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The usual instrument for telling time in antiquity was the sundial. This employed the shadow of a pointer cast on a surface marked with lines indicating the seasonal hours (one seasonal hour was 1/12 of the length of daylight at a given place: hours of constant length were used only by astronomers). The sundial was not commonly used until the 3rd cent bc, when the mathematical theory necessary for correctly drawing the hour‐lines had been developed. Before then the popular way to tell time was a crude shadow‐table, using the measured length of a man's own shadow.

At night the ancients used the water‐clock, which measured time by the flow of water from a vessel. A primitive form was in use in Athenian courts of the 5th cent. bc, but accuracy was not achieved until Ctesibius (3rd cent.) invented a device to ensure a uniform flow. See time‐reckoning.

Subjects: Medieval and Renaissance History (500 to 1500).

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