Five who counted

The Roman general Marcus Claudius Marcellus had ordered that Archimedes — the famous engineer who had defended Syracuse with siege machines for two years — be spared. A junior Roman soldier found the 75-year-old man drawing diagrams in the sand. Archimedes told him not to disturb his circles. The soldier killed him. Marcellus was reportedly furious and ordered Archimedes given an honourable burial.

Eratosthenes measured the angle of the noon sun at Alexandria on the summer solstice (about 7°12') and compared it with the well at Syene (where the sun was directly overhead). He calculated the Earth's circumference at 250,000 stadia. Using the Egyptian stadion of 157.5 m — which most scholars now believe is what he used — that's about 39,375 km, against the actual 40,008 km. Under 2% off.

Comparing his own star positions with Alexandrian measurements 150 years older, Hipparchus found that the celestial coordinate system was drifting westward at approximately one degree per century. Precession is the slow gyroscopic wobble of Earth's axis; the full cycle takes ~26,000 years. He was the first observer to detect any systematic motion of the entire celestial sphere.

Heiberg, working at the library of the Metochion of the Holy Sepulchre in Istanbul, identified the faint mathematical underwriting beneath a 13th-century Greek prayer book as Archimedes — including the only surviving copy of the *Method of Mechanical Theorems*. The manuscript later disappeared, resurfaced at Christie's in 1998, and was recovered by multispectral imaging at the Walters Art Museum.

The Antikythera mechanism — a bronze geared device recovered from a Roman shipwreck in 1900-1901 — was a hand-cranked analog computer for astronomy. Its 37 gears modelled the lunar orbit, predicted solar and lunar eclipses, tracked planetary positions, and included a 4-year Olympiad counter. It is roughly 1,500 years more sophisticated than the next known geared mechanism.