Thales of Miletus, who lived around 585 BCE, is regarded as the first Western philosopher. He asked what the basic stuff of the universe is. According to Aristotle, he answered that it was water. Pythagoras was born around 580 BCE on the island of Samos. He is best known for the Pythagorean theorem about the sides of a right triangle. Euclid wrote The Elements, the most widely used mathematics and geometry textbook in history. Around 300 BCE he ran his own school in Alexandria, Egypt. Aristarchus of Samos, who lived about 310 to 230 BCE, first proposed a model of the universe with the Sun, not the Earth, at the center.

The Greeks already knew the Earth was round. They thought of it as a circle of 360 degrees. Eratosthenes, head of the great library in Alexandria, was the first to calculate its circumference. He heard that at noon on the summer solstice the Sun shone straight down a well at Syene. In Alexandria, a pole cast a shadow. The angle of that shadow, with the distance between the two cities, gave him the size of the Earth. He also wrote a three-volume work on geography, a term he is credited with coining. In his city, every ship that arrived in the port was boarded and searched for books, which were copied.

Archimedes was born in Syracuse, Sicily, around 287 BCE. He is one of the greatest mathematicians and inventors of antiquity. He invented a screw for drawing water from a lower level up to a higher one. It is a cylinder enclosing a twisted blade that turns when a crank is turned. In a famous story, he worked out how to test whether a crown was pure gold by using water displacement. This is the Eureka story. Greek inventors also built a machine for the sky. The Antikythera mechanism could help track the motions of the Sun, Moon and five of the planets. It could even tell when athletic competitions such as the Olympics were set to take place.

Hippocrates was born on the island of Kos in the 5th century BCE. He became the most famous physician of ancient times, and he set up a school of medicine on Kos. He stressed careful observation of the patient and treating the whole person, not religious or superstitious explanations for illness. Socrates was born in Athens around 469 BCE. He believed that true wisdom lies in knowing how much you do not know, and in examining your own beliefs. Aristotle founded a school in Athens called the Lyceum, which housed the first research library.

Words to know

• philosopher: lover of wisdom
• circumference: distance around
• physician: doctor
• Eureka: I found it
• Lyceum: Aristotle's school