Universal time
The Universal Time ( UT ; German Universal Time ) is the obtained by astronomical observation mean solar time of through the Greenwich Observatory leading prime meridian . It is a universal time scale that reflects the measured earth rotation . It is the most widely used time system in geosciences and astrometry .
history
Greenwich Mean Time (GMT) served as the world time from 1884 to 1928 . Since astronomers counted the GMT from noon, while it was counted from midnight in civil life, confusion arose, so that in 1928 a new world time was agreed and it was called Universal Time (UT). In 1968 the Universal Time was split into several time systems (UT0, UT1 and UT2) in order to take into account small irregularities in the rotation of the earth.
The UT1 variant is directly the phase angle of the earth's rotation and serves as the reference time for the Coordinated Universal Time (UTC) introduced in 1972 . This completely uniform time scale, independent of the rotation of the earth as atomic time, is adapted to the UT1 as required by occasional leap seconds and thus forms the contribution of the astronomical chronology to the precise determination of time .
Measurement and meaning
As defined as mean solar time , UT measures the rotation angle of the earth with respect to the mean sun . In practice, however, one determines the rotation angle of the earth with respect to the vernal equinox ( sidereal time in the equatorial system ), with respect to suitable fixed stars ( galactic system , with astrolabes or zenith cameras ) or with respect to distant radio sources (in the fundamental system , with VLBI (interferometry)) and calculates the resulting Angle around using suitable formulas in solar time (86400 solar time seconds correspond to approximately 86636.6 sidereal time seconds). The advantage is that the positions of stars and radio sources can be observed much more precisely than those of the sun. Radio sources can also be measured during the day so that UT can be continuously determined.
Since the UT is obtained from an observation of the rotation angle of the earth and is therefore ultimately derived from the