Antikythera mechanism

A hand-cranked bronze gearwork device built around 150–100 BC — the oldest known analog computer. Turning a single input crank advances 37 meshing gears whose tooth-count ratios encode the periods of the Sun, Moon, and planets. A differential gear (rediscovered in the 16th century) models the Moon's elliptical speed variation. Front dials show the zodiac position of Sun and Moon and display lunar phase via a half-silvered sphere; rear spiral dials track the 19-year Metonic cycle (235 lunations), the 18-year Saros eclipse cycle, and the 4-year Olympiad. Setting a date predicts eclipses and planetary positions decades ahead. Speed: instantaneous (gears turn as fast as the crank). Capacity: ~10 astronomical cycles simultaneously; eclipse prediction decades in advance.