in the ancient astronomy, the eighth heaven or sphere, with respect to the seven spheres of the planets which it surrounds. It is supposed to have two motions; a diurnal motion, given to it by the primum mobile, from east to west, about the poles of the ecliptic; and another opposite motion, from west to east; which it completes, according to Tycho, in 25,412 years; according to Ptolemy, in 36,000; and according to Copernicus, in 25,800; when the fixed stars return to the same precise points in which they were at the commencement. This period is commonly called the Platonic or great year.