Classics of Modern Science, Copernicus to Pasteur

Author: Galileo Galilei

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IV Galileo Galilei 1564–1642



Formerly I used frequently to visit the marvelous city of Venice and to meet there Signore Giovan Francesco Sagredo, a man of most distinguished ancestry and remarkable intelligence. Thither also came from Florence, Signore Filippo Salviati, whose least claim to renown was his noble blood and great wealth; a noble mind, that held no enjoyment of greater price than that of study and thought. With both of these men I often discussed these questions, in the presence of a Peripatetic philosopher, who apparently valued the acquisition of knowledge in no way in so high a degree, as he did the renown which his interpretations of Aristotle had gained for him.

Now that cruel death has robbed the cities of Venice and Florence of these two enlightened men in the bloom of their years, I have endeavored, as far as my weak powers may permit, to perpetuate their fame in these pages by making them the speakers in this dialogue. The valiant Peripatetic also shall not fail to appear; because of his over-weaning love for the commentary of Simplicius, it seemed permissible to omit his own name and let him pass under that of his favorite author. May the souls of these two great men accept this public testimony of my undying love; may the recollection of their eloquence aid me in setting down for posterity the spoken discussions.


SALVIATI: We departed yesterday so often and so far from the direct path of our discussion, that I can scarcely return to the right point and proceed without your help.

SAGREDO: I find it quite intelligible that you are somewhat at a loss, since you have had your head so full of both the things already brought forward and things still to be discussed. I, however, who as merely a listener have in mind only the things already discussed, may I hope set our investigation straight by a brief summary of what has been gone over. So, if my memory fails not, the chief result of our yesterday’s conversation was that we tested thoroughly which of the two theories was the more probable and better grounded; that according to which the substance of the heavenly bodies is unproducible, indestructible, unchangeable, intangible, in brief not subject to any variation aside from change of location, and so presents a fifth element which is entirely distinct from our elementary, producible, destructible, changeable bodies; or the other view, according to which an incongruity between parts of the universe is rejected, our earth rather enjoys the same privileges as the rest of the constituent bodies of the universe, in a word, is a freely moving ball just as the moon, Jupiter, Venus, or any other planet. Finally we noticed the many similarities in particular between the earth and the moon, and of course with the moon more than any other planet because of the closer and more definite knowledge which we possess of it by reason of its less distance. Since we agreed that this second opinion possessed the greater probability, the logical consequence, it seems to me, is that we should investigate the question whether we should hold the world inmovable, as has been formerly believed in general, or movable as some ancient philosophers believed and as some recent ones suppose: and if movable, how its movement could have been produced.

SALV.: Let us begin our discussion with the admission that whatever sort of motion may be ascribed the earth, we, as its inhabitants and therefore partakers in the movement, would be unconscious of it, as if it did not occur, since we can only take into consideration earthly things. Therefore it is necessary that this movement should seem to belong to all the other bodies and visible objects in common which, separated from the earth, have no share in its movement. The correct method of determining whether movement is to be attributed to the earth, and what movement, is that one should inquire and observe whether an apparent movement can be ascribed to the bodies outside of the earth, which belongs to all of them in the same degree. So a movement which, for example, can be supposed of the moon, and not of Venus or Jupiter or other stars, cannot be peculiar to the earth. Now there is such a general movement governing all other objects, namely that which the sun, moon, planets, fixed stars, in a word the whole universe with the single exception of the earth, seems to follow from east to west within the space of twenty-four hours. This, at least at first glance, may be just as well attributed to the earth alone, as to the rest of the entire universe except the earth.

SAGR.: I understand clearly that your suggestion is correct. An objection, however, forces itself upon me that I cannot solve. That is, since Copernicus ascribes to the earth a further movement aside from the daily one, according to the above mentioned principle this should be apparently un-noticeable on the earth, but should be visible in the rest of the universe. I come then to the conclusion that either he plainly erred when he ascribed to the earth a movement to which no counterpart is apparent in the firmament, or else such a movement exists, and then Ptolemaus is guilty of a second error in that he did not refute with arguments this movement as well as that daily rotation.

SALV.: Your objection is very just. If we take up this other movement, you shall see how much superior in intelligence was Copernicus to Ptolemaus, in that he saw what this one did not, namely how wonderfully this second motion is reflected in the rest of the heavenly bodies. For the present, however, we will leave this aside and return to our first consideration. Proceeding from the most general suppositions, I will present the arguments which seem to favor the motion of the earth, in order then to hear the opposing arguments of Signore Simplicio. First, then, when we consider the immense circumference of the stellar sphere in comparison with the smallness of the earth, which is contained in that several million times, and therefore regard the velocity of motion which would be necessary for an entire revolution in the course of a day and night, I am unable to understand how any one could hold it more reasonable and credible that it is this whole stellar sphere that moves and that the earth remains still.

