1. BOOK II. CHAPTER 25. Afterwards he [Aristotle] sets forth another opinion.

Text: And some say the earth is at the center of the universe and revolves and moves circularly around the pole established for this, as written in Plato's Timaeus.

Gloss: This was the opinion of one called Heraclides of Pontus who proposed that the earth is moved circularly and that the heavens are at rest. Aristotle does not here refute these opinions; it perhaps seemed to him that they have little [root in] appearance and are well refuted elsewhere in philosophy and astronomy.

But it seems to me, subject to correction, that one could well support and give luster to the last opinion, namely that the earth, and not the heavens, is moved with a daily movement. Firstly, I wish to state that one could not demonstrate the contrary by any experience (expenence) Secondly, [I will show that the contrary cannot be demonstrated] by reasoning. And thirdly, I will put forth reasons in support of it (that is, the diurnal rotation of the earth).

2. As for the first point, one experience [commonly cited in support of the daily motion of the heaven is the following]: We see with our senses the sun and moon and many stars rise and set from day to day, and some stars turn around the arctic pole. This could not be excepted by the movement of the heavens, as was demonstrated in Chapter 26. Thus [the argument runs], the heaven is moved with a diurnal movement. Another experience [cited] is [this]: If the earth is so moved, it makes a complete turn in a single natural day. Therefore, we and the trees and houses are moved toward the east very swiftly, and so it should seem that the air and wind blow continuously and strongly from the east, [much] as it does against a quarrel shot, [only] much more strongly. But the contrary appears by experience. The third [experience] is that which Ptolemy advances: If a person on a ship moved rapidly eastward and an arrow were shot directly upward, it ought not to fall on the ship but a good distance westward from the ship. Similarly, if the earth is moved so swiftly in turning from west to east, and it has been posited that one throws a stone directly above, then it ought to fall, not on the place it left, but a good distance to the west. But in fact the contrary is clear.

It seems to me that by [using] what I shall say regarding these experiences, one could respond to all the other [experiences] which might be adduced in this matter

3.... Again, I make the supposition that local motion can be sensibly perceived only in so far as one may perceive one body to be differently disposed with respect to another. In support of this [I give the following illustration]: If a person is in one ship called a, which is moved very carefully [that is, without pitching or rolling]--either rapidly or slowly--and this person sees nothing except another ship called b, which is moved in every respect in the same manner as a in which he is situated, I say that it will seem to this person that neither ship is moving. And if a is at rest and b is moved, it will appear and seem to him that b is moved.

[On the other hand], if a is moved and b is at rest, it will appear to him as before that a is at rest and b is moved. And thus, if a were at rest for an hour and b were moved, and then immediately in the following hour the situation were reversed, namely, that a were moved and b were at rest, this person [on a] could not perceive this mutation or change. Rather it would continually seem to him that b was moved; and this is apparent by experience. The reason for this is that these two bodies, a and b, are continually changing their dispositions with respect to each other in the same manner throughout when a is moved and b is at rest as they were conversely when b is moved and a is at rest. This is apparent in the fourth book of The Perspective of Witelo, [who says] that one can perceive movement only in such a way as one perceives one body to be differently disposed in comparison with another. I say, then, that if the upper of the two parts of the cosmos mentioned above should today move with a diurnal movements while the upper (that is, the heavens) should not, we could not perceive this change in any way, but everything would seem the same today and tomorrow. It would seem to us continually that the part where we are situated was at rest and that the other part was always moved, just as it seems to a person who is in a moving ship that the trees outside are moved. Similarly, if a person were in the heavens and it were posited that they were moved with a diurnal movement, and [furthermore] that this man who is transported with the heaven could see the earth clearly and distinctly and its mountains, valleys, rivers, towns, and chateaux, it would seem to him that the earth was moved with a diurnal movement, just as it seems to us on the earth that the heavens move. Similarly, if the earth and not the heavens were moved with a diurnal movement, it would seem to us that the earth was at rest and the heavens moved. This can be imagined easily by anyone with good intelligence. For this [reasoning] is evident the response to the first experience, since one could say that the sun and the stars appear thus to set and rise and the heavens to turn as the result of the movement of earth and its elements where we are situated.

