<P> For thousands of years, devices have been used to measure and keep track of time . The current sexagesimal system of time measurement dates to approximately 2000 BC from the Sumerians . </P> <P> The Egyptians divided the day into two 12 - hour periods, and used large obelisks to track the movement of the sun . They also developed water clocks, which were probably first used in the Precinct of Amun - Re, and later outside Egypt as well; they were employed frequently by the Ancient Greeks, who called them clepsydrae . The Zhou dynasty is believed to have used the outflow water clock around the same time, devices which were introduced from Mesopotamia as early as 2000 BC . </P> <P> Other ancient timekeeping devices include the candle clock, used in ancient China, ancient Japan, England and Mesopotamia; the timestick, widely used in India and Tibet, as well as some parts of Europe; and the hourglass, which functioned similarly to a water clock . The sundial, another early clock, relies on shadows to provide a good estimate of the hour on a sunny day . It is not so useful in cloudy weather or at night and requires recalibration as the seasons change (if the gnomon was not aligned with the Earth's axis). </P>

When was the first clock invented and by whom
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