Coordinated Universal Time

World map with current time zones

Coordinated Universal Time or UTC is the primary time standard by which the world regulates clocks and time.

It is one of several closely related successors to Greenwich Mean Time (GMT). For most common purposes, UTC is synonymous with GMT, since GMT is no longer the defined standard for the scientific community.

Etymology

The Coordinated Universal Time System was designed by an international group of experts from the International Telecommunication Union. Since it was desired that the same acronym be used in all languages, it was decided on UTC, an intermediate solution between the English Coordinated Universal Time CUT and the French Temps universel coordonné TUC.

History

In 1928, the term Universal Time (UT) was introduced by the International Astronomical Union to refer to GMT, with the day beginning at midnight. Until the 1950s, broadcast time signals were based on UT and, therefore, in the rotation of the Earth.

In 1955 the cesium atomic clock was created. This clock offered a more stable and comfortable way of measuring time than astronomical observations. In 1956, the National Bureau of Standards and the US Naval Observatory began developing atomic frequency time scales; in 1959, these time scales were used to generate the WWV time signals, named for the shortwave radio station that broadcast them. In 1960, the US Naval Observatory, the Royal Greenwich Observatory and the UK National Physical Laboratory coordinated their radio emissions so that the time steps and frequency changes were coordinated, and the resulting time scale was called informally "Coordinated Universal Time"..

In a controversial decision, the frequency of the signals was initially set to match that of UT, but was later kept at the same frequency through the use of atomic clocks and deliberately allowed to drift away from UT. When the divergence increased significantly, the signal was shifted 20 ms out of phase so that it would again coincide with UT. Twenty-nine of these steps were used before 1960.

In 1958, data were published linking the newly established frequency for the cesium transition to the second of ephemeris. The ephemeris second is a system unit of time that, when used as an independent variable in the laws of motion governing the motion of the planets and moons in the solar system, enables the laws of motion to accurately predict observed positions. of the bodies of the solar system. Within the limits of observable precision, ephemeris seconds have a constant length, just like atomic seconds. This publication made it possible to choose a value for the length of the atomic second that would agree with the celestial laws of motion.

In 1961, the Bureau International de l'Heure began coordinating the UTC process internationally (but the name Coordinated Universal Time was not formally adopted by the International Astronomical Union until 1967). they produced time jumps every few months and frequency changes at the end of each year. Jumps increased in size up to 0.1 seconds. This UTC was intended to allow a very close approach to UT2.

In 1967, the SI second was redefined in terms of the frequency supplied by a cesium atomic clock. The length of the second thus defined was practically equal to the ephemeris time second. This was the frequency provisionally used in the TAI since 1958. It was soon decided that having two types of seconds with different lengths, namely the UTC second and the second SI used in the TAI, was a bad idea. It was thought that it was better that the time signals maintain a constant frequency, and that this frequency coincide with the SI second. Thus, it would be necessary to rely solely on the time steps to maintain the approximation of the UT. This was tested experimentally on a service known as "Stepped Atomic Time" (SAT), which clocked the same frequency as the TAI and used 0.2 second steps to keep in sync with UT2.

There was also discontent with the frequent jumps in UTC (and SAT). In 1968, Louis Essen, the inventor of the cesium atomic clock, and G. M. R. Winkler independently proposed that the steps be only 1 second. This system was eventually approved, along with the idea of keeping the second UTC equal to the second TAI. In late 1971, there was one last irregular jump of exactly 0.107758 TAI seconds, bringing the total of all small time steps and frequency offsets in UTC or TAI during 1958-1971 to exactly ten seconds, so that Jan 1, 1972 00:00:00 UTC was Jan 1, 1972 00:00: 10 TAI exactly, and an integer number of seconds thereafter. At the same time, the UTC cadence was changed to exactly match the TAI. UTC also began to follow UT1 instead of UT2. Some time signals began to issue the DUT1 (UT1 - UTC) correction for applications that required a closer approximation to UT1 than UTC now offered.

Current number of leap seconds

The first leap second occurred on June 30, 1972. Since then, leap seconds have occurred on average once every 19 months, always on June 30 or December 31. As of July 2022, there have been 27 leap seconds in total, all positive, putting UTC 37 seconds behind TAI.

UTC and other standards

UTC is derived from International Atomic Time, a time standard calculated from a weighted average of atomic clock signals located in nearly 70 national laboratories around the world. Because the Earth's rotation is stable, but not constant, and lags behind atomic time, UTC is synchronized with Greenwich Mean Time (derived from the length of the solar day), to which is added or removes a leap second when necessary, always at the end of June or December. The decision on leap seconds is determined by the International Service for Earth Rotation and Reference Systems, based on their measurements of the Earth's rotation.

UTC presents problems for computer systems like Unix, which store time as a number of seconds from a reference time. Due to leap seconds, it is impossible to determine what representation a future date will have, because the number of leap seconds to include in the date is still unknown.

UTC is the time system used by many Internet and World Wide Web standards. In particular, the Network Time Protocol has been designed as a way to distribute UTC time on the Internet.

The Wikipedia server itself uses UTC as the basis for referencing when article updates are posted.

UTC

GMT time is based on the mean position of the Sun and was defined for the first time from Greenwich noon, but on January 1, 1925 the convention was adopted that the day began at midnight, moving that time back day 12 hours and since then the GMT continues to be defined as of Greenwich midnight. This time lacks certain reliability since it is based on the average movement of the Sun. It was for this reason that UTC time was defined, which has great precision, since it is given by atomic clocks.

Time zones with respect to UTC

Worldwide time zones are expressed as either positive or negative deviations from UTC, based on the Greenwich Mean Time or Zero Meridian.

Since the Earth rotates from west to east, when moving from one time zone to another in an easterly direction, one hour must be added. On the contrary, to the west you have to subtract one hour. The 180° meridian, known as the International Date Line, marks the change of day.

Daylight Saving Time

UTC does not vary with a change in the seasons, but local time may change if a jurisdiction uses the daylight saving time system. For example, the official time is five hours behind (local time) on the East Coast of the United States, during the winter, with respect to Greenwich Mean Time; but it is four hours behind during daylight saving time in that zone, with respect to Greenwich Mean Time.

Zulu Time

ZULU or Zulu Time designates in military uses, and in air navigation, Coordinated Universal Time. Zulu represents the letter Z in the Interco code. Its main merit is that it allows a common time to be used as a reference and not the local times with which a transformation process would be required.

Each time zone has a correlative letter as identification, starting with Greenwich. When a time is expressed in CUT, in UTC, or in Zulu, it is actually the time at longitude 0° through Greenwich, England. All time zones on the planet are set in reference to 0° longitude, also known as the Greenwich meridian.

As an example, if we are in Peru and the aeronautical time is 22:30Z, then the local time will be 17:30R (R for ROMEO in the aeronautical phonetic alphabet and a difference of -05:00 hours).