Universal Time

Universal Time (UT) is a time standard based on Earth's rotation. It is a modern continuation of Greenwich Mean Time (GMT), i.e., the mean solar time on the Prime Meridian at Greenwich. In fact, the expression "Universal Time" is ambiguous (when accuracy of better than a few seconds is required), as there are several versions of it, the most commonly used being Coordinated Universal Time (UTC) and UT1 (see below). All of these versions of UT, except for UTC, are based on Earth's rotation relative to distant celestial objects (stars and quasars), but with a scaling factor and other adjustments to make them closer to solar time. UTC is based on International Atomic Time, with leap seconds added to keep it within 0.9 second of UT1.

Universal Time and standard time

Prior to the introduction of standard time, each municipality throughout 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 rail travel in Britain, which made it possible to travel fast enough over long distances to require continuous re-setting of timepieces as a train progressed in its daily run through several towns. Greenwich Mean Time, where all clocks in Britain were set to the same time, was established to solve this problem. Chronometers or telegraphy was used to synchronize these clocks.