regarded Mercury and Venus as satellites of the sun. Some have maintained that the obelisks which they erected served the purpose of gnomons, for determining the obliquity of the ecliptic, the altitude of the pole, and the length of the tropical year. It is thought that even the Pyramids, by the position of their sides toward the cardinal points, attest their acquaintance with a meridional line. The Chinese boast of having noticed and recorded a series of eclipses extending over a period of three thousand eight hundred and fifty-eight years, and it is probable that they anticipated the Greeks two thousand years in the discovery of the Metonic cycle, or the cycle of nineteen years, at the end of which time the new moons fall on the same days of the year. They determined the obliquity of the ecliptic, one thousand one hundred years before our era, to be 23 degrees 54' 3-15". The Indians, at a remote antiquity, represented celestial phenomena with considerable exactness, and constructed tables by which the longitude of the sun and moon are determined. Bailly thinks that astronomy was cultivated in Siam three thousand one hundred and two years before Christ, which hardly yields in accuracy to that which modern science has built on the theory of universal gravitation. The Greeks divided the heavens into constellations fourteen centuries before Christ. Thales, born 640 B.C., taught the rotundity of the earth, and that the moon shines with reflected light. He also predicted eclipses. Anaximander, born 610 B.C., invented the gnomon, and constructed geographical charts. [Sidenote: The early Greek investigators.]