Clepsydrae

Clepsydra, or clepsydrae in the plural, is a timekeeping device that measures hours or portions of an hour by the regulated flow of liquid, typically water. The word derives from Greek klepsydra, from kleptein “to steal” and hudor “water,” a reference to time being “stolen” as the water leaves or is added.

There are two principal forms: outflow and inflow. In an outflow clepsydra, water drains from a vessel through a narrow orifice into a receiving container, and the vessel is marked with a scale so that the decreasing water level indicates elapsed time. In an inflow clepsydra, water is moved into a vessel at a controlled rate or into a tank that rises; the rising level is calibrated against a time scale. Some designs combine both approaches or employ a float pointer to track time more reliably. Accuracy depends on a steady flow, which can be affected by hole size, temperature, and the surrounding conditions.

Historically, clepsydrae are among the oldest artificial clocks. They appear in ancient Egypt and Mesopotamia and were later refined by Greek engineers such as Ctesibius in the Hellenistic period. The device continued to be used in the Roman world and was discussed by authors such as Hero of Alexandria and Philo of Byzantium. In the medieval Islamic world, water clocks were studied and improved, and similar concepts persisted in Europe before the widespread adoption of mechanical clocks.

Clepsydrae contributed to the development of time measurement and informed later horological technology, while also serving practical roles in daily life, astronomy, and ceremonial contexts.