Timekeeping Timeline
3500 B.C., a simple device called a gnomon, consisting of a vertical pillar, gives the time of day by the length of its shadow.
8th century B.C., the Egyptians bring the sundial into use. Later, they also introduce simple water clocks comprising an alabaster bowl with a hole at the bottom and horizontal ``hour'' marks inside.
300 B.C., Babylonian astronomer Berosus creates the hemicycle, a wooden cube with a pointer in its hemispherical center that, also using shadows, breaks the day into 12 units.
3rd century B.C., Egyptian astronomers are the first to recognize that the solar year is close to 365 days, and that even this calculation needed adjustment by adding one day every four years. Unfortunately, the Egyptian rulers could not be persuaded to add the extra days, and so the seasons and the calendar slowly drifted out of phase.
46 B.C., Julius Caesar institutes the 365-day year adjusted for leap years, but even this adjustment over corrects by about 12 minutes every solar year.
8th to 11th centuries A.D., the Chinese introduce a water clock with some characteristics of later mechanical clocks. It becomes so popular that by the 13th century, a special guild for its makers exists in Germany. Besides the fact that it doesn't keep very good time, this clock also tends to freeze in the western European winter.
14th century A.D., the first mechanical clocks appear, powered by weights attached to a cord wrapped around a cylinder. They also employ horizontal arms called foliots with moveable weights to adjust the clock rate. The ``foliot'' clock is accurate to 15 minutes a day.
1335, the first clock publicly sounds the hour in Milan, Italy.
1500, Robert Henlein, a German locksmith, develops the first clock powered by a coiled brass or steel spring, replacing weights suspended from ropes or chains, but the rest of the clock was essentially the ``foliot'' mechanism.
1582, Pope Gregory XIII modifies the Roman calendar adopted in 46 B.C. and the rules for generating it, reducing the error to about 1 day in 3,300 years. To bring the calendar back in step with the seasons, Pope Gregory removes 10 days from the year 1852, following October 4 with October 15. The Gregorian calendar is still in use today.
1582, Galileo theorizes about the timekeeping qualities of the pendulum.
1648, Irish Archbishop Ushe asserts the universe was formed on Sunday, October 23, 4004 B.C.
1656, Christian Huygens, a Dutch physicist and astronomer, builds the first pendulum-based clock, accurate to 10 seconds a day, a dramatic improvement over the ``foliot'' clock.
1660, Robert Hooke, and English physicist, toys with the idea of using a straight metal spring instead of a pendulum.
1670, English clock master William Clement adds a second pendulum and encases the clock in a wooden cabinet, creating the first grandfather clock.
1675, Christian Huygens employs Hooke's principle and builds the first successful spring-controlled clock, replacing the pendulum with a spiral spring connected to a rotating balance wheel.
1675, King Charles II orders that an astronomical observatory be built in Greenwich, England to develop a method for determining a ship's longitude.
Late 1600s, the minute hand is added to clocks because of their improved accuracy.
1687, 6000 watches are produced in Geneva, Switzerland.
1704, Nicholas Facio, of Basel, Switzerland, introduces the jeweled bearing, replacing the holes punched in brass to support gear axles.
1713, the British government offers an award of £20,000 to anyone who could build a chronometer that would serve to determine longitude to within 1/2 degree.
1761, William Harrison sails to Jamaica to test a chronometer based on more than 40 years of work by his father, John. The timepiece is accurate to within 54 seconds over a 5-month period, making it possible to determine his ship's longitude to within 1/3 degree and to claim the £20,000 award.
1767, nearly a century after being established, the observatory at Greenwich, England, publishes its first annual nautical almanac predicting the position of the moon and particular stars relative to a fixed line (or meridian) at Greenwich, enabling a navigator with a sextant and an accurate clock to calculate his longitude.
1793, the ``decimalized clock'' is introduced in France. With 100 seconds to the hour, 10 hours to the day, and so on, this reform movement lasted less than a (non-decimalized) year.
Late 1700s, about 50,000 watches per years are being produced in Geneva, Switzerland.
1833, the Royal Greenwich Observatory takes it upon itself to be the world's timekeeper, lowering a large canvas ball from the top of the observatory tower at precisely 1 pm every day.
1840, Alexander Bain theorizes that time signals could be sent over wires, but it wasn't until a decade or so later that serious progress was made in this direction, driven primarily by the railroads' need for better time information and dissemination.
