The Sphere of the World

...of the fixed stars and of the ultimate heaven. Any one of these spheres surrounds the lower ones in a spherical manner. There are indeed two motions of these. One is that of the ultimate heaven The Primum Mobile or "First Mover," the outermost sphere in the medieval cosmos that drives the motion of all others. over the two extremities of the axis, namely the arctic and antarctic poles, from the east through the west, returning again to the east; this is divided through the middle by the equinoctial circle The celestial equator.. There is also another motion of the lower spheres along an oblique path The ecliptic, the path the sun appears to follow across the sky. which is opposite to the first, revolving on its own poles which are distant from the first by 23 degrees and 33 minutes. But the first motion carries all other spheres with it by its own force once within a day and a night around the earth; nevertheless, they strive against it, just as the eighth sphere The sphere of the fixed stars. moves one degree in 100 years. Indeed, the zodiac The circle of twelve constellations through which the planets appear to travel. divides this second motion through the middle, beneath which each of the seven planets has its own sphere in which it is carried by its own motion against the motion of the ultimate heaven. Each planet measures this motion in various spaces of time: Saturn in 30 years; Jupiter in 12; Mars in two; the Sun in 365 days and almost six hours; Venus and Mercury move almost similarly; the Moon, however, in 27 days and 8 hours.

On the revolution of the heavens.

¶ That the heaven revolves from east to west is proven by a sign. The stars which rise in the east are always raised gradually and successively until they come to the middle of the sky; and they are always at the same proximity and distance to one another; and thus, maintaining themselves, they tend toward their setting continuously and uniformly. There is also another sign. The stars which are near the arctic pole, which never set for us, move continuously and uniformly around the pole, describing their own circles; and they are always at an equal distance and proximity to one another. Thus, by these two continuous motions of the stars—both those tending toward the setting and those which do not—it is clear that the firmament The sky or the heavens, particularly the sphere of the fixed stars. moves from east to west.

On the roundness of the heavens.

¶ That the heaven is round, there is a triple reason: similarity, convenience, and necessity. Regarding similarity: because the sensible world The physical world that can be perceived by the senses. was made in the likeness of the archetypal world The divine or ideal blueprint of the universe, a concept from Platonic and Neoplatonic philosophy., in which there is no beginning...