THEORICA

The text discusses the arrangement of the celestial spheres, questioning why astronomers posited multiple, non-physical spheres for the planets.

...not disorderly, but when it is uniform, it is therefore necessary that it should proceed from a circle declining from the aequinoctialis equinoctial/celestial equator. From whence that circle was derived, it is the one proper to the wandering stars planets, and the motion of the sun was discovered in it according to truth, truly through the two sides of this circle, and upon it run the moon and the five wanderers, and through it they pass from north to south and from south to north, without any of them exceeding the space of distance determined for it from the two sides, not even by a little. And this is exactly what he said about the diversity of the motions of the seven stars, also positing the obliquus oblique/ecliptic circle the same for all.

We, however, ask: what prevents one from positing that all the spheres are only two? One is that which is moved upon the poles of the universe, and the other is that which is moved upon the other two poles, namely, the poles of the circle designated by the sun. And how can it be that there are many spheres, all of which have no more than only two poles, and yet they are moved by diverse motions? But I opine that the reason which led him to think that the motion of all the spheres which are below the superior sphere, which is moved by the daily motion, is one, and is only upon the two poles of the circle designated by the sun by its motion, is because the poles of the seven spheres are very close to one another, and there is no great excess of distance between the stars placed in them.

And since those observing the places of the fixed stars found them to also be changed from their proper places, both in longitude and latitude, they therefore judged concerning these that their sphere also moved upon the two poles of the same oblique circle. Wherefore they posited the primary motions of all these spheres—namely, of that which is moved by the daily motion and of all those which are beneath it—as two motions. One of these is the motion of the superior sphere moving the universe with a daily motion, and the two poles of this motion are the poles of the equinoctial. The other, however, is the motion which they opined to be contrary to this, and it is the motion of the remaining spheres—namely, the sphere of the fixed stars and the seven spheres which are beneath it—and the two poles of this motion are the poles of the circle designated by the sun, which is called the circle of signs the zodiac. It is as if this motion among them were that of one sphere existing below the first sphere and above the sphere of the fixed stars, which would be moved against the motion of the first sphere and would move everything that is below it upon its own poles, whereas the spheres which are below it do not have poles upon which they might move.

And all this, in truth, is something imaginary, and to call this imagined sphere the sphere of signs, which in reality is not found. However, for each of their spheres there are indeed two poles, and the poles of each are diverse, and the motions of all the wandering stars are not upon the two poles of the circle designated by the sun; how much more so, how will the motion of the sphere of the fixed stars be upon them? And that from which it is perceived that this oblique circle, which he said to be the root of the motion of the sphere of the fixed stars and the spheres of the other stars, is not the root of their motion, is that its declination is not remaining in the same manner. For we find the two points of intersection with the equinoctial to be changed, and thus also the points of the two solstices, and this indeed according to the change of the sphere of the fixed stars. And it appears that the sphere called by them the sphere of signs follows in its motion the sphere of the fixed stars. How, therefore, can it happen that it is following, and the cause that another should follow, and this in truth is among the greatest lies and impossibilities.

But that which brought this error into his roots to posit this circle as the root of all those motions, and led them to great complexity and to posit that which is impossible to exist in the nature of things—and it is that they would require also the motion of declination which they affirmed to be of the spheres of the superiors, and Venus, and Mercury—is in my judgment because it was perceived by the senses by everyone that all the stars were moved in a single revolution by day and by night from east to west upon two fixed poles and equidistant circles, as far as the eye can see, the largest of which is called the equinoctial. Further observing these stars, he testified that he had seen some of them to be postponed to others, as he said, and it is the distance of one from another toward the eastern part; and they saw with them, beyond slowness, an emergence in latitude and declination from that middle circle, sometimes to the north and sometimes to the south, whence they judged...