The main advances in Hellenistic science came in astronomy, geography, and mechanics. The most notable of the Hellenistic astronomers was Aristarchus (a-ruh-STAHR-kuhs) of Samos (ca. 310–230 B.C.E.). Aristarchus concluded that the sun is far larger than the earth and that the stars are enormously distant from the earth. He argued against the commonsense observation, which Aristotle had supported, that the earth was the center of the universe. Instead, Aristarchus developed the heliocentric theory — that the earth and planets revolve around the sun. His theory was discussed for several centuries, but was later forgotten when another astronomer working in Alexandria, Claudius Ptolemy (ca. 90–ca. 168 C.E.) — probably no relation to the ruling Ptolemies, as the name was a common one — returned to an earth-centered universe. Aristarchus’s heliocentric theory was resurrected in the sixteenth century C.E. by the brilliant Polish astronomer Nicolaus Copernicus.

In geometry Hellenistic thinkers discovered little that was new, but Euclid (YOU-kluhd) (ca. 300 B.C.E.), a mathematician who lived in Alexandria, compiled a valuable textbook of existing knowledge. His Elements of Geometry rapidly became the standard introduction to geometry. Generations of students from the Hellenistic period to the twentieth century learned the essentials of geometry from it.

The greatest thinker of the Hellenistic period was Archimedes (ca. 287–212 B.C.E.), a native of Syracuse who was interested in nearly everything. (See “Individuals in Society: Archimedes, Scientist and Inventor.”) A clever inventor, he devised new artillery for military purposes. In peacetime he perfected the water screw to draw water from a lower to a higher level. 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 volume of a solid floating in a liquid is equal to the volume of the liquid displaced by the solid.

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Archimedes was willing to share his work with others, among them Eratosthenes (ehr-uh-TAHS-thuh-neez) (285–ca. 204 B.C.E.). Like Archimedes, he was a man of almost universal interests. From his native Cyrene in North Africa, Eratosthenes traveled to Athens, where he studied philosophy and mathematics. He refused to join any of the philosophical schools, for he was interested in too many things to follow any particular dogma. Around 245 B.C.E. King Ptolemy III invited Eratosthenes to Alexandria and made him the head of the library there. Eratosthenes continued his mathematical work and by letter struck up a friendship with Archimedes.

Eratosthenes used mathematics to further the geographical studies for which he is most famous. He concluded that the earth was a spherical globe and calculated the circumference of the earth geometrically, estimating it as about 24,675 miles. He was not wrong by much: the earth is actually 24,860 miles in circumference. He drew a map of the earth and discussed the shapes and sizes of land and ocean and the irregularities of the earth’s surface. His idea that the earth was divided into large landmasses influenced other geographers and later shaped ordinary people’s understanding of the world as well. Using geographical information gained by Alexander the Great’s scientists, Eratosthenes declared that to get to India, a ship could sail around Africa or even sail directly westward, an idea that would not be tested until the end of the fifteenth century.

Other Greek geographers also turned their attention southward to Africa. During this period the people of the Mediterranean learned of the climate and customs of Ethiopia and gleaned some information about sub-Saharan Africa from Greek sailors and merchants who had traveled there. (See “Evaluating the Evidence 4.3: The Periplus of the Erythraean Sea.”) Geographers incorporated these travelers’ reports into their more theoretical works.

As the new artillery devised by Archimedes indicates, Hellenistic science was used for purposes of war as well as peace. Theories of mechanics were used to build machines that revolutionized warfare. Fully realizing the practical possibilities of the first effective artillery in Western history, Philip of Macedonia had introduced the machines to the broader world in the middle of the fourth century B.C.E. The catapult became the first and most widely used artillery piece, shooting ever-larger projectiles. Generals soon realized that they could also hurl burning bundles over the walls to start fires in the city. 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. Once there, archers stationed on top of the siege towers swept the enemy’s ramparts with arrows, while other soldiers manning catapults added missile fire. As soon as the walls were cleared, soldiers from the siege towers swept over the enemy’s ramparts and into the city. To augment the siege towers, generals added battering rams that consisted of long, stout shafts housed in reinforced shells. Inside the shell the crew pushed the ram up to the wall and then heaved the shaft against the wall. Rams proved even more effective than catapults in bringing down large portions of walls.

Diodorus provided a description of these machines in his discussion of Philip’s attack on the city of Perinthos in 340 B.C.E.:

For the Perinthians this grim story had a happy ending when their allies arrived to lift the siege, but many cities were successfully besieged and conquered with the new machines. Over time, Hellenistic generals built larger, more complex, and more effective machines. The earliest catapults could shoot only large arrows and small stones. By the time Alexander the Great besieged Tyre in 332 B.C.E., his catapults could throw stones big enough to knock down city walls.

If these new engines made waging war more efficient, they also added to the misery of the people, as war often directly involved the populations of cities. As it had in Periclean Athens (see Chapter 3), war often contributed to the spread of disease, and battlefields gave surgeons and physicians plenty of opportunities to test their ideas about how the human body would best heal.