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Sundial first used in Egypt to measure the time of day by the sun's shadow. Hours are shorter in winter and longer in summer.Greeks use a water clock, which measures the outflow of water from a vessel, to measure timeKing Charles V of France decrees that all Paris church bells must ring at the same time as the Royal Palace, helping end the ringing of bells at the canonical hours (prayer times) decreed by the church.Alfred the Great (a Saxon king) uses burning candles to measure time.Mechanical clocks are built in Europe, using a mainspring and balance wheel.Galileo Galilei realizes that the frequency of a pendulum's swing depends on its length.Christiaan Huygens invents the first pendulum clock, capable of far greater accuracy than any preceding timekeeper. But the clock does not work at sea.John Harrison builds a clock, that loses only 5 seconds on a voyage from England to Jamaica. Navigators cheer, and Harrison gets richTelegraph invented, allowing instant transmission of time signals.Time ball is dropped at noon each day at the U.S. Naval Observatory. Ships in the harbor use the ball to set their clocks.Twenty-five countries accept Greenwich, England, as the prime meridian (0 degrees longitude). The prime meridian gradually becomes the basis for time throughout the world. Liberia finally adopts it in 1972.Salespeople for the R.W. Sears Watch Company fan out across America selling affordable timepieces. The firm is later renamed Sears, Roebuck and Co.A radio time signal starts being transmitted from Washington DC to help ships find longitude.Physicist Isador Rabi suggests making a clock based on the study of atoms, using a method called atomic-beam magnetic resonanceNational Bureau of Standards (now the National Institute of Standards and Technology, or NIST) builds the first atomic clock, using ammonia.A second is formally defined as 9,192,631,770 vibrations of the cesium atom. For the first time, time is not defined by the movement of astronomical bodies.Time is more popular than ever: about half-a-billion watches are sold each year.
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