The Immobility of the Earth in the Center of the World

Author: Thomas Aquinas

The Immobility of the Earth in the Center of the World

Thomas Aquinas

1. Inasmuch as the Philosopher [that is, Aristotle] has described the opinions of others concerning the earth, he here considers it according to the truth [of the matter]. First he solves the problem of the earth’s place and rest; and secondly, where he says "Its shape must necessarily be spherical" (297a.9), he considers its shape. He makes two points concerning the first part: First he determines the truth by means of natural reasons; and secondly, where he says "This view is further supported . . ." (297a.3), he resorts to astronomical appearances. With regard to the first of these [two parts], he makes two points: First he shows that it is impossible for the earth to be moved; secondly, where he says ["From what we have said the explanation of the earth’s immobility] is also apparent" (296b.28), he gives the true explanation for the earth’s immobility from premises.

Concerning the first part, he makes three points. First he directs attention to what must be discussed first, namely whether the earth moves or rests.If it moves, then we must move on to other things that have to be considered about the earth. And thus he posits this first in order to assume it as a beginning for what is to follow.

Secondly, where he says "For, as we said, [there are some who make it one of the stars] . . ." (296a.25), he shows the necessity for the aforesaid inquiry. For, as stated above, some, namely the Pythagoreans, assumed that the earth was moved around the center of the world—as if it were one of the planets; others, indeed, assuming that the earth is in the center, as it is written in the Timaeus, say that it is rotated around the middle of the pole—that is, around the axis dividing the sky down the middle.

2. Thirdly, where he says "That both views are untenable . . ." (296a.27), he shows by means of four reasons that it is impossible for the earth to be moved in this way. In the first of these [reasons] he takes it as a starting point that if the earth were moved circularly whether in the center of the world or outside of it—such a motion would be necessarily violent. Now, it is obvious that circular motion is not a proper and natural motion for the earth; for if it were, every particle of earth should also have this motion, since, as stated above, a natural motion is the same for whole and part. But we see that this is false, because all parts of the earth are moved with a rectilinear motion toward the center of the world. If, however, the motion of the earth were circular, it would be violent and contrary to nature, and [therefore] could not be eternal, since, as was said earlier, nothing violent is eternal. But if the earth were moved circularly it would be necessary that such a motion be eternal assuming, [of course,] that the world is eternal. In his [that is, Aristotle’s] opinion this is so, because it is necessary that the universal order be eternal, and the motion or rest of the principal parts of the world [that is, heavens and earth] are part of this order. It follows, therefore, that the earth is not moved with a circular motion.

3. Where he says "Again, everything [that moves with the circular movement] . . ." (296a.34), he posits the second reason, which is this. All bodies that are moved circularly seem to drop back, that is, do not maintain a constant position, because every one of them except the first sphere, which according to Aristotle is the sphere of the fixed stars is moved with several motions and not by one only. Therefore, if the earth—whether it is in the center or outside the center—has a circular motion, it would necessarily be moved with several motions, namely with the motion of the first sphere around the equinoctial poles [that is, the poles of the equator], and with some other proper motion around the poles of the zodiac. But this cannot be, for if it were so, the fixed stars would exhibit changes and turnings with respect to the earth, which, in virtue of its proper motion, would lag behind and not return to the same point together with a fixed star—just as happens with the planets. This applies either to the whole earth or any part that may be designated. And so it would follow that the fixed stars would not always be seen to rise and set at the same part of the earth. But this does not happen, because the fixed stars rise and set at the same designated places. Therefore, the earth is not moved circularly.

4. Where he says "Further, the [natural] movement [of the earth] . . ." (296b.7), he asserts the third reason, which depends upon the motion of parts of the earth and of the whole. He makes three points concerning this. In the first place, he proposes what the natural motion of the earth and its parts must be; secondly, where he says "but it might be [questioned] . . ." (296b.9), he raises a certain doubt concerning this; and thirdly, he proves what he proposed.

In the first place, then, he says that motion of parts of the earth—in accordance with the nature of earth—is toward the center of the whole world; and, similarly, if the whole earth were outside the center of the world, it would, by its very nature, be moved to the center of the world, because the natural motion of the whole is the same as that of a part.