4.  To the second experience, according to this opinion, the response is [this]: Not only is the earth so moved [diurnally], but with it water and the air, as was said, in such a way that the water and lower air are moved differently than they are by winds and other causes. It is like this situation: If air were enclosed in a moving ship, it would seem to the person situated in this air that it was not moved.

5. To the third experience, which seems more effective, that is, the experience concerning the arrow or stone projected upward, etc., one would say that the arrow is trajected upwards and [simultaneously] with this trajection it is moved eastward very swiftly with the air through which it passes and with all the mass of the lower part of the universe mentioned above, it all being moved with a diurnal movement. For this reason the arrow returns to the place on earth from which it left. This appears possible by analogy: If a person were on a ship moving toward the east very rapidly without his being aware of the movement, and he drew his hand downward, describing a straight line against the mast of the ship, it would seem to him that his hand was moved with rectilinear movement only. According to this opinion [of the diurnal rotation of the earth], it seems to us in the same fashion that the arrow descends or ascends in a straight line . . .

. . . In support of this [position, consider the following]: If a man in that ship were moving westward less swiftly than the ship was moving eastward, it would seem
to him that he was approaching the east, when actually he would be moving west. Similarly, in the case described above, all the movements would seem consistent with a stationary earth.

Also, in order to clarify the response to the third experience, I wish to add a natural example verified by Aristotle to the artificial example already given. It posits in the upper region of the air a portion of pure fire called a. This latter is of such a degree of lightness that it mounts to its highest possible point b near the concave surface of the heavens. I say that just as with the arrow in the case posited above, there would result in the case [of the fire] that the movement of a is composed of rectilinear movement, and, in part, of circular movement, because the region of the air and the sphere of fire through which a passes are moved, according to Aristotle, with circular movement. Thus, if it were not so moved, a would ascend rectilinearly in path ab, but because b is meanwhile moved to point c, by the circular daily movement, it is apparent that a in ascending describes the line ac and the movement of a is composed of a rectilinear and a circular movement. So also would be the movement of the arrow, as was said. Such composition or mixture of movements was treated in chapter three of book I [of the De caelo] . . . I conclude that one could not by any experience whatsoever demonstrate that the heavens, and not the earth, are moved with a diurnal motion.

6. As to the second point relative to the rational demonstration [of the diurnal movement of the heavens, I first note the following]: It seems to me that this [ratlonal demonstration] proceeds from these arguments which follow and to which I shall respond in such a fashion that, using the same reasoning, one could respond to all other arguments pertaining to it . . .

Again, if the heavens were not moved with diurnal movement, all astronomy (as~e) would be false and a great part of natural philosophy where one supposes this movement of the heavens.

Also, this seems to be against the Holy Scripture which says [in Eccles. I:5-6]: "The sun riseth, and goeth down, and returneth to his place: and there rising again, maketh his round by the south, and turneth again to the north: the spirit goeth forward surveying all places round about, and returneth to his circuits" [Douay translation of Vulgate]. And so it is written of the earth that God made it immobile: "For [God] created the orb of the earth, which will not be moved."

Also, the Scrlptures say that the sun was halted in the time of Joshua (see Josh. 10:12-14) and that it returned (that ls, turned back) in the time of King Ezechias (Hezekia; see Isa. 38:8; II Kings 20:11 [Vulgate, IV Kings 20:11]). If the earth were moved and not the heavens, as was said, such an arrestment would have been a returning and the returning of which lt speaks would rather have been an arrestment. And this is against the Scriptures . . .

7. To the flfth argument, where lt ls said that if the heavens would not make a rotation day to day, all astronomy would be false, etc., I answer this is not so because all aspects, conjunctlons, oppositions, constellations, figures, and lnfluences of the heavens would be completely just as they are, as ls evident clearly from what was said ln response to the first experlence. The tables of the movements and all other books would be just as true as they are, except ln regard to the daily movement one would say of lt that lt ls ln the heavens "apparently" (selon apparence) but in the earth 'actually' (selon vente). There is no effect which follows from the one [assumption] more than from the other. Apropos of this is the statement of Aristotle in chapter 16 [of book II of Aristotle's De caelo], namely that the sun appears to us to turn and the stars to sparkle and twinkle because, he says, it makes no difference whether the thing one sees is moved or the viewer is moved. Also one would say apropos of this matter [of diurnal rotation] that our sight is moved with diurnal rotation.

