A Source Book in Medieval Science

Author: Claudius Ptolemy  | Date: 1952

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On the Possible Diurnal Rotation of the Earth

Introduction by Edward Grant

1. Ptolemy: The Immobility of the Earth in the Center of the World

Translated by R. Catesby Taliaferro1

Annotated by Edward Grant


By the same arguments as the preceding it can be shown that the earth can neither move in any one of the aforesaid oblique directions, nor ever change at all from its place at the centre. For the same things would result as if it had another position than at the centre. And so it also seems to me superfluous to look for the causes of the motion to the centre when it is once for all clear from the very appearances that the earth is in the middle of the world and all weights move towards it. And the easiest and only way to understand this is to see that, once the earth has been proved spherical considered as a whole and in the middle of the universe as we have said, then the tendencies and movements of heavy bodies (I mean their proper movements) are everywhere and always at right angles to the tangent plane drawn through the falling body’s point of contact with the earth’s surface. For because of this it is clear that, if they were not stopped by the earth’s surface, they too would go all the way to the centre itself, since the straight line drawn to the centre of a sphere is always perpendicular to the plane tangent to the sphere’s surface at the intersection of that line.

All those who think it paradoxical that so great a weight as the earth should not waver or move anywhere seem to me to go astray by making their judgment with an eye to their own affects and not to the property of the whole. For it would not still appear so extraordinary to them, I believe, if they stopped to think that the earth’s magnitude compared to the whole body surrounding it is in the ratio of a point to it. For thus it seems possible for that which is relatively least to be supported and pressed against from all sides equally and at the same angle by that which is absolutely greatest and homogeneous. For there is no "above" and

"below" in the universe with respect to the earth, just as none could be conceived of in a sphere. And of the compound bodies in the universe, to the extent of their proper and natural motion, the light and subtle ones are scattered in flames to the outside and to the circumference, and they seem to rush in the upward direction relative to each one because we too call "up" from above our heads to the enveloping surface of the universe; but the heavy and coarse bodies move to the middle and centre and they seem to fall downwards because again we all call "down" the direction from our feet to the earth’s centre.2 And they properly subside about the middle under the everywhere-equal and like resistance and impact against each other. Therefore the solid body of the earth is reasonably considered as being the largest relative to those moving against it and as remaining unmoved in any direction by the force of the very small weights, and as it were absorbing their fall. And if it had some one common movement, the same as that of the other weights, it would clearly leave them all behind because of its much greater magnitude. And the animals and other weights would be left hanging in the air, and the earth would very quickly fall out of the heavens. Merely to conceive such things makes them appear ridiculous.

Now some people, although they have nothing to oppose to these arguments, agree on something, as they think, more plausible. And it seems to them there is nothing against their supposing, for instance, the heavens immobile and the earth as turning on the same axis from west to east very nearly one revolution a day,3 or that they both should move to some extent, but only on the same axis as we said, and conformably to the overtaking of the one by the other.

But it has escaped their notice that, indeed, as far as the appearances of the stars are concerned, nothing would perhaps keep things from being in accordance with this simpler conjecture, but that in the light of what happens around us in the air such a notion would seem altogether absurd.4 For in order for us to grant them what is unnatural in itself, that the lightest and subtlest bodies either do not move at all or no differently from those of contrary nature, while those less light and less subtle bodies in the air are clearly more rapid than all the more terrestrial ones; and to grant that the heaviest and most compact bodies have their proper swift and regular motion, while again these terrestrial bodies are certainly at times not easily moved by anything else—for us to grant these things, they would have to admit that the earth’s turning is the swiftest of absolutely all the movements about it because of its making so great a revolution in a short time, so that all those things that were not at rest on the earth would seem to have a movement contrary to it, and never would a cloud be seen to move toward the east nor anything else that flew or was thrown into the air. For the earth would always outstrip them in its eastward motion, so that all other bodies would seem to be left behind and to move towards the west.

