Hellenistic thinkers made advances in mathematics, astronomy, and mechanical design. The most notable of the Hellenistic astronomers was Aristarchus of Samos (ca. 310–230 B.C.E.). Aristarchus rightly concluded that the sun is far larger than the earth and that the stars are enormously distant from the earth. He also argued against Aristotle’s view that the earth is the center of the universe, instead propounding the heliocentric theory — that the earth and planets revolve around the sun.

In geometry Euclid (YOO-kluhd) (fl. ca. 300 B.C.E.), a mathematician living in Alexandria, compiled a valuable textbook of existing knowledge. His The Elements of Geometry became the standard introduction to the subject.

The greatest thinker of the Hellenistic period was Archimedes (ahr-kuh-MEE-deez) (ca. 287–212 B.C.E.). A clever inventor, he devised new artillery for military purposes. In peacetime he created the water screw to draw water from a lower to a higher level. (See “Individuals in Society: Archimedes, Scientist and Inventor.”) He also invented the compound pulley to lift heavy weights. His chief interest, however, lay in pure mathematics. He founded the science of hydrostatics (the study of fluids at rest) and discovered the principle that the weight of a solid floating in a liquid is equal to the weight of the liquid displaced by the solid.

Eratosthenes (ehr-uh-TOSS-thuh-neez) (285–ca. 204 B.C.E.), who was the librarian of the vast Ptolemaic royal library in Alexandria, used mathematics to further the geographical studies for which he is most famous. He concluded that the earth is a spherical globe and calculated the circumference of the earth geometrically with remarkable accuracy.

As the new artillery devised by Archimedes indicates, Hellenistic science was used for purposes of war as well as peace. Theories of mechanics were applied to build military machines. The catapult became the most widely used artillery piece. As the Assyrians had earlier, engineers built siege towers, large wooden structures that served as artillery platforms, and put them on wheels so that soldiers could roll them up to a town’s walls. Generals added battering rams to bring down large portions of walls. If these new engines made warfare more efficient, they also added to the misery of the people, as war often directly involved the populations of cities. War and illness fed the need for medical advances, and doctors as well as scientists combined observation with theory during the Hellenistic period. Herophilus, who lived in the first half of the third century B.C.E., worked in Alexandria and studied the writings attributed to Hippocrates. He approached the study of medicine in a systematic, scientific fashion: he dissected dead bodies and measured what he observed. His students carried on his work, searching for the causes and nature of illness and pain.