Sundial first used in Egypt to measure the time of day by
the sun's shadow. Hours are shorter in winter and longer
Greeks use a water clock, which measures the outflow of
water from a vessel, to measure time (scroll down this page
for pics of sundials and water clocks. Click for information
on making a sundial.
Alfred the Great (a Saxon king) uses burning candles to
King 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.
clocks are built in Europe, using a mainspring and balance
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 rich (see "Longitude" in the bibliography)
Telegraph invented, allowing instant transmission of 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.
Isador Rabi suggests making a clock based on the study of
atoms, using a method called atomic-beam magnetic resonance
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. 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.