FIRMAMENT, in the Ptolemaic 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 last it finishes, according to Tycho, in 25412 years, according to Ptolemy in 26000, and according to Copernicus in 25800, in which time the fixed stars return to the same points in which they were at the beginning. This period is commonly called the Platonic year, or the great year.