SAGR.: Even if universal phenomena which depend upon these movements could be explained as readily by the one hypothesis as by the other, yet by the first general impression I would regard as more unreasonable the view that the whole universe moves; just as if any one should climb to the top of your dome for the purpose of getting a view of the city and its environs and then should demand that the whole region be made to move around him to save him the trouble of turning his head. In any event, there would have to be great advantages connected with this theory, which were lacking in the other, in order that such an absurdity should be balanced and outweighed and should appear more credible than the opposite opinion. But Aristotle, Ptolemaus, and Signore Simplicio must find such advantages in their theory, and I should be glad if we might hear these advantages if they exist, or if they do not, that some one would explain to me why they do not and cannot exist.

SALV.: If, in spite of every sort of investigation, I am able to find no such differences, I believe I have thereby discovered that such difference does not exist. So in my opinion it is useless to pursue this further: rather let us proceed. Motion is only so far motion and acts as such, if it stands in relation to things which lack motion. In relation to things that are all in the same degree affected by it, it is as much without effect as if it did not take place. The wares with which a ship is loaded move, when they depart from Venice and arrive at Aleppo, passing Korfu, Candia, Cyprus etc; since Venice, Korfu and Candia remain fixed and do not move with the ship. But in respect to the bales, chests, and other pieces of baggage which are on the ship as cargo or ballast, the movement of the ship itself from Venice to Syria is as good as non-existent, since their position in relation to one another does not change; and this is due to the fact that the movement is a common one in which they all take part. If of the wares on the ship one bale moves only an inch away from the chest, this is for it a greater movement in relation to the chest, than the whole journey of 2,000 miles which they undergo in common.

Therefore, since plainly the motion which many movable bodies undergo in common is without effect and, with regard to their mutual position toward one another, it is as if it did not exist, for there is no change among them; and since it only affects the relative position of such bodies as do not share in the movement, for in this case the mutual relation is changed; since we have divided the universe into two parts, of which one must be movable and the other immovable; then for all purposes this movement will be of the same effect whether it is ascribed to the earth alone or to all the rest of the universe. For the working of such a motion is on nothing but the relative position in which the earth and the heavenly bodies stand to one another, and aside from this relative position nothing changes. If now it is indifferent for accomplishing this result whether the earth alone moves and the whole universe rests, or the earth rests and the whole universe is subject to one common movement, who can believe that Nature—who by common agreement does not employ great means when she can obtain the same result by smaller ones—would have undertaken to set in motion an immeasurable number of mighty bodies, and that with incredible velocity, to accomplish what could be obtained by the moderate motion of one single body around the center?

SIMPL.: I do not agree that that mighty movement would be as if it did not happen in regard to the sun, the moon, the innumerable host of fixed stars. Do you call it nothing that the sun goes from one meridian to another, rises from one horizon, sinks under another, brings now day, now night; that the moon goes through similar changes and likewise the other planets, as well as the fixed stars?

SALV.: All the changes mentioned by you are such only with respect to the earth. To demonstrate this, only imagine yourself away from the earth; there is then no rising or setting of the sun, no horizons, no meridians, no day, no night; in a word, by the movement mentioned no change in the relation of the moon to the sun or to any other star is evoked. All these changes have reference to the earth; they are supposed only because the sun is first visible in China, then Egypt, Greece, France, Spain, America, and so on, and so also for the moon and the other heavenly bodies. The same process would occur in the same way, if, without disturbing so vast a part of the universe, the earth alone should be revolved.

The difficulty is however doubled since a second very important one is added. That is, if one attributes to the firmament this mighty motion, one must regard it as necessarily opposed to the particular movements of all the planets, all of which indisputably have their own movements from west to east, and in comparison very moderate movements at that. One is then forced to the conclusion that they depart from that rapid daily motion, namely from east to west, to go in the opposite direction. But, if we suppose that the earth moves, the opposition of motions disappears and the single movement from west to east fits in with all the facts and explains them most satisfactorily.

SIMPL.: As far as this opposition of motions is concerned that has little importance, since Aristotle proves that the circular motions are not opposed to one another and that the apparent opposition cannot actually be called so.

SALV.: Does Aristotle prove that or merely suppose it, because it aids him for a certain purpose? If, according to his own declaration, those things are opposed which mutually destroy one another I do not see how two moving bodies which meet one another in a circular motion should do one another less harm than if they meet on a straight line.

SAGR.: Wait a moment, I pray. Tell me, Signore Simplicio, if two knights run into one another with leveled lances on the open field, if two squadrons or two streams on their way to the sea break through and unite with one another, would you call such collisions opposed movements?

SIMPL.: Of course we would call them opposed.

SAGR.: How then is there no opposition in circular motions? For the movements mentioned take place upon the surface of the earth or water, both of which are recognized to be circular in form and so the motions must be circular. Do you understand, Signore Simplicio, what circular motions are not opposed to one another? Two circles which touch each other on the outside and of which the revolution of one is in a reverse direction from that of the other. If, however, one circle is within the other, then motions in different directions must be opposed to one another.

SALV.: Whether opposed or not opposed is merely a strife of words. I know that in fact it is simpler and more natural to accomplish everything with one motion than to call in two. If you do not wish to call them opposite, then call them reverse. Moreover, I mention this introduction of a double movement not as something impossible, and in no way propose to deduce from it a strong proof for the motion of the earth, but merely a high degree of probability for it.

