematical discussions about astronomical controversies and innovations), by Locher, at the instigation of Scheiner (Ingolstadt, 1614).

In this text, some really imaginative thought experiments are undertaken:

• If a ball is dropped from the height of the moon, how long will it take to fall to earth?

• If the earth is spinning on its axis, and God suddenly stops it from spinning, what will happen to everything that is on the earth’s surface?

• Were there to exist a giant cavern in the center of the earth, and two stones were placed therein, the first at the very center of the earth, and the second at the top of the cavern, what motions would these stones be subject to?

Salviati handles these puzzles quite well, but what stood out for me was this exchange between Simplicio and Salviati.

Simp: Very well, but as heavy and light bodies can have neither an internal nor an external principle of moving circularly, then neither does the earth move circularly. And thus we have [the author’s] meaning.

Salv: I did not say that the earth has neither an external nor an internal principle of moving circularly; I say that I do not know which of the two it has. My not knowing this does not have the power to remove it.

But if this author knows by which principle other world bodies are moved in rotation, as they certainly are moved, then I say that that which makes the earth move is a thing similar to whatever moves Mars and Jupiter, and which he believes also moves the stellar sphere. If he will advise me as to the motive power of one of these movable bodies, I promise I shall be able to tell him what makes the earth move. Moreover, I shall do the same if he can teach me what it is that moves earthly things downward.

Simp: The cause of this effect is well known; everybody is aware that it is gravity.

Salv: You are wrong, Simplicio; what you ought to say is that everyone knows that it is called “gravity.” What I am asking you for is not the name of the thing, but its essence, of which essence you know not a bit more than you know about the essence of whatever moves the stars around. I except the name which has been attached to it and which has been made a familiar household word by the continual experience that we have of it daily. But we do not really understand what principle or what force it is that moves stones downward, any more than we understand what moves them upward after they leave the thrower's hand, or what moves the moon around. We have merely, as I said, assigned to the first the more specific and definite name “gravity,” whereas to the second we assign the more general term “impressed force” (virtù impressa), and to the last-named we give “spirits” (intelligenza), either “assisting” (assistente) or “abiding” (informante); and as the cause of infinite other motions we give “Nature.” [pp.234-235, emphasis mine]

Salviati is here taking a stand in one of the most fundamental debates of philosophy: nominalism vs. essentialism. Simplicio simply names gravity as a cause, but Salviati insists that we do not know what gravity is. It should be noted that even today, it is not clear what is the nature of gravity.

The second anti-Copernican text is De tribus novis stellis quae annis 1572, 1600, 1609 comparuere (About the three new stars that appeared in the years 1572, 1600, and 1609), by Chiaramonti (Cesena, 1628). Simplicio begins reading:

The first argument begins here: “And first, if Copernicus's opinion is embraced, the criterion of science itself will be badly shaken if not completely overturned.” By which criterion he means, in agreement with philosophers of every school, that the senses and experience should be our guide in philosophizing. But in the Copernican position, the senses much deceive us when they visually show us, at close range and in a perfectly clear medium, the straight perpendicular descent of very heavy bodies. Despite all, according to Copernicus, vision deceives us in even so plain a matter and the motion is not straight at all, but mixed straight-and-circular. [p.248]

This argument has already been addressed above. Once again, the Peripatetics refuse to understand that should the earth spin on its axis, everything attached to the earth spins along with the earth, and so there should be no visible motion of the earth.

But it gets better. Chiaramonti did not even read Copernicus correctly! Here is Salviati calling him out:

I am grieved to learn that he set himself up to dispute a position which he did not even understand, for these are not the movements which Copernicus attributes to the earth. Where did he get the idea that Copernicus made the earth's annual motion to be along the ecliptic opposite to its motion around its own center? p.261]

Salviati continues:

Judge, then, how much study this opponent may be supposed to have spent on the books of Copernicus, when he gets this basic and principal hypothesis backwards, upon which are founded all the dissents of Copernicus from the doctrine of Aristotle and Ptolemy. [p.262]

In conclusion, Salviati states:

It is true that the Copernican system creates disturbances in the Aristotelian universe, but we are dealing with our own real and actual universe. [p.268]

Day Two ends with the analysis of the part of Chiaramonti’s book pertaining to diurnal motion. Day Three will cover the question of whether the earth orbits around the Sun, a topic also dealt with by Chiaramonti.

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1. 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.