Sundial first used in Egypt
to measure the time of day by the sun's shadow.
Candles and burning incense mark time in China.
1400s mechanical clocks are built in Europe, using a mainspring and balance wheel.
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
1583 Galileo Galilei realizes that the frequency of a pendulum's swing depends on its length.
John Harrison builds a clock, that loses only 5 seconds on a voyage from England
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.
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.
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.
A second is formally defined as 9,192,631,770 vibrations of the cesium atom.
Time is more popular than ever: about half-a-billion watches are sold each year.