1859, Big Ben, one of the most accurate tower clocks ever constructed, is installed at London's Houses of Parliament.
1870, Connecticut school teacher Charles Ferdinand Dowd invents the idea of time zones, dividing the United States by meridians into zones one hour, and 15 degrees, apart, using Greenwich, England, as the basis.
1883, Upon the recommendation of Dowd, the railroads adopt four time zones for the continental United States, plus one time zone for the easternmost provinces of Canada. Prior to this, a traveler on a cross-country railroad trip would have to change a watch 20 times or so to stay in step with the ``railroad time.''
1884, Greenwich, England, is officially named the world's prime meridian by a vote at an international conference in Washington, D.C. Other contenders were Jerusalem, the Great Pyramid of Egypt and Paris.
Late 1800s, R. H. Ingersoll employs machine-produced, interchangeable parts to produce the famous ``dollar watch,'' a pocket watch at a price the average person could afford, selling millions over the next several decades.
1904, the United States Naval Observatory experimentally broadcasts time signals from Boston.
1910, time signals are broadcast from the Eiffel Tower in Paris, France.
1918, the United Stated Congress passes the Standard Time Act, authorizing the Interstate Commerce Commission to establish standard time zones within the United States, and establishing ``daylight-saving time.''
1918, the application of synchronous electric motors to clocks brings electric timekeeping to American homes.
1923, the National Bureau of Standards begins broadcasting its own time signals on shortwave radio station WWV in Washington, D.C., helping other radio stations to stay on their assigned frequencies.
1929, Dr. Warren A. Marrison develops the first quartz crystal clock, requiring a cabinet about 9 feet high, 7 1/2 feet high and 3 feet deep to accommodate the various necessary components.
1949, the National Bureau of Standards (now called the National Institute for Standards and Technology) announced the world's first atomic clock, a time source linked to the natural frequency of atomic particles, in this case the ammonia molecule.
1955, the International Astronomical Union culminates age-old attempts to discover an invariant astronomical reference by adopting the Ephemeris Second, based on the motion of the earth about the sun at the beginning of the year 1900. The Ephemeris Second is defined as ``1/31,556,925.9747 of the tropical year for January 0, 1900 at 12 hours.''
1957, the first electric wristwatch is introduced, powered by a tiny battery instead of a spring.
1958, the ``rubber second,'' a varying slight alteration of the length of the second relative to the atomic second, is introduced to allow for variations in the earth's rotation. It is later abandoned as a nuisance.
1959, the Bulova Accutron replaces the balance wheel found in all watches with a tiny tuning fork.
1964, astronomers at Cambridge University's Mullard Radio Observatory discover the first ``stellar clock,'' the pulsar or neutron star, emitting radio-frequency pulses at the extremely regular rate of 1.3330113 pulses per second.
1964, Hewlett-Packard Company introduces the first commercially available cesium-beam atomic clock, the HP model 5060A.
1966, the U.S. Congress enacts the Uniform Time Act, ruling that the entire nation should use daylight-saving time from 2:00 am on the last Sunday in April until 2:00 am on the last Sunday in October, but individual states could, by legislative action, refuse to conform, as did Hawaii in 1967, Arizona in 1968 and Indiana in 1971.
1967, the XIII General Conference of Weights and Measures replaces the Ephemeris Second by formally adopting the International Second, also called the atomic second, defined as ``the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium atom 133.''
1971, the XIV General Conference of Weights and Measures formally defines International Atomic Time (IAT), and responsibility is given to the Bureau International de L'Heure (BIH) in Paris to keep the unit interval of the IAT as close as possible to the definition of the standard second.
1972, the ``leap second'' is instituted as a means of adjusting time to correspond with the rotation of the earth. Two leap seconds are added this year, also a leap year, making it the ``longest'' year in modern times.
1974, 200 million clocks and watches are sold worldwide for a price of $4 billion.
1976, the National Bureau of Standards completes its first laboratory cesium-beam primary frequency standard. Weighing over 6600 pounds and measuring about 18 feet long, it is housed in its own special room in Boulder, Colorado.
1992, HP introduces the world's most accurate commercially available atomic clock, the HP 5071A.
Late 21st century A.D., the International Space Commission establishes standard time zones for other planets in the galaxy. Gamma ray clocks are introduced as primary frequency standards, replacing the hydrogen maser.
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