BİRİNCİ FEN · İKİNCİ BÂB · DOKUZUNCU FASIL · Üçüncü Madde
Third Article
It describes the epicycle of the Moon, along with its form and movements.
O Noble One! Know that astronomers have said: The fourth sphere of the Moon is also its epicycle—a small sphere not encompassing the earth, which carries the Moon itself. It is fixed and embedded in the belt of the eccentric sphere. The diameter of the epicyclic sphere is tangent to and equal to the two surfaces of the carrier sphere.
The epicyclic sphere is a solid, spherical body enclosed by a single surface. Within the carrier sphere, it revolves from east to west around its own center, without conforming to the order of the signs of the zodiac, carrying the Moon along with it, adhering to one side. The Moon’s epicycle moves contrarily to the epicycles of the other five planets. And this epicycle, through its continued movement around its own center from east to west, moves the Moon approximately thirteen degrees from three hundred sixty degrees of its belt in a day and night, completing one revolution after twenty-eight days, just as the carrier sphere does. This is also called differential motion or special motion.
The Moon itself is a solid, spherical body enclosed by a single surface, dark on the inside and luminous on the outside. It appears within its epicycle, as do the “five perplexed ones,” fixed between the two poles of the epicycle and alongside its belt, in such a way that it is tangent to the surface of the epicycle at a common point. That is, the body of the Moon is entirely upon the body of the epicycle, with its face touching its surface.
Just as there is movement of the epicycle on one side of the carrier sphere, observers have also noted that the Moon’s sphere has its own motion from east to west around its center, contrary to the rising of the signs of the zodiac, on the side of the epicycle. Based on this observation and verification, they have said, “There is no stillness in the spheres.” The central movement of the Moon upon the circumference of the epicycle is slight; likewise, the movement of the carrier sphere’s circumference is less than that of the central movement of the epicycle on it, so the Moon never appears to stop or turn back. Only its slow movement can be observed when it is at the solstice point.
The belt of the Moon's carrier sphere is tangent to the belt of its inclined sphere, and since the apex of the epicyclic sphere has reached the belt of the carrier sphere (as we have explained above), the Moon’s inclined sphere does not possess a belt other than five degrees north and south from the signs of the zodiac. It also lacks any difference in latitude, as the Moon's inclined, carrier, and epicyclic spheres are all tangent to each other with a single surface and never separate or incline in any way.
In this manner, the conditions of the Moon in these four spheres have settled into a system.