3rd sphere: that of the planet Venus.
4th sphere: that of the Sun; regarded as a planet.
5th sphere: that of the planet Mars.
6th sphere: that of the planet Jupiter.
7th sphere: that of the planet Saturn.
8th sphere: that of the fixed stars.
The eighth sphere included all the fixed stars, and was called the firmament, because it was believed to impart steadiness to the inner spheres, and, by its diurnal revolution, to carry them round the Earth, causing the change of day and night.
The separate motions of the spheres, revolving with different velocities, and at different angles to each other, accounted for the astronomical phenomena associated with the orbs attached to each. According to Ptolemy’s scheme, the eighth sphere formed the outermost boundary of the universe; but later astronomers added to this system two other spheres—a ninth, called the Crystalline, which caused Precession of the Equinoxes; and a tenth, called the Primum Mobile, or First Moved, which brought about the alternation of day and night, by carrying all the other spheres round the Earth once in every twenty-four hours. The Primum Mobile enclosed, as if in a shell, all the other spheres, in which was included the created universe, and, although of vast dimensions, its conception did not overwhelm the mind in the same manner that the effort to comprehend infinitude does.