For if they should say that the air is also carried around with the earth in the same direction and at the same speed, none the less the bodies contained in it would always seem to be outstripped by the movement of both.5 Or if they should be carried around as if one with the air, neither the one nor the other would appear as outstripping, or being outstripped by, the other. But these bodies would always remain in the same relative position and there would be no movement or change either in the case of flying bodies or projectiles. And yet we shall clearly see all such things taking place as if their slowness or swiftness did not follow at all from the earth’s movement.6

1. Reprinted with the permission of the Encyclopaedia Britannica from Ptolemy’s Almagest, Book I, chapter 7, as translated by R. Catesby Taliaferro in Great Books of the Western World (Chicago: Encyclopaedia Britannica, 1952), XVI, 10–12.

2. Unlike Aristotle, Ptolemy seems here to deny an absolute "up" and "down" with respect to the earth. These directions are only relative.

3. This interpretation was held by Heraclides of Pontus and Aristarchus of Samos (see sections 3 and 5 of this selection and n. 22). By reporting this important minority opinion, Ptolemy joins Simplicius (Commentary on Aristotle’s De caelo) as one of three primary sources introducing this conception to medieval authors (the third is Aristotle’s De caelo, where Aristotle seems to attribute to Plato a belief in the axial rotation of the earth at II.13.293b.30–32 and to mention it again, rather vaguely, in II. 14.296.b.1–4). Aquinas probably derived it from Simplicius’ commentary in the translation made available by William Moerbeke (see number 47 of Moerbeke’s list in Selection 8); Oresme probably knew all three sources (see section 5 of this selection).

4. It is of interest to note that Ptolemy concedes that the earth’s diurnal rotation is compatible with the purely astronomical phenomena, while holding it absurd in terms of physics.

5. Buridan was unquestionably familiar with this argument, for he reports that adherents of diurnal rotation say that "the earth, water, and the air in the lower region are moved simultaneously with diurnal motion (see pars. 7 and 8 of section 4 of this selection). While Buridan appears to accept this, he rejects diurnal rotation by arguing that bodies moved violently ought to resist the motion of the air as it accompanies a moving earth. Experience shows that they do not (see par. 9). Copernicus explicitly reports and rejects Ptolemy’s opinion on the disruptive effects of a terrestrial motion (see Bk. I, ch. 7 and ch. 8, of Copernicus’ De revolutionibus, in section 6 of this selection).

6. This last argument, that there would be no detectable motion if the air and everything in it rotated with the earth, seems to have been answered indirectly by Oresme and Copernicus. Thus Oresme says (in section 5) that "according to this opinion, not only the earth moves, but also with it the water, and the air, as we stated above, although the water and air here below may be moved in addition by the winds or other forces." Perhaps influenced by Oresme, Copernicus conjectures that despite the rotation of the earth, "the air nearest Earth, with the objects suspended in it, will be stationary unless disturbed by wind or other impulse which moves them this way or that—for a wind in the air is as a current in the sea" (Bk. I, ch. 8, in section 6).


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Chicago: Claudius Ptolemy, "On the Possible Diurnal Rotation of the Earth," A Source Book in Medieval Science, trans. R. Catesby Taliaferro in A Source Book in Medieval Science, ed. Edward Grant (Cambridge: Harvard University Press, 1974), 494–496. Original Sources, accessed February 20, 2020, http://originalsources.com/Document.aspx?DocID=JL22R28VFMUI9SU.

MLA: Ptolemy, Claudius. "On the Possible Diurnal Rotation of the Earth." A Source Book in Medieval Science, translted by R. Catesby Taliaferro, Vol. XVI, in A Source Book in Medieval Science, edited by Edward Grant, Cambridge, Harvard University Press, 1974, pp. 494–496. Original Sources. 20 Feb. 2020. originalsources.com/Document.aspx?DocID=JL22R28VFMUI9SU.

Harvard: Ptolemy, C, 'On the Possible Diurnal Rotation of the Earth' in A Source Book in Medieval Science, trans. . cited in 1974, A Source Book in Medieval Science, ed. , Harvard University Press, Cambridge, pp.494–496. Original Sources, retrieved 20 February 2020, from http://originalsources.com/Document.aspx?DocID=JL22R28VFMUI9SU.