Water Clock or Clepsydra

The earliest known example of a water clock or clepsydra, which dates to 1500 BC and is from the tomb of the Egyptian pharaoh Amenhotep I, remains unknown as to when or where it was initially created.

There were two water clocks in the ancient world: inflow and outflow. The inside of a container was marked with measurement lines in an outflow water clock. Water was placed inside the container and was allowed to seep out slowly. By observing the water level change, observers could determine the time. The continuous dropping of water was the same technique in outflow and inflow water clocks. The former’s measurements, however, were in a different container. One could tell how much time had elapsed by the amount of water that trickled from the first container.

First Functioning Water Clock or Clepsydra

Greeks first employed water clocks in 325 BC; they named it the clepsydra (literally, “water thief”) at that time. The timing of utterances in legal proceedings was one of the purposes of the water clock in Greece, particularly in Athens. Aristotle, the playwright Aristophanes, and the statesman Demosthenes, among other well-known Greeks, are said to have utilized the water clock during their speeches, according to specific Attic sources. In addition to timing their talks, the water clock also limited the length of their speeches. Different amounts of water would be poured into the vessels according to the speech or trial taking place.

But there were certain drawbacks to the water clock. A constant water pressure was first required to maintain a consistent rate of water flow. To address this issue, a large reservoir maintained at a consistent level of water was used to supply water to the water clock. A prime example is the “Tower of the Winds,” which the Greek astronomer Andronikos constructed in Athens in the first century BC. It is an octagonal marble building that is still standing, 42 feet (12.8 m) high and 26 feet (7.9 m) in circumference. The frieze of figures in relief that depict the winds that blow from each of the building’s eight sides, each facing a point on the compass, is ornamented; below, on the sides facing the sun, are the lines of a sundial. The Horologium had a water clock (clepsydra) to keep track of the time when the sun wasn’t shining and was topped with a weather vane in the shape of a bronze Triton.

Another issue with the water clock was that it required monthly calibration since the duration of the day and night changed with the seasons. There were several approaches used to address this issue. For instance, a disc with 365 holes of various sizes was employed to control the flow of water. The winter solstice, when the day would be the shortest, related to the greatest hole, while the summer solstice, when the day would be the longest, is connected to the smallest hole. The other holes, which were positioned between them and varied in size, were at the opposite ends of the disc from these two holes. After each day, one hole would be added to the holes’ rotation, representing the days of the year.


Although the basic tenet of water is rather straightforward, the ancients faced difficulties with the physics of water pressure and the cyclical nature of the seasons, which led to the development of more sophisticated water clocks over time. It appears we have gone a long way, given how simple it is for us to keep track of time now.

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