Greenwich Mean Time
The Greenwich Mean Time or GMT (Greenwich Mean Time /ˌgren.ɪʧ'mi:n.taɪm/) is a time standard that originally referred to mean solar time at the Royal Greenwich Observatory in Greenwich, near of London, England, which in 1884 was chosen by the International Meridian Conference as the prime meridian.
Prior to the introduction of Coordinated Universal Time (UTC) on January 1, 1972, Greenwich Mean Time (also known as Zulu Time) was considered Universal Time (UT), which is a standard astronomical concept used used in many technical fields. Astronomers no longer use the term Greenwich Mean Time.
In the UK, GMT is the official time only during winter; in summer daylight saving time is used.
For many years the most accurate clocks that existed were the movement of the Earth around its axis and around the Sun. Everything else related to time was defined from them. One turn of the Earth around the Sun was a year, one turn of the Earth on itself was a day, which was divided into 24 hours, the hour into 60 minutes, and the minute into 60 seconds. In 1900 a second was defined as 1/86,400 of a mean solar day.
This was accurate enough for everyday activities, but it gradually became clear that Earth was not the best clock. The tides make its turn decrease with some regularity but, in addition, there are other influences that mean that the duration of that turn is not constant. The differences do not affect everyday life but they can affect the precision of navigation or the position of artificial satellites, for example.
Later developments
With the advancement of knowledge of the atom, a more precise clock was discovered, the resonance frequency of certain atoms when they passed from one state to another. In 1950, at the National Physical Laboratory, an American built the first clock based on this property of atoms, which was called the atomic clock. Cesium atoms were used in its construction. Its precision was so high that in 1967 international standards bodies changed the definition of the second based on the movement of the Earth to a definition based on the cesium atom: the second, a unit of time in the International System of Units, is the duration of 9,192,631,770 periods of the radiation associated with the hyperfine transition of the ground state of the cesium 133 atom, with the following observation: the ground state is defined with zero magnetic field.
The error of cesium-based atomic clocks is one part in 1013. They are much more precise than the Earth's spin and supplanted it in the definition of international time. A universal measurement using the atomic definition of a second was adopted in 1972. This time is called in all languages UTC (coordinated universal time).
Since the Earth's spin is less uniform than the behavior of atomic clocks, there is some discrepancy between mean solar time, the base of GMT, and UTC. In order for there to be synchrony between the two times, what is done is to monitor the rotation of the Earth with extreme precision. UTC and GMT are admitted to be correct if they do not differ by more than 0.9 seconds. If they differ by more than that amount, one second is added or subtracted from the atomic clocks.
Leap second
The first time the adaptation was made was on June 30, 1972 at twelve o'clock at night, with which the UTC and GMT times were synchronized on July 1. The next few times it has been done, up to today, it has always been to add a second, but it could also have been to subtract a second if the Earth's rotation had varied otherwise.