The Hellenistic period, following Alexander's conquests, continued and built upon this knowledge. Hipparchus is considered the pre-eminent astronomical observer of the ancient world, and was probably the first to develop an accurate method for the prediction of solar eclipse, while Aristarchus of Samos was the first known astronomer to propose a heliocentric model of the solar system, though the geocentric model of Ptolemy was more commonly accepted until the seventeenth century. Ptolemy also contributed substantially to cartography and to the science of optics. For his part Archimedes was the first to calculate the value of "π" and a geometric series, and also the earliest known mathematical physicist discovering the law of buoyancy, as well as conceiving the irrigation device known as Archimedes' screw.