The improbability of the movement of the universe about the earth is tripled, however, by the complete upsetting of that arrangement which governs all the heavenly bodies whose circular motion is accepted not doubtfully but with full assurance. That is, that in such cases the larger the orbit the longer the time required for its completion, and the smaller, the shorter. Saturn, whose course surpasses all the planets in extent, completes it in thirty years. Jupiter revolves in a smaller circle in twelve years. Mars in two, the moon in a month. We see clearly in the case of the Medicean stars [the moons of Jupiter] that the one nearest Jupiter goes through its orbit in a very short time, namely, forty-two hours, the next nearest in three and a half days, the third in seven days, and the farthest removed in sixteen days. This thoroughly constant rule remains unchanged if we ascribe the twenty-four hour movement to the revolution of the earth, but if we suppose the earth to remain unmoved, we must proceed from the short period of the moon to increasingly greater periods, to the two year period of Mars, the twelve year period of Jupiter, the thirty year period of Saturn, and then abruptly to a disproportionately larger orbit, to which must also be ascribed the revolution in twenty-four hours. And these suppositions entail the smallest part of the disturbance of the otherwise constant law. For when one passes from the orbit of Saturn to those of the fixed stars and attributes to them even greater orbits, which correspond to the period of revolution of many thousands of years, one must pass from this by a much more disproportionate transition to that other movement and ascribe to them a period of revolution about the earth of twenty-four hours. But if the movement of the earth is supposed, the regularity of the period is accounted for in the best possible way; from the slow period of Saturn we arrive at the immovable fixed star.

A fourth difficulty also is encountered which must be added if we suppose the motion of the smaller sphere. I mean the great dissimilarity in movements of these stars, some of which must revolve at a tremendous rate in immense circles, others slowly in smaller circles, according as they are placed at greater or smaller distances from the pole. And not only the size of the different circles and so the velocity of movement varies greatly in different fixed stars, but also the same stars change their courses and their velocity; herein is the fifth difficulty. That is, those stars which 2,000 years ago stood on the equator of the stellar sphere and thereafter moved in the greatest circles, must now, since to-day they have moved several degrees from it, move more slowly and in smaller circles. Within a conceivable time it will happen that one of those which have been continually moving will eventually reach the pole and cease to revolve, then later, after a period of rest, begin to move again. The other stars, however, which undoubtedly move, all have, as has been said, as orbit an immense circle and move in it without change.

The improbability is increased (and this may be called a sixth difficulty) for him who investigates basic principles, by the fact that one cannot imagine the firmness which that immense sphere must possess, in whose depths so many stars are so solidly fixed that in spite of such varieties of motions they are held together in the revolution without in any way changing their relative positions. But if according to the most probable view the heavens are fluid, so that each star may describe its own orbit, by what law and according to what principles are their orbits governed, so that seen from the earth they appear as if held in one sphere? To accomplish this it seems to me it would be easier and more convenient to make them stationary instead of movable, just as the paving stones in the market place are kept in order more easily than the troops of children who race over them.

Finally the seventh objection; if we ascribe the daily revolution to the highest heavens we must suppose this to be of such power and force that it bears along the innumerable crowd of fixed stars, every one a body of immense mass and much larger than the earth, further, all the planets, although these by their nature move in an opposite direction. Moreover, we must suppose that the element of fire and the greater portion of the air is also borne along; therefore, singly and alone the little earth ball withstands stubbornly and independently this mighty force: a supposition that seems to me to have much against it. I cannot explain how the earth, a body freely suspended and balanced on its axis, inclined by nature as much toward motion as the rest, surrounded by a fluid medium, is not seized on by this general revolution. We do not encounter this difficulty, however, if we suppose the earth to move, a body so small, so inconsiderable in comparison with the whole universe that it could have no effect at all upon this.

* Translated from the Dialogo dei due Massima Systemi del Mondo (1632).


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Chicago: Galileo Galilei, "The Copernican Versus the Ptolemaic Astronomies," Classics of Modern Science, Copernicus to Pasteur in Classics of Modern Science, Copernicus to Pasteur, ed. William S. Knickerbocker (New York: Alfred A. Knopf, 1927), 36–45. Original Sources, accessed July 22, 2024,

MLA: Galilei, Galileo. "The Copernican Versus the Ptolemaic Astronomies." Classics of Modern Science, Copernicus to Pasteur, in Classics of Modern Science, Copernicus to Pasteur, edited by William S. Knickerbocker, New York, Alfred A. Knopf, 1927, pp. 36–45. Original Sources. 22 Jul. 2024.

Harvard: Galilei, G, 'The Copernican Versus the Ptolemaic Astronomies' in Classics of Modern Science, Copernicus to Pasteur. cited in 1927, Classics of Modern Science, Copernicus to Pasteur, ed. , Alfred A. Knopf, New York, pp.36–45. Original Sources, retrieved 22 July 2024, from