I declare, since Saturn has four motions, that it has four spheres in which it moves. The first motion that of the very great sphere from the east of the world to the west; the second motion, its own natural motion from the west of the world to the east, the third motion148, the motion of the sphere in which it itself is fixed, and in which it moves in a direct line, or backwards, swiftly or slowly; the fourth motion, the motion of the eccentric sphere, and it is in that motion (lit. on that sphere) every planet is raised as high as possible from the earth, and is lowered as near as possible to the earth; and these are the four motions that all the planets have, except the sun, which has two spheres and two motions.
I will again describe those four spheres together with their motions themselves; and first I will make a figure of the very great sphere, and the figure of the earth in the middle of it, and I will place A in the east of it and B at the top of it and C in the west of it and D at the bottom of it, and thus is the motion of the very great sphere from A to B, from B to C, from C to D, and from D to A.
I make a figure (Fig 26) of the second sphere, which moves from the west of the world to the east, and which is under the very great sphere, and in the direct line beneath the Zodiac; and the Zodiac is situated obliquely, and the very great sphere is directly over our heads, because, as I mentioned, their poles and their pivot are far apart.
I will make a figure (fig 27) of the third sphere149150, the eccentric sphere, inside the two preceding spheres. The centre of this sphere is south of the centre of the earth by two and a half degrees, according to the measurement of the diameter of the sphere, and is divided into one hundred and twenty parts;150 and this sphere is near the earth on one side, and distant from it on another.
I will make a figure (fig 28) of the fourth sphere151 which confines firmly the body of the planet within itself, inside of the other three spheres.
The centre of that planet152 forms the centre of that sphere in which it is, and it moves from the west to the east of the world; and at the top of the eccentric sphere is the centre of those planets like a firm immovable nail in a sphere. It is not a straight course like that of an arrow that the planets have, but a circular natural course like that of a cartwheel, moving from the west of the world to the east, and if there was a nail in the upper rim of the cart wheel153moving from the west of the world to the east, whilst the nail would move downwards towards the earth, it would not move westwards or eastwards, and when it would reach the earth, it would incline its course from east to west; and when it would rise up from the earth it would not move westwards or eastwards, but when it would reach the extreme top, then it would move eastwards; and this is what causes the planets to perform a forward course at one time and a backward course at another, and a swift course at one time and a slow one at another.