But the splendid progress made in the scientific acquaintance with the celestial bodies at that time, is most worthy of note. Aristyllus and Timochares determined the position of fixed stars. Aristarchus of Samos, the cotemporary of Cleanthes, was acquainted with the ancient Pythagorean ideas, attempted to explore thoroughly the construction of the universe, and guessed at the double movement of the earth round its axis, as well as its progress round a central sun. Seleucus of Euthræ, a century later attempted to confirm the opinion of the Samian writer; and Hipparchus, the founder of scientific astronomy, became the greatest original observer of the stars in the whole of antiquity. He was the first author of astronomical tables, and the discoverer of the precession of the equinoxes. His own observations were made at Rhodes, and upon comparing them with those of Timochares and Aristyllus, he was led to this great discovery. In the same hands, celestial phenomena were first employed to determine the geographical position of certain places. The new map of the world, constructed by Hipparchus, touched upon eclipses, and the measurement of shadows, for the determination of the geographical latitudes and longitudes. Improvements cluster, and a new aid of great value soon appeared, in the hydraulic clock of Ctesibius, which measured time much more accurately than the Clypsydra, or water-glasses, formerly in use. For a corresponding improvement in the determination of space, better instruments were invented from time to time, dating from the ancient sun-dial and the scaphæ to the discovery of the Astrolabes, the solstitial rings, and the dioptric lines. Wider views and keener organs were afforded to increased scientific skill, which gradually led to a closer acquaintance with the loftiest planetary movement. But the knowledge of the absolute size, form, and physical properties of these bodies, made no progress whatever, that being reserved as the leading glory of a posterior age. The Augustan period, though it attained not to true astronomical science in the highest form, was yet remarkable in some departments of mathematics. Euclid, Appollonius of Perga, and Archimedes, were geometers of the highest class, who were intermediate between Plato and the Menæchmean figures and the age of Kepler and Tycho, Galileo and Laplace.