POSTED 25 AUGUST 2005
On time?
Let's take a trip through time, from the epoch when farmers told time by looking at the sun to today, when atoms tell us the time. Some of these highlights come from "Time's Pendulum..." (see bibliography).

1500-1300 BC: Sundial first used in Egypt to measure time of day by the sun's shadow. Hours are shorter in winter and longer in summer. 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?: (Sung dynasty, China) Candles and burning incense mark time.

1370: King Charles V of France decrees that all Paris church bells ring at the same time as the Royal Palace, helping end bell ringing at the canonical hours (prayer times) decreed by the church.

1400s: Mechanical clocks are built in Europe, using mainsprings and balance wheels.

1567: As ships are sinking because they don't know their location, Philip II of Spain offers a reward for a method of finding longitude at sea. Finding longitude -- the east-west location -- from the position of the sun or stars requires precise knowledge of 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. Galileo Illustration: NASA

1657: Christiaan Huygens invents the first pendulum clock, capable of far greater accuracy than any preceding timekeeper. But the clock does not work at sea.

1759: John Harrison's clock loses only 5 seconds on a voyage from England to Jamaica. Navigators cheer, and Harrison gets rich (see "Longitude" in the bibliography).

1839: Telegraph invented, allowing instant transmission of time signals.

Time ball courtesy the U.S. Naval Observatory.

1840s: Time ball is dropped at noon each day at the U.S. Naval Observatory in Washington, D.C. Ships in the harbor use it 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.

In 1865, the Naval Observatory started its time service, sending a telegraph signal to the Navy Department. This service was later extended via Western Union telegraph lines to provide accurate time to railroads across the nation. Photo: US Naval Observatory

1884: 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 is last to adopt it in 1972.

1886: Salespeople for the R.W. Sears Watch Company fan out across America selling affordable timepieces. The firm is later renamed Sears, Roebuck and Co.

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

1928: W.A. Marrison of Bell Laboratories builds the first quartz clock, accurate to within 1-2 thousandths of a second per day. Quartz technology is later adapted for wristwatches.

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. For the first time, time is not defined by the movement of astronomical bodies.

NIST-7, from The National Institute of Standards and Technology

1993: NIST-7 -- an updated latest atomic clock -- comes on line, with an accuracy of five parts in 10 ¹⁵.

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

1999: NIST-F1 goes into operation. This atomic fountain is accurate to 1 second in 30 million years!

Got time for our bibliography?