Firmament (Lat. firmamentum, support), in ancient astronomy, the eighth sphere or heaven which surrounded the seven spheres of the planets and supported the fixed stars. Two motions were attributed to it: the diurnal motion from east to west, and a motion from west to east, which was completed, according to Ptolemy, in 36,000 years, when the fixed stars were again in precisely the same position as at the beginning. This period was called the Platonic or great year. In common language, firmament signifies the sky or heaven.