As that philosopher says, we see two kinds of motion in the firmamentone motion from east to west and the other from west to east of the world. The motion of the sun, moon, and each of the other five planets corresponds to the extent of the amplitude of their own spheres in the eastward motion. The westward motion moreover carries the planets with it westwards in a contrary direction, in opposition to their natural motion which is eastward.
I repeat that the sun, moon, and other five planets and all the fixed stars have the same equal motion, for of them all individually there is no star which moves more swiftly or more slowly than the other. Therefore, there is no difference in the world between the motion of the sun and moon, and the motion of the other stars, because it is certain that they have the same nature and form. Although Saturn appears to be slower than the moon in cosequence of the reason I shall now relate, their motion is equal.
As Ptolemy and the other philosophers declare, there are ten large spheres, and the largest sphere of those, which is called the very great sphere, possesses the same motion as the sphere of the signs, since both move westward.
The motion of the eight spheres moreover, i.e., the sphere of the fixed stars and that of the sun and of the moon and of the other five planets, is from the west to the east of, the world, as I have frequently remarked, and those spheres are situated within each other; and the sphere of the moon is the nearest to the earth, and then the spheres of Mercury and Venus respectively, and that of the sun outside those, and the spheres of Mars and Jupiter outside those, and the sphere of the fixed stars outside those. It is not because they do not move that they are called fixed stars, for
p.117they move from the west of the world to the east, as do the other planets, but because they do not incline from the north of the firmament to the south, as do those others. The sphere of the signs is the ninth sphere, and outside those one and all is the tenth sphere called the very great sphere, or by another name, the straight (?) sphere, (orbis rectus). Here without is a figure which represents them all.
I said above that the moon appears swifter than Saturn. If the moon were in the orbit of Saturn, it would be thirty years travelling as Saturn travels. Similarly Saturn would traverse the orbit of the moon, if he were in it, in twenty-eight days, and seven weeks less one day23, as it does itself. Thus it is the narrowness of the orbit they have, or the wideness of the other orbit (sic)24, which causes the planets that are in them to appear swift or slow and not that they are really so, for they have exactly the same course and nature, swiftness and slowness. If the sphere of Saturn were divided into three hundred and sixty equal parts to the centre of the earth and if each of those parts were given a circular form, each part would be equal to the sphere of the moon. If the sphere of the moon was opened out so that three hundred and fifty-nine times25 as much were added to it, and the whole made into the shape of a sphere, none the less would it be equal to the sphere of Saturn. Thus it is proved that it is the narrowness and the wideness of the orbits of the planets that makes some of them appear to have a swift and some a slow movement, although as I have repeatedly stated, such is not the case.
Ptolemy gave a clear example to explain the two motions I mentioned above, from east to west and from west to east of the world. Imagine that a wheel revolved from the east of the world to the
p.119west in a day and a night, and that there was a small circle around the centre of that wheel, and a circle twice as large outside it, and a third circle outside that three times as large as the first circle, the fourth circle outside of that four times larger than the first circle, and so on up to the eighth circle, each separate circle being a sphere, moving from the west of the world to the east. This wheel is like the very great sphere of the world and the small circles I mentioned are like the inner circles of that great sphere. Then, when the large first circle completes its first revolution the second circle is on the second part of its round, and the third circle on the third part, and the fourth circle on the fourth and the fifth on the fifth, and the sixth on the sixth, and the seventh on the seventh, and the eighth on the eighth. Thus when the eighth circle would have traversed its whole course the first circle would have made eight revolutions. Whilst those eight circles would be fulfilling their circular course, the wheel would revolve very frequently between those revolutions from the east to the west of the world and the eight circles would begin their own motion; and to enlighten the mind of the reader I have set down this diagram.