Universal Time

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Universal Time (UT) is a measurement of time based on the rotation of the Earth as determined by astronomical observations. In Britain UT is usually called Greenwich Mean Time, after the Prime Meridian at Greenwich, near London, England.

UT usually does not coincide exactly with Coordinated Universal Time (UTC), but UTC is defined in such a way that it is never more than 0.9 seconds behind or ahead of UT.

There are actually several slightly differing versions of UT (UT0, UT1, UT1R, UT2, and others), each of which takes into account different factors that can affect either the rotation of the earth or the results of astronomical observations, such as ocean tides and polar motion.