- BOOK, II, QUESTION 22. It is sought consequently whether the earth always is at rest in the center of the cosmos . . .
This question is difficult. For in the first place there is significant doubt as to whether the earth is directly in the middle cf the cosmos so that its center coincides with the center of the cosmos. Further, there is serious doubt as to whether it is not sometimes moved rectilinearly as a whole, since we do not doubt that often many of its parts are moved, for this is clear through our senses. There is also another serious doubt as to whether the following conclusion of Aristotle is sound, namely, if the heaven is by necessity to be moved circularly and eternally, then it is necessary that the earth be at rest forever in the middle. There is also a fourth doubt whether, in positing that the earth is moved circularly around its own center and its poles, all the phenomena that are apparent to us can be saved (po6sent salvari omnia nobis apparenffa). Concerning this last doubt let us now speak.
- It should be known that many people have held as probable that it is not contradictory to appearances for the earth to be moved circularly in the aforesaid manner, and that on any given natural day it makes a complete rotation from west to east by returning again to the west–that is, if some part of the earth were designated [as the part to observe]. Then it is necessary to posit that the stellar sphere would be at rest, and then night and day would result through such a motion of the earth, so that motion of the earth would be a diurnal motion (motus diurnus). The following is an example of this [kind of thing]: If anyone is moved in a ship and imagines that he is at rest, then, should he see another ship which is truly at rest, it will appear to him that the other ship is moved. This is so because his eye would be completely in the same relationship to the other ship regardless of whether his own ship is at rest and the other moved, or the contrary situation prevailed. And so we also posit that the sphere of the sun is totally at rest and the earth in carrying us would be rotated. Since, however, we imagine we are at rest, just as the man on the ship moving swiftly does not perceive his own motion nor that of the ship, then it is certain that the sun would appear to us to rise and set, just as it does when it is moved and we are at rest.
- It is true, however, that if the stellar sphere is at rest, it is necessary to concede generally that the spheres of the planets are moving, since otherwise the planets would not change positions relative to each other and to the fixed stars. And therefore this opinion imagines that any of the spheres of the planets moved evidently like the earth from west to east, but since the earth has a lesser circle, hence it makes its rotation (circulatio) in less time. Consequently, the moon makes its rotation in less time than the sun. And this is universally true, so that the earth completes its rotation in a natural day, the moon in a month, and the sun in a year, etc.
- It is undoubtedly true that, if the situation were just as this position posits, all celestial phenomena would appear just as they now appear. We should know likewise that those persons wishing to sustain this opinion, perhaps for reasons of disputation, posit for it certain arguments . . . The third argument is this: To celestial bodies ought to be attributed nobler conditions, and to the highest sphere, the noblest. But it is nobler and more perfect to be at rest than to be moved. Therefore, the highest sphere ought to be at rest . . .
The last argument is this: Just as it is better to save the appearances through fewer causes then through many (if this is possible) so it is better to save [them] by an easier way than by one more difficult. Now it is easier to move a small thing than a large one. Hence it is better to say that the earth (which is very small) is moved most rapidly and the highest sphere is at rest, than to say the opposite.
- Yet this opinion is not to be followed. In the first place because it is against the authority of Aristotle and all of the astronomers (astrologi). But these people respond that authority does not demonstrate, and that it suffices astronomers that they posit a method by which appearances are saved, whether or not it is so in actuality. Appearances can be saved in either way; hence they posit the method most pleasing to them.
- Others argue [against the theory of the earth’s diurnal rotation] by many appearances (apparentiic). One of these is that the stars sensibly appear to be moved from the east to the west. But they solve this [by saying] that it would appear the same if the stars were at rest and the earth were moved from west to east.
- Another appearance is this: If anyone were moving very swiftly on horseback, he would feel the air resisting him. Therefore, similarly, with the very swift motion of the earth in motion, we ought to feel the air noticeably resisting us. But these [supporters of the opinion] respond that the earth, the water, and the air in the lower region are moved simultaneously with diurnal motion. Consequently there is no air resisting us.
- Another appearance is this: Since local motion heats, and therefore since we and the earth are moved so swiftly, we should become hot. But these [supporters] respond that motion does not produce heat except by the friction (confricatio), rubbing, or separation of bodies. These [causes] would not be applicable there, since the air, water, and earth would be moved together.
