Having spoken of the sphere and motion of Saturn and the other planets, I shall now tell how they turn backwards, which is called retrogressio, i.e., a back turning, i.e., when the planet turns back from Aries to Piscis.
To explain that, I will make a figure of the two spheres of Saturn, and outside of them I will place the sphere of the signs and divide it into twelve parts, and then inside of it I will place the eccentric sphere of Saturn, and above at the top of it I will place the sphere in which the body of the planet is fixed, and then I will place the earth in its own position with E in the middle of it and Saturn in four small circles around its own circumference. I will place the first of those small circles at the top of its own circle and a line through it between Aries and Piscis; the second circle in the first stopping place, with B in the middle of it ; the third circle in the place where it turns back, with C in the middle of it; the fourth circle in the second stopping place with D in the middle of it, and I will draw three lines from the centre of the earth up through Saturn to the figure of the signs which are in the sphere of the fixed stars. Those lines represent the sight of the eyes up from the earth towards Saturn, and I will draw the vertical line up
p.131from the earth towards A, and thus when Saturn is in position A, it is in the beginning of Aries in the Zodiac, and its motion is eastwards full and direct, and when it has moved thus eastwards some degrees in Aries it inclines towards B, and when it arrives there it moves neither to the east nor west, and, therefore, that is its first halt, and when it departs from that position it moves back to D, and that is the place of its retrogression, and there it has a full direct motion from the east of the world to the west; and as it moves westwards, when it reaches position C, that is its second halting place, because there it moves neither to the east or the west. Whosoever would then look up from the middle of the earth he would find then in Piscis the same Saturn which was previously in Aries, and on its leaving D in the east it will mount by degrees again to A. That is the reason why the planets appear larger at one time than at another, because, as they make a retrogressive motion towards the earth, they appear at their largest.
All that I have said concerning spheres and motions and every other quality which Saturn possesses, ought to be understood with regard to Jupiter and Mars, since there is no difference between them in their course, or in their motion, or in their actions. The three planets that are above the sun experience the same things, although they are not evident from the moon on account of the excessive speed of its eccentric sphere, because the sphere which holds the moon firmly moves eastwards, and when it turns on its backward course it moves westwards, and, therefore, that revolution is not evident although its other course and its halting are evident, because one day it moves twelve degrees and another it moves fourteen degrees.