The Hellenistic period's scientific achievements have been systematically undervalued by a scholarly tradition that treats the Greek intellectual story as running from the pre-Socratics through the Sophists to Plato and Aristotle and then declining. The Hellenistic period that follows — from Alexander's death to Augustus' reign — actually produced the most practically sophisticated science of the ancient world.

Eratosthenes of Cyrene, chief librarian at Alexandria under Ptolemy III, calculated the earth's circumference using the difference in sun angles between Aswan and Alexandria at summer solstice. His method was sound and his result was accurate to within a few percent. He also compiled a catalog of over 700 stars, proposed a calendar reform including leap years, and wrote a systematic geography of the known world.

Aristarchus of Samos proposed a heliocentric model of the solar system in the 3rd century BC — nearly 1800 years before Copernicus. His argument was geometric: he estimated the sun was much larger than the earth (correctly), which made it more logical for the smaller body to orbit the larger. His estimate of the sun-earth distance was wrong, but his logical framework was sound.

The pneumatic devices of Hero of Alexandria (1st century AD) — steam engines, vending machines, mechanical theater, odometers — represent the most systematic application of scientific principles to engineering in antiquity. The fact that these devices remained curiosities rather than being developed into practical technology is one of the more puzzling aspects of ancient technological history.