Time Line

1500-1300 BC

Sundial first used in Egypt to measure the time of day by the sun's shadow.

  400 BC Greeks use a water clock, which measures the outflow of water from a vessel, to measure time.

  980 Alfred the Great (a Saxon king) uses burning candles to measure time.

1000 Candles and burning incense mark time in China.

1400s mechanical clocks are built in Europe, using a mainspring and balance wheel.

1567 Philip II of Spain offers a reward for a method of finding longitude at sea. To obtain longitude -- the east-west location -- from the position of the sun or stars, you must know the precise local time, which was impossible with the day's poor clocks.

1583 Galileo Galilei realizes that the frequency of a pendulum's swing depends on its length.

1759 John Harrison builds a clock, that loses only 5 seconds on a voyage from England to Jamaica.  

1840s 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.

1850s Regional time zone is established in New England to coordinate railroad schedules, halting confusion due to using local (sun) time at every station.

1884 Twenty-five countries accept Greenwich, England, as the prime meridian. The prime meridian gradually becomes the basis for time throughout the world.

1905 A radio time signal starts being transmitted from Washington DC to help ships find longitude.

1945 Physicist Isador Rabi suggests making a clock based on the study of atoms, using a method called atomic-beam magnetic resonance.

1949 National Bureau of Standards (now the National Institute of Standards and Technology, or NIST) builds the first atomic clock, using ammonia.

1967 A second is formally defined as 9,192,631,770 vibrations of the cesium atom.

1998 Time is more popular than ever: about half-a-billion watches are sold each year.

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