Coordinated Universal Time

Ferro Meridian

The solar time is different for different places on Earth, because the sun rises and sets in different places at different times (and it also occurs at different times in different places at noon). More precisely, a different time applies to each Earth meridian. However, it would not be practical for virtually every place on Earth to have a different time. Therefore, it was decided to use the same time for a larger area. The key was to agree on where on Earth the base time would be measured. The westernmost island of the Canary Islands, El Hierro (or Ferro), was first used as the optimal one.

This choice was already proposed by Klaudios Ptolemaios in 150 AD due to the fact that all places are to the east of this meridian, it is possible to suffice with positive numbers for longitudes, but it was not realised until 1634 by King Louis XIII of France, after ordering, on the basis of meetings of the most important European astronomers and geographers, to redraw all the maps in France in order to comply with the designation of the Ferro meridian as the basic meridian. However, the agreement was valid not only for France, but practically for the whole of Europe (with the exception of England).

Greenwich Meridian

The Ferro meridian was used as the basic one in France until 1792, when it was replaced by the Parisian meridian, but elsewhere in Europe until 1884, when the zero Greenwich meridian was established by international agreement as the basic one, passing through the observatory in the English city of Greenwich. For the selection of this meridian, used in England as the basic one since 1721, the fact that the base meridian passes through the observatory played an important role, which enabled a more accurate measurement of time. Global unification at the basic meridian was important, because until then, virtually every major country had established its own basic meridian. For example, Portugal determined longitudes from the island of Terceira, Spain from the city of Toledo, France (as mentioned above) from Paris, the Netherlands from Tenerife, Denmark from Copenhagen, Italy from Rome or Pisa, Russia from St. Petersburg (or more precisely from the Pulkovo Observatory). ) etc. The basic meridians were also the meridians passing through Jerusalem or Philadelphia.

The designation of the Greenwich meridian as the official basic meridian was decided at a special conference (International Meridian Conference) organised at the initiative of US President Chester A. Arthur in October 1884 in Washington with the participation of 41 delegates from 25 countries. At that time, two-thirds of all ships were already using this meridian as a base (the meridian was promoted for English ships by the prominent English astronomer Sir George Airy as early as 1851). Representatives of 22 countries spoke in favour of setting the Greenwich meridian as zero, France and Brazil abstained, and Haiti (then San Domingo) opposed it.

In addition to the determination of the basic meridian, the conference also adopted other conclusions regarding the measurement of time, such as that the basis for the universal unit of time will be a mean solar day, the beginning of which will be midnight of the mid-solar time. The zero meridian was long symbolised by a brass band in the courtyard, later replaced by stainless steel, and from 16 December 1999 the meridian marks a green laser aimed north at the night sky of London. However, GPS devices in Greenwich show that the zero meridian marking strip in Greenwich is not exactly zero at longitude, but it is 102.478 m from the meridian (the astronomical longitude of the Greenwich meridian is 5.3" west longitude), which is given by the latest analysis from 2015, especially in that the inclination of the vertical measured from the aircraft to the Earth's surface at a given location is not equal to 90 degrees. Other phenomena also occur, but their influence is less significant.

Coordinated Universal Time

The time from which the derived civil time is then used all over the world is called Coordinated Universal Time (UTC). Coordinated Universal Time follows Greenwich Mean Time (GMT). However, while Greenwich Mean Time is determined from the apparent motion of celestial bodies, i. e. the motion of the Earth relative to distant celestial objects (stars, quasars), the Coordinated Universal Time is tied to the Temps Atomique International (TAI), i. e. the time measured by atomic clocks. If the difference between Coordinated Universal Tine and Mean Solar Time approaches one second, this difference is eliminated by inserting a leap second at midnight of 30 June to 1 July or of 31 December to 1 January (The last one was added at midnight of 31 December 2016 to 1 January 2017). The main systematic cause of the difference between the two times is the slowing down of the Earth's rotation, and therefore until now leap seconds are always added, but in theory it is also possible to remove the leap second. The total difference increased from 1972, when the difference was 10 s, to 2019 (27 leap seconds were included in this time interval) to 37 seconds.

Note: To refine Universal Time (UT), several Universal Time Systems were introduced on January 1, 1956, depending on which inequalities or inequalities they compensate.

• The simplest version of universal time is UT0 time, designed for Greenwich from astronomical observations of the daily motions of stars, the Moon, Earth satellites and extragalactic radio sources (especially quasars). This time system does not include any compensation.

• The basic system of universal time is called UT1. Unlike UT0, it compensates for the motion of the earth's poles inside the Earth, which causes changes in the positions and planes of individual meridians. It is determined from the rotation of the Earth relative to distant celestial objects. Like UT0, UT1 is uneven due to the unevenness of the Earth's rotation. Its inaccuracy is 3 ms per day.

• The subtype of UT1 is the time UT1R, in which the irregularities of the Earth's rotation caused by tidal phenomena (tides), i. e. irregularities with a period of less than 35 days, are compensated.

• Another time system is the UT2 system, in which the uneven rotations of the Earth during the year are compensated with respect to UT1. However, this time system is not widely used for time measurement at present.

• The subtype of UT2 is the time UT2R, in which the irregularities of the Earth's rotation caused by tidal phenomena (tides) are compensated. The inequalities in UT2R are then caused only by unpredictable changes in the speed of the Earth's rotation caused, for example, by manifestations of plate tectonics, earthquakes, but also other significant mass movements in the Earth's mantle, water movements in the oceans (tsunami) or air in the Earth's atmosphere.

• The above-mentioned Coordinated Universal Time UTC is essential for the measurement of time, which, unlike all the above-mentioned systems, is relieved of the unevenness of the Earth's rotation, due to its connection to the Temps Atomique International TAI. Coordinated Universal Time UTC is determined from time measurements in many national time laboratories equipped with an atomic clock; from these measurements, the International Bureau of Weights and Measures then determines the UTC value retrospectively, because the individual atomic clocks may differ from each other by a few nanoseconds.

Note: In addition to Coordinated Universal Time, a “faster” UTC time preliminary value is also issued for the purpose of indicative time knowledge – UTCr (UTC rapid).

The leap second is inserted according to the difference between Coordinated Universal Time and UT1 Universal Time. If their difference increases to 0.9 s, it is decided to insert (or possibly remove) one second at midnight of 30 June or 31 December, as already mentioned. Interestingly, the GPS system does not use the insertion of leap seconds. The difference between UTC and UT1 thus still corresponds to 19 s, as it was in 1980. However, everything is compensated by software in the GPS signal receivers.