A water clock moves figures and doors before Christ's birth

Water clocks from 1,500 BC

Water clocks were among the earliest timekeepers that did not rely on the observation of celestial bodies. One of the oldest was found in the tomb of Amenhotep I, who was buried around 1500 BC. Later called clepsydras, or "water thieves," by the Greeks, they were discovered around 325 BC.

Other clepsydras were cylindrical or bowl-shaped containers designed to slowly fill with water flowing in at a constant rate. Marks on the inner surfaces measured the passage of "hours" as the water level reached them. These clocks were used to determine the hours at night, but they may also have been used during daylight hours. Another version consisted of a metal bowl with a hole in the bottom. The bowl would fill and sink in a set time when placed in a container of water. These are still in use in North Africa in the 21st century.

More sophisticated and impressive mechanized water clocks were developed by Greek and Roman clockmakers and astronomers between 100 BC and 500 AD. The added complexity was intended to make the flow more constant by regulating water pressure and to represent the passage of time more elegantly. Some water clocks rang bells and gongs. Others opened doors and windows to reveal small figures of people, or moved hands, dials, and astrological models of the universe.

The water flow rate is very difficult to control precisely, so a clock based on this flow rate could never achieve excellent accuracy. Naturally, people were led to other approaches.

The water clock of Ctesibius: The clock works with a system of two fluid-filled containers. A figure that indicates the time floats in the larger container. The fluid level continuously rises as the incoming water flows out of the smaller container at a constant rate. Hours are thus displayed on a vertical scale.