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). 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.
History
Sir Sandford Fleming was a Scottish-born Canadian engineer and inventor. He proposed worldwide standard time zones,designed Canada's first postage stamp, left a huge body of surveying and map making, engineered much of the Intercolonial Railway and the Canadian Pacific Railway, and was a founding member of the Royal Society of Canada and founder of the Royal Canadian Institute, a science organization in Toronto.
After missing a train in 1876 in Ireland because its printed schedule listed p.m. instead of a.m., he proposed a single 24-hour clock for the entire world, located at the centre of the Earth and not linked to any surface meridian.At a meeting of the Royal Canadian Institute on February 8, 1879 he linked it to the anti-meridian of Greenwich (now 180°). He suggested that standard time zones could be used locally, but they were subordinate to his single world time, which he called Cosmic Time. He continued to promote his system at major international conferences including the International Meridian Conference of 1884. That conference accepted a different version of Universal Time, but refused to accept his zones, stating that they were a local issue outside its purview. Nevertheless, by 1929 all of the major countries of the world had accepted time zones.
