Note 628 — BOOK X.

Hence motion darts its force.—This is the tenth sphere, the Primum Mobile of the ancient system. To account for the appearances of the heavens, the Peripatetics ascribed a double motion to it. While its influence drew the other orbs from east to west, they supposed it had a motion of its own from west to east. To effect this, the ponderous weight and interposition of the ninth sphere, or crystalline heaven, was necessary. The ancient astronomers observed that the stars shifted their places. This they called the motion of the crystalline heaven, expressed by our poet at the rate of one pace during two hundred solar years. The famous Arab astronomer, Abulhasan, in his Meadows of Gold, calculates the revolution of this sphere to consist of 49,000 of our years. But modern discoveries have not only corrected the calculation,{*} but have also ascertained the reason of the apparent motion of the fixed stars. The earth is not a perfect sphere; the quantity of matter is greater at the equator; hence the earth turns on her axis in a rocking motion, revolving round the axis of the ecliptic, which is called the procession of the equinoxes, and makes the stars seem to shift their places at about the rate of a degree in 72 years; according to which all the stars seem to perform one revolution in the space of 25,920 years, after which they return exactly to the same situation as at the beginning of this period. However imperfect in their calculations, the Chaldean astronomers perceived that the motions of the heavens composed one great revolution. This they called the annus magnus, which those who did not understand them mistook for a restoration of all things to their first originals.

{*} However deficient the astronomy of Abulhasan may be, it is nothing to the calculation of his prophet Mohammed, who tells his disciples, that the stars were each about the bigness of a house, and hung from the sky on chains of gold.