In 1967, the SI second was redefined in terms of the frequency supplied by a caesium atomic clock. The length of second so defined was practically equal to the second of ephemeris time. This was the frequency that had been provisionally used in TAI since 1958. This UTC was intended to permit a very close approximation of UT2, within around 0.1 s. The jumps increased in size to 100 ms, with only one 50 ms jump having ever occurred. UTC was officially initiated at the start of 1961 (but the name Coordinated Universal Time was not adopted by the International Astronomical Union until 1967). The TAI instant 1 January 1961 00:00:01.422818 exactly was identified as UTC instant 1 January 1961 00:00:00.000000 exactly, and UTC ticked exactly one second for every 1.000000015 s of TAI. Time steps occurred every few months thereafter, and frequency changes at the end of each year. This publication allowed a value to be chosen for the length of the atomic second that would work properly with the celestial laws of motion. Within the limits of observing accuracy, ephemeris seconds are of constant length, as are atomic seconds. In 1958 data was published linking the frequency for the caesium transition, newly established, with the ephemeris second. The ephemeris second is the duration of time that, when used as the independent variable in the laws of motion that govern the movement of the planets and moons in the solar system, cause the laws of motion to accurately predict the observed positions of solar system bodies. The frequency of the signals was initially set to match the rate of UT, but then kept at the same frequency by the use of atomic clocks and deliberately allowed to drift away from UT. When the divergence grew significantly, the signal was phase shifted (stepped) by 20 ms to bring it back into agreement with UT. Twenty-nine such steps were used before 1960. In 1960 the U.S. Naval Observatory, the Royal Greenwich Observatory, and the UK National Physical Laboratory coordinated their radio broadcasts so time steps and frequency changes were coordinated, and the resulting time scale was informally referred to as 'Coordinated Universal Time'. National Bureau of Standards and U.S. Naval Observatory started to develop atomic frequency time scales by 1959 these time scales were used in generating the WWV time signals, named for the shortwave radio station that broadcasts them. By National Physical Laboratory - Public Domain In 1955 the caesium atomic clock was invented. This provided a form of timekeeping that was both more stable and more convenient than astronomical observations. In 1928, the term Universal Time (UT) was introduced by the International Astronomical Union (IAU) to refer to GMT with the day starting at midnight. Until the 1950s, broadcast time signals were based on UT, and hence on the rotation of the Earth. It persisted much longer elsewhere because it was mentioned at the 1884 conference. In 1884 the Greenwich Meridian was used for two-thirds of all charts and maps as their Prime Meridian. In contrast, astronomical GMT began at mean noon, 12 hours after mean midnight of the same date until 1 January 1925, whereas nautical GMT began at mean noon, 12 hours before mean midnight of the same date, at least until 1805 in the British Navy. It started from 0 hours at mean midnight. This agreed with civil Greenwich Mean Time (GMT), used in Great Britain since 1847. Greenwich Mean Time (GMT) is still the standard time for the Prime Meridian (Zero Longitude) and civil time in UK when Daylight Saving Time is not in use.Īt the 1884 International Meridian Conference held in Washington, DC, USA, local mean solar time at the Royal Observatory, Greenwich in England was chosen to define the Universal Day. Coordinated Universal Time ( UTC ) replaced Greenwich Mean Time (GMT) as the World Standard for time in 1972. UTC is based on atomic measurements, while GMT is based on Earth's rotation.

Universal time gmt