8. To the sixth argument concerning the Holy Scripture which says that the sun revolves, etc., one would say that it is in this part [simply] conforming to the manner of common human speech, just as in several places, for example, where it is written that God is 'repentent' and is 'angry' and 'pacified' and other such things which are just as they sound. Also appropriate to our question, we read that God covers the heavens with clouds-- 'who covereth the heavens with clouds' (Ps. 146: 8 --and yet in reality the heavens cover the clouds.) Thus one would say that according to appearances the heavens and not the earth are moved with a diurnal motion, while in actuality the contrary is true. Concerning the earth, one would say it is not moved from its place in actuality, nor in its place apparently, but that it is moved m its place actually. To the seventh argument, one would answer in the same way, that according to appearances in the time of Joshua the sun was arrested and advanced or speeded up its movement in the time of Ezechias. It would make no difference as to effect whichever opinion was followed. This latter opinion [supporting the diurnal rotation of the earth] seems more reasonable than the former, as we shall make clear.

As to the third [main] point [of this gloss], I wish to put forth arguments or reasons by which it would appear that the earth is moved as was indicated . . .

9. Again, all philosophers say that something done by several or large-scale operations which can be done by fewer or smaller operations is done for nought. And Aristotle says in the eighth chapter of the first book that God and Nature do not do anything in vain. But if it is so that the heavens are moved with a diurnal movement, it becomes necessary to posit in the principal bodies of the world and in the heavens two contrary kinds of movement, one east-to-west, and others of the opposite kind, as has been said often. With this [theory of the diurnal movement of the heavens] it becomes necessary to posit an excessively great speed. This will become clear to one who considers thoughtfully the height or distance of the heaven, its magnitude, and that of its circuit; for if such a circuit is completed in one day, one could not imagine nor conceive of how the swiftness of the heaven is so marvelously and excessively great. It is so unthinkable and inestimable. Since all the effects which we see can be accomplished, and all the appearances saved, by substituting for this [diurnal movement of the heavens] a small operation, that is, the diurnal mvoement of the earth, which is very small in comparison with the heavens, and [since this can be done] without making the [number of necessary] operations so diverse and outrageously great, it follows that [if the heaven rather than the earth is moved] then God and Nature would have made and ordained Things for nought. But his is not fitting, as was said.

Again, when it has been posited that the whole heavens are moved with daily movement and in addition that the eighth sphere is moved with another movement, as the astronomers posit, it becomes necessary, according to them, to assume a ninth sphere which is moved with a daily movement only. But when it has been posited that the earth is moved as was said, the eighth sphere is moved with a single slow movement and thus it is not necessary with this theory to dream up or imagine a ninth natural sphere, invisible and without stars; for God and Nature would not have made this sphere for nought, since all things can be as they are by using another method . . .

10. It is apparent, then, how one cannot demonstrate by any experience whatever that the heavens are moved with daily movement, because, regardless of whether it has been posited that the heavens, and not the earth, are so moved or that the earth and not the heavens is moved, if an observer (ouyl) is in the heavens and sees the earth clearly, it (the earth) would seem to be moved; and if the observer were on the earth, the heavens would seem to be moved. The sight is not deceived in this, because it senses nothing except movement. But if it is relative to any such body, this judgment is made by the senses from inside that body, just as he [Witelo] stated in The Perspective; and such senses are often deceived in such cases, as was said before concerning the person in the moving ship. Afterwards it was demonstrated how it cannot be concluded by reasoning that the heavens are so moved. Thirdly, reasons have been put forth in support of the contrary position, namely that [the heavens] are not so moved. Yet, everyone holds, and I believe, that they (the heavens), and not the earth, are so moved, for "God created the orb of the earth, which will not be moved" (Ps. 92:1), notwithstanding arguments to the contrary. [This is] because they are "persuasions" which do not make the conclusions evident. But having considered everything which has been said, one could by this believe that the earth and not the heavens is so moved, and there is no evidence to the contrary. Nevertheless, this seems prima facie as much, or more, against natural reason as are all or several articles of our faith. Thus, that which I have said by way of diversion (esbatement) in this manner can be valuable to refute and check those who would impugn our faith by argument.

[Excerpt quoted from the translation of Menut and Denomy, printed  in The Science of Mechanics in the Middle Ages by Marshall Clagett, pp. 600-06, with modifications and minor variations.]