UT1
Earth. It is a modern continuation of the Greenwich Mean Time (GMT), i.e. the mean solar time on the meridian of Greenwich, England, which is the conventional 0-meridian for geographic longitude. GMT is sometimes used, incorrectly, as a synonym for UTC.
Universal Time and standard time
On November 2, 1868 New Zealand officially adopted a standard time to be observed nationally, and was perhaps the first country to do so. It was based on the longitude 172° 30' East of Greenwich, that is 11 hours 30 minutes ahead of Greenwich Mean Time. This standard was known as New Zealand Mean Time.
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, Canadian railway engineer Sir Sandford Fleming invented standard time to fix the problem.
Standard time divides the world into 24 "time zones", 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.
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 International Atomic Time (TAI) is determined by Very Long Baseline Interferometry (VLBI) observations of distant quasars, which method has an accuracy of micro-seconds.
The rotation of the Earth and UT are monitored by the International Earth Rotation and Reference Systems 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 It has been replaced by ephemeris time, which has since been replaced by International Atomic Time. 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.
In 1928 the term Universal Time (UTO) was adopted internationally as a more appropriate term than Greenwich Mean Time for the basis of an international standard of time. This was again prompted by expansion of rail transport and also shipping and air transport. But the term Greenwich Mean Time persisted in common usage, probably because at first the two were the same.
Versions
There are several versions of Universal Time:
- UT0 is the rotational time of a particular place of observation. It is observed as the diurnal motion of stars or extraterrestrial radio sources, and also from ranging observations of the Moon and artificial Earth satellites. If the geographic longitude of the observatory with respect to Greenwich is known, a simple subtraction yields UT0. However, because of Besselian year.
- UTC (Coordinated Universal Time) is the international standard on which civil time is based, according to time zone. It is measured with atomic clocks, and is kept within 0.9 seconds of UT1 by the introduction of one-second steps to UTC, the "leap second." To date these steps have always been positive. When an accuracy better than one second is not required, UT1 can be used as an approximation of UTC.
In celestial navigation applications, Universal Time is obtained from UTC by applying increments determined by the U.S. Naval Observatory.
- See also: Coordinated Universal Time, time scale
References
