Its About Time
by Steven Craig
Egyptians development the first portable timepiece, the sundial about 1500 B.C.
The Egyptians develop the merkhet, a device that could tell the time at night by stars crossings of the meridian.
About 100 A.D, the Greeks and Romans develop mechanical clocks run by a constant flow of water to regulate time.
About 1330, large mechanical gear driven, weight powered clocks appears in Italy.
About 1500, spring driver clocks appear in Germany.
1656, the first pendulum driver clocks appear. For the next 300 years, variations improve accuracy.
In the 1840s a Greenwich standard time for all of England, Scotland, and Wales was established, replacing several "local time" systems. The Royal Greenwich Observatory was the focal point for this development because it had played such a key role in marine navigation based upon accurate timekeeping. Greenwich Mean Time (GMT) subsequently evolved as the official time reference for the world and served that purpose until 1972.
The United States established the U.S. Naval Observatory (USNO) in 1830 to cooperate with the Royal Greenwich Observatory and other world observatories in determining time based on astronomical observations. The early timekeeping of these observatories was still driven by navigation. Timekeeping had to reflect changes in the earth's rotation rate; otherwise navigators would make errors. Thus, the USNO was charged with providing time linked to "earth" time, and other services, including almanacs, necessary for sea and air navigation.
The United States developed the ideal of time zones to accommodate railroad travelers. A minute of error from local time was introduced for every 12 ½ miles one traveled east/west. Thus, at noon, on November 18, 1883, telegraph lines transmitted GMT time to major cities where authorities adjusted their clocks to their zone's proper time.
In the 1930s, piezoelectric quartz crystals clocks appear.
In the 1960s, atomic clocks were developed.
In 1967 the cesium atom's natural frequency was formally recognized as the new international unit of time: the second was defined as exactly 9,192,631,770 oscillations or cycles of the cesium atom's resonant frequency replacing the old second that was defined in terms of the earth's motions. The second quickly became the physical quantity most accurately measured by scientists. The best primary cesium standards now keep time to about one-millionth of a second per year.
With the advent of highly accurate atomic clocks, scientists and technologists recognized the inadequacy of timekeeping based on the motion of the earth which fluctuates in rate by a few thousandths of a second a day. The redefinition of the second in 1967 had provided an excellent reference for more accurate measurement of time intervals, but attempts to couple GMT (based on the earth's motion) and this new definition proved to be highly unsatisfactory. A compromise time scale was eventually devised, and on January 1, 1972, the new Coordinated Universal Time (UTC) became effective internationally.
GPS time started on January 6, 1980, at 0000 hours GMT. GPS time is measured on the second interval.
And here you are.
Posted on 06/14/2008
Copyright © 2020 Steven Craig