People have always measured time by watching things that repeat. The turning of day and night, the phases of the moon, and the yearly return of the seasons gave the first calendars their shape. Sundials divided the daylight, and water clocks and candles marked the hours when the sky was dark. Mechanical clocks appeared in European monasteries in the thirteenth century, and the pendulum later made them accurate enough to lose only seconds a day. Today atomic clocks count the vibrations of cesium atoms and would drift by less than a second in millions of years. Yet the sense of time is stranger than any instrument suggests.
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