# Universal Time

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Universal Time (UT) is a timescale based on the rotation of the Earth. It is a modern continuation of Greenwich Mean Time (GMT), i.e., the mean solar time on the Prime Meridian at Greenwich, and GMT is sometimes used loosely as a synonym for UTC. In fact the expression "Universal Time" is ambiguous, as there are several versions of it, the most commonly used being UTC and UT1 (see below). All of these versions of UT are based on the rotation of the earth compared to distant celestial objects (stars and quasars), but with a scaling factor and other adjustments to make them closer to solar time.

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### Universal Time and standard time

Prior to the introduction of standard time, every municipality around the civilized world set its official clock, if it had one, according to the local position of the sun (see solar time). This served adequately until the introduction of the steam engine, the telegraph, and rail travel, which made it possible to travel fast enough over long distances to require almost constant re-setting of timepieces, as a train progressed in its daily run through several towns. Standard time, where all clocks in a large region are set to the same time, was established to solve this problem. Chronometers or telegraphy were used to synchronize these clocks.

Standard time, as originally proposed by Sir Sandford Fleming in 1879, divided the world into twenty-four time zones, each one covering exactly 15 degrees of longitude. All clocks within each of these zones would be set to the same time as the others, but differed by one hour from those in the neighboring zones. The local time at the Royal Greenwich Observatory in Greenwich, England was chosen as standard at the 1884 International Meridian Conference, leading to the widespread use of Greenwich Mean Time to set local clocks. This location was chosen because by 1884 two-thirds of all charts and maps already used it as their prime meridian. The conference did not adopt Fleming's time zones because they were outside the purpose for which it was called, to choose a prime meridian. Nevertheless, by 1929 all major countries had adopted standard time zones. Political considerations have now increased the number of standard time zones to 40.

In 1928, the term Universal Time was adopted internationally as a more precise term than Greenwich Mean Time, because GMT could refer to either an astronomical day starting at noon or a civil day starting at midnight. However, the term Greenwich Mean Time persists in common usage to this day in reference to civil timekeeping.