5. Then, when he says "but it might be [questioned] . . ." (296b.9), he raises a certain doubt. First, he proposes the doubt, saying that if the earth were assumed in the middle or center of the world—he means by this that the center of the whole world and of this earth would be the same one can ask to which of these are heavy bodies especially parts of the earth—moved naturally. That is, are they moved to the center because it is in the center of the world, or because it is the center of the earth?

Secondly, where he says "The goal, surely . . ." (296b.13), he resolves the doubt, saying that heavy bodies are moved to the center because it is the center of the whole world. For the motion of heavy bodies is contrary to the motion of light bodies; but light bodies, especially fire, are moved to the extremity of the [innermost] celestial body. Therefore, heavy bodies, especially earth, are moved to the center of the earth, not, however, per se but accidentally, because the center of the earth and the center of the world coincide. . . .

6. Thirdly, where he says "That the center of the earth is the goal of their movement . . ." (296b.18), he proves what he assumed, namely that heavy bodies and parts of the earth are moved to the center. . . .

7. Where he says "It is clear, then . . ." (296b. 21), he concludes what he has proposed. But he infers two propositions. The first of these is that the earth is in the center of the world, which is deduced from premises in the following way: All heavy bodies are self-moved toward the center of the world; furthermore, as was [just] demonstrated, they are also moved toward the center of the earth; therefore, the center of the earth is the center of the world. And so the earth is in the center of the world.

The second proposition is that the earth is immobile. This is deduced from premises in the following way "Nothing is moved [when it is] in a place to which it is moved naturally, since it is natural for it to rest there; but earth is moved naturally toward the center of the world; therefore, it is not moved in the center. Moreover, as has been shown, the earth cannot but be in the center of the world; therefore, the earth is in no way moved.

8. He posits the fourth reason where he says "because heavy bodies forcibly thrown . . ." (296b.23). We see that if a stone were suspended above some table, it would fall down in a straight line; and as long as its motion is downward it will fall along the same straight line. Now, if the table were not moved, the stone would fall to the [very] same place where it fell before; but if the table were moved, the stone would fall to another place that would be as far removed [from the previous place] in proportion as the height from which the stone is dropped increases, since a greater time will elapse between the beginning and the end of the fall. However, we see that heavy bodies fall downward in a regular manner—that is, in a straight line—and that they fall to the same place on earth when dropped repeatedly from a particular place. Now, someone might say that because of the slowness of the earth’s motion, there is an imperceptible distance between each of these places. But this overlooks the fact that if someone were to cause the stone to fall downward time after time for an infinite number of times, the same thing would occur, so that the extent of the time [involved] should make the distance between the places perceptible. And so it is obvious that the earth is not moved.

Then, in conclusion, he deduces from premises that the earth can neither be moved nor located outside the center of the world. . . .

11. That the earth is not movable from place to place with a translatory motion results from the fact that it is always in the center. But should it be moved with any motion whatever, it would follow, again, that in virtue of its velocity all other motions—whether of clouds or animals—would be concealed from us. For that does not appear to be moved which is moved more slowly near a body that is moved more quickly.

Therefore, in closing, the Philosopher concludes that enough has been said on the relation between the position, motion, and rest of the earth.

Related Resources

Writings on Astronomy

Download Options


Title: The Immobility of the Earth in the Center of the World

Select an option:

*Note: A download may not start for up to 60 seconds.

Email Options


Title: The Immobility of the Earth in the Center of the World

Select an option:

Email addres:

*Note: It may take up to 60 seconds for for the email to be generated.

Chicago: Thomas Aquinas, The Immobility of the Earth in the Center of the World in A Source Book in Medieval Science, ed. Edward Grant (Cambridge, MA: Harvard University Press, 1974), 496–499. Original Sources, accessed February 21, 2024, http://originalsources.com/Document.aspx?DocID=E847WGAPL4RHJNM.

MLA: Aquinas, Thomas. The Immobility of the Earth in the Center of the World, in A Source Book in Medieval Science, edited by Edward Grant, Cambridge, MA, Harvard University Press, 1974, pp. 496–499. Original Sources. 21 Feb. 2024. http://originalsources.com/Document.aspx?DocID=E847WGAPL4RHJNM.

Harvard: Aquinas, T, The Immobility of the Earth in the Center of the World. cited in 1974, A Source Book in Medieval Science, ed. , Harvard University Press, Cambridge, MA, pp.496–499. Original Sources, retrieved 21 February 2024, from http://originalsources.com/Document.aspx?DocID=E847WGAPL4RHJNM.