in the ancient astronomy, the eighth heaven or sphere, in which the fixed stars were supposed to be placed. It is called the eighth, with respect to the seven heavens or spheres of the planets which it surrounds.

The firmament 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 258,000; when the fixed stars return to the same precise points in which they were at the commencement of this grand revolution. This period is commonly called the Platonic or great year.