Coordinated Universal Time
Coordinated Universal Time (abbreviated UTC) is the basis for standard time in time zones around the world. It is a compromise between a measurement of time based solely on an atomic clock and a one based solely on the rotation of the Earth.
Because the day is defined as exactly 86,400 seconds (where the second is defined in terms of a physical property of atoms of the element cesium), but the Earth takes about 86,400.002 seconds to make exactly one rotation, time kept solely according to an atomic clock would eventually get out of synchronization with time as measured by astronomical observations. But to keep time strictly according to the period of the Earth's rotation -- which varies slightly from day to day -- would require resetting clocks by differing tiny fractions of a second every day. As a compromise, UTC, introduced in 1972, is defined so as to never be more than 0.9 seconds behind or ahead of the time as determined by astronomical observation (called Universal Time, UT or, unofficially, Greenwich Mean Time).
Because the difference between UT and UTC builds up to about one second about every 500 days, leap seconds are added to the end of the last minute of July or December in certain years. There have been 34 leap seconds added since UTC began. There can also be negative leap seconds, if UTC were to get ahead of rather than behind UT, but so far this has never happened.
UTC is also, by definition, always an integral number of seconds (no fractions) different from International Atomic Time (TAI), a measurement based solely on atomic clocks, which is never adjusted to account for variations in the Earth's rotational speed.
- U.K. National Maritime Museum, Time fact files Technically there are three slightly different calculations of UT, called UT0, UT1, and UT2. "UT" in this article refers to UT1.
- U.S. Naval Observatory, Leap Seconds
- International Earth Rotation and Reference Systems Service (IERS, Bulletin C 36
- The call sign of the National Institute of Standards and Technology's shortwave radio station located in Fort Collins, Colorado.
- U.S. Naval Observatory, Astronomical Applications Dept., Universal Time