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2 Measurement 3 Versions 4 Sources 5 External links |
Prior to the introduction of standard time, every municipality set their clock, if they had one, by the local position of the sun. This served well until the introduction of the train, when it became possible to travel fast enough to require almost constant re-setting of clocks. After missing a train for just this reason in 1878, Sir Sandford Fleming invented standard time to fix the problem.
Standard time divides the world into 24 "timezones", each one covering, in theory at least, 15 degrees. All clocks within each of these zones would be set to the same time. The local time at the Royal Greenwich Observatory in Greenwich, England was chosen as standard, leading to the widespread use of Greenwich Mean Time in order to set local clocks. The standard time system used today, called Universal Standard Time, is standard time based on Universal Time.
The rotation of the Earth and UT are monitored by the International Earth Rotation Service (IERS).
Because the rotation of the Earth is somewhat irregular and the length of the day increases due to tidal acceleration, UT is not a perfect clock time. It has been replaced by ephemeris time which has since been replaced by International Atomic Time (TAI). However, because universal time is synchronous with night and day, and more perfect clocks drift away from this, UT is still used as a correction to atomic time in order to obtain civil clock time.
Universal Time and standard time
Measurement
One can measure time based on the rotation of the Earth by observing celestial bodies cross the meridian every day. Astronomers have preferred observing meridian crossings of stars over observations of the Sun, because these are more accurate. Nowadays, UT in relation to TAI is determined by VLBI observations of distant quasars, which has an accuracy of micro-seconds.Versions
There are several versions of Universal Time:
UT2 = UT1 + 0.0220*sin(2*pi*t) - 0.0120*cos(2*pi*t) - 0.0060*sin(4*pi*t) + 0.0070*cos(4*pi*t) seconds
In celestial navigation applications, Universal Time is obtained from UTC by applying increments determined by the U.S. Naval Observatory. Sources
External links