By international agreement, all UTC time scales must agree with the UTC time scale operated by BIPM and IERS/CB to within 1 millisecond. The International Bureau of Weights and Measures (BIPM) and the Central Bureau of International Earth Rotation Services (IERS/CB) are the agencies responsible for international co-ordination of time and frequency standards. Although UTC will differ from TAI by an integral number of second, its seconds "markers" will always coincide with those of TAI. As TAI is currently gaining on UTC positive leap-seconds as required. To ensure that UTC and UT1 would never differ by more that 0.9 second, provision was made for the introduction from time to time of "leap seconds" in the UTC scale. Also on 1 January 1972, the duration of 1 second UTC was shortened by about 30 nanoseconds so as to equal the second on the TAI scale. This was accomplished by a step of 0.107 757 7 second being taken out of UTC. in days, hours, minutes and seconds from the origin 1958 January 1 d 0 h 0 min 0 s.Īt the beginning of 1972 a compromised time scale known as Co-ordinated Universal Time (UTC) was changed to a new standard so that 1972 January 1 d 0 h 0 min 0 s UTC coincided precisely with 1972 January 1 d 0 h 0 min 10 s TAI. It is in the form of a continuous scale, i.e. The International Atomic Time Scale (TAI) is the international reference scale of atomic time based on the seconds as defined above in the International System of Units (SI). The atomic time scale is accurate to a few billionths of a second as compared with a few thousandths of a second in one day in the case of Universal Time. "The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom." Research on atomic frequency standards indicated that an alternative basis for a definition of the 'second' might be possible and in 1967 the Thirteenth General Conference of Weights and Measures adopted a resolution to replace the astronomical definition of the second by :. But this time scale is unsuitable for general use because of the delay in the astronomical observations used to define it. Astronomers avoided the difficulty by establishing a time scale based on the earth's movement around the sun, the Ephemeris Time. Corrections were made to remove their effect and an even more uniform time UT2 was obtained. With further improvements in the precision of pendulum and quartz clocks, it was discovered that UT1 had periodic variations caused by the seasonal fluctuations in the rate of rotation of the earth. After careful measurements at various observatories throughout the world, the effect of the wobble was eliminated by the introduction of a new time designation called UT1. This was later found to be caused by a wobble in the axis of the earth. Universal Time (UT0) is mean solar time at the Greenwich Meridian and is called Greenwich Mean Time (GMT).Īs better clocks were developed, astronomers began to notice a discrepancy in UT0 measured at different locations. A more uniform time called mean solar time was therefore introduced. There is also an effect due to the inclination of the earth's equatorial plane to the plane of its orbit. However, apparent solar time is not uniform because the earth's orbit is not circular and its speed changes with its distance from the sun. The first timing system in the world was based on the sundial, and the time measured was apparent solar time.
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