From the dawn of written history, when the first men, watching through the nights, observed the regular motions of the moon and stars, humanity has been striving to obtain a correct understanding of the relation of the earth to the. First it was believed that the sun, moon, and stars revolved in circles around the earth (which for a time was supposed to be flat instead of spherical). The great Greek philosopher, Hipparchus, after observing the movements of the heavenly bodies, suggested that the earth was not exactly in the middle of the circles. Three hundred years later, Ptolemy discovered a number of facts concerning the movements of the sun, moon and planets, which were unknown to Hipparchus, and which led him to suggest that the sun and moon move in circles around the earth, but that the planets move around the earth in circles, whose centres again move around the earth. This somewhat complex theory explained very well what was known of astronomy in the days of the ancients. In fact, the views of Ptolemy were quite generally accepted for 1300 years. About 1500, A. D., Copernicus, a Dutch astronomer, having still more facts in his possession than had Ptolemy, concluded that the simplest manner in which the apparent movements of the sun, moon, and planets could be explained, was to assume that the sun is the center of the planetary system, and that the earth, with the moon and planets, revolves according to definite laws around the sun. This theory, supported by numerous confirmatory observations, was generally accepted by astronomers, and really did explain very simply and clearly many of the facts of planetary motion. Fifty years after the death of Copernicus, the celebrated astronomer, Kepler, proposed extensions and improvements of the Copernican doctrine, which made the theory that the planets revolve about the sun more probable than ever before. He suggested first that the planets move around the sun in closed curves, resembling flattened circles, and known as ellipses. By assuming this to be true, and assisted by other discoveries, he was also able to state the times required by the planets for their revolutions around the sun, and the velocity of their motions at different times of the year. Later investigations have proved the great laws proposed by Copernicus and Kepler to be true; and from their days is dated the birth of modern astronomy. [Sidenote: The law of gravitation is universal and explains many of the motions of celestial bodies.]