A. All Astronomers Agree that there are at least Nine Heavens

31. Here the supposition must be made that no star [sc. planet] has a proper local motion, that is, that it is not moved with a motion other than with the motion of the sphere in which it is located; for if it left the part of the sphere where it is and moved to another part, either nothing would succeed to it (and so there would be a vacuum), or something else would succeed to it (and so the heavenly body would be made thinner or denser, or it could be rent and, when the rending body recedes, be again continuous), or if none of these is conceded, then the result is that a moved star is always in the same place as another moving body. - This is what Aristotle means in On the Heaven 2.8.290a29-35, “If nature had given a power of progress to the stars, etc.”

32. With this supposition it follows that all the stars that are not at the same distance from each other are not in the same heaven, for different distances at different times cannot be by a motion proper to the star but only by the motion of the heaven in which it is; and if one star is at different distances from another, then the heaven of the first is differently moved from the heaven of the second, and so the heaven of the first is other than that of the second.

33. Now seven stars [sc. the seven planets] are moved differently, so that they are not always at the same distance from the fixed stars, which are for this reason called fixed that they are always at the same distance from each other and keep the same place and local arrangement; and so there is no need to posit more than one heaven in respect of all the fixed stars. But the seven planets are not always at the same distance from each other.

34. These two points, about the varying distance of the planets from each other and from the other stars that are called fixed, are the supposition for the second consideration of the astronomers; for it is possible to be sure of the positions of the planets by means of instruments, one of which - namely armillary spheres - is dealt with by Ptolemy in the Almagest statement 5 ch.1.

35. And if it is objected that ‘the visual ray is refracted because of the diversity of the mediums, and so does not give certainty about the position of the stars’, then at least it will give certainly about the visible position of a star; and if the stars be at the same distance according to the visible position, then they are so also according to their true position, because their varying visible positions are disposed proportionally to their true positions (or at least they are not so disproportionally disposed that there could be so great a distance in their visible positions without some distance in their true positions), and this distance suffices for the matter at issue.

36. So, in addition to the sphere posited for all the fixed stars that are always at the same distance from each other, at least seven other spheres are posited proper to the seven planets that are moved differently both from the fixed stars and from each other; therefore there are eight heavens.

37. Further it is also commonly conceded that there is a ninth heaven higher than the starry heaven, because there is only one proper motion to one celestial body; but the diurnal motion is not proper to the motion of the starry heaven, since the starry heaven is moved by another motion, as has been proved by observations (for no fixed star is always at the same distance from the stationary poles, nor is it even at the same distance at the same times from the houses of Aries and Libra); therefore this diurnal motion will be proper to some other body - and only to a higher body, because the eighth heaven [sc. that of the fixed stars] is moved by this motion (but no heaven is moved by the motion proper to another unless that other is higher than it). There is therefore some movable body that is uniformly moved with a diurnal motion, higher than the starry heaven; Avicenna says this in his Metaphysics 9 ch.2 and Ptolemy in his Almagest statement 1 ch.8, statement 7 ch.3.