Astronomy, DESCRIPTIVE

Plain account of the appearances, motions, and general characteristics of the heavenly bodies. That the Chaldeans observed celestial phenomena with great diligence is attested by their discovery, in a purely empirical manner, of the period, called by them the saros, comprising 223 lunations, or 18 years and 10 days. At the end of this period the earth, sun and moon are found in very nearly the same relative positions as at the beginning, and eclipses of the sun and moon follow each other in the same order in succe ive periods. Thus they were able to predict the future occurrence of eclipses by a simple arithmetical operation.