The Sphere of the World with Commentaries

Around the elemental region, indeed, the ethereal region—lucid and immune to variation in its immutable essence—proceeds in continuous circular motion; and this is called by philosophers the fifth essence. There are nine spheres of this, as was discussed previously: namely, those of the Moon, Mercury, Venus, the Sun, Mars, Jupiter, Saturn, the fixed stars, and the outermost heaven. Each of these superior spheres surrounds the inferior one spherically. Of these, there are two motions. One is the motion of the outermost heaven upon the two extremities of the axis—namely, the arctic pole and the antarctic pole—returning from the east through the west to the east again, which the equinoctial circle divides through the middle. There is also another motion of the inferior spheres along the oblique, opposite to this one, upon their own axes, which are distant from the first by 23 degrees and 33 minutes. But the first [motion] sweeps all other spheres with it by its own force once within a day and a night around the Earth, though they struggle against it, so that the eighth sphere [moves] by one degree in 100 years. The zodiac divides this second motion through the middle, under which each of the seven planets has its own sphere in which it is carried by its own proper motion against the motion of the outermost heaven, and measures it out in different spaces of time: as Saturn in 30 years, Jupiter in 12, Mars in two, the Sun in 365 days and almost six hours. Venus and Mercury do so similarly, while the Moon [does so] in 27 days and eight hours.

ON THE REVOLUTION OF THE HEAVEN.

That the heaven revolves from east to west is a sign: Stars that rise in the east always elevate themselves gradually and successively until they arrive at the middle of the heaven, and they are always at the same proximity and distance from each other; and thus, always remaining as they are, they tend toward the setting continuously and uniformly. There is also another sign: The stars that are near the arctic pole, which never set for us, move continuously and uniformly around the pole, describing their circles, and they are always at an equal distance and proximity to each other. Hence, from these two continuous motions of the stars—both those tending toward setting and those that do not—it is evident...