- But the last appearance which Aristotle notes is more demonstrative in the question at hand. This is that an arrow projected from a bow directly upward falls to the same spot on the earth from which it was projected. This would not be so if the earth were moved with such velocity. Rather, before the arrow falls, the part of the earth from which the arrow was projected would be a league’s distance away. But still supporters would respond that it happens so because the air that is moved with the earth carries the arrow, although the arrow appears to us to be moved simply in a straight line motion because it is being carried along with us. Therefore, we do not perceive that motion by which it is carried with the air. But this evasion is not sufficient because the violent impetus of the arrow in ascending would resist the lateral motion of the air so that it would not be moved as much as the air. This is similar to the occasion when the air is moved by a high wind. For then an arrow projected upward is not moved as much laterally as the wind is moved, although it would be moved somewhat . . .
- Then I come to the other doubts. One would be whether the earth is situated directly in the middle of the cosmos. It should be answered in the affirmative. For we suppose that the place [designated] absolutely (simpliciter) as “upward, ” insofar as one looks at this lower world, is the concave [surface] of the orb of the moon. This is so because something absolutely light, that is, fire, is moved toward it. For since fire appears to ascend in the air, it follows that fire naturally seeks a place above the air, and this place above the air is at the concave [surface] of the orb of the moon; because no other element appears to be so swiftly moved upward as fire. Now the place downward ought to be the maximum distance from the place upward, since they are contrary places. Now that which is the maximum distance from the heaven is the middle of the cosmos. Therefore the middle of the cosmos is absolutely downward. But that which is absolutely heavy–and earth is of this sort–ought to be situated absolutely downward. Therefore, the earth naturally ought to be in the middle of the cosmos or be the middle of the cosmos.
- But a significant difficulty is whether the center of magnitude in the earth is the same as the center of gravity (medium gravitatis). According to some statements it is not. This is because a large region of the earth is not covered with water due to the habitation of animals and plants, and the opposite part is covered with water, it is clear that the air which is naturally hot, and the sun, make the noncovered part hot, and thus they make it to some degree more subtle, rare, and light. The covered part remains more compact and heavy. Now if a body in one part is lighter and in an opposite part heavier, the center of gravity will not be the center of magnitude. Rather, with the center of gravity given, the greater magnitude will be in the lighter part, just as in the case of balances if on one side is placed a stone and on the other side wool [and they balance], the wool will be of a much greater volume.
- With this understood, it should be seen which of those centers is the center of the cosmos. It should be answered immediately that the center of the cosmos is the center of gravity of the earth. This is because, as Aristotle says, a]l parts tend toward the center of the cosmos through their gravity, and a part which is heavier would displace another, and thus finally it is necessary that the center of the cosmos coincide with the center of gravity. From these arguments it follows that the earth is nearer to the heaven in the part not covered with waters than in the covered part, and thus at the covered part there is greater declivity, and so the waters flow to that part. So the earth, therefore, with respect to its magnitude, is not directly in the center of the cosmos. However, we commonly say that it is in the center of the cosmos because its center of gravity is the center of the cosmos.
- From this another doubt is evidently solved, whether the earth is sometimes moved in its entirety in a straight line. We can answer affirmatively because from this higher [part of] the earth many parts of the earth continually flow down rivers to the bottom of the sea, and thus the earth is augmented in the covered part and diminished in the uncovered part. Consequently, the center of gravity does not remain the same as before. Now, therefore, with the center of gravity changed, that which has newly become the center of gravity is moved so that it will coincide with the center of the cosmos, and that point which was the center of gravity before ascends and recedes, and thus the whole earth is elevated toward the uncovered part so that the center of gravity might always become the center of the cosmos. And just as I have said elsewhere, it is not clear how it could be saved unless the mountains were consumed and destroyed sometimes, nay infinite times, if time were eternal. Nor is any other way apparent by which such mountains could be generated. This was treated elsewhere, so I now desist . . .
[From Medieval Philosophy, ed. Herman Shapiro, pp. 542-47, modified with minor variations from the Latin.]