The Ancient Greek Water Clock That Kept the Most Accurate Time for 1,800 Years

Ctesibius’ third-century BC clepsydra used a vertically mounted water-filled cylinder whose level moved a pointer to indicate the hour, while a water wheel and gear train rotated the cylinder to match the variable daylight hour length. A siphon automatically emptied the reservoir every 24 hours, allowing the device to keep accurate time for 1,800 years until the pendulum clock’s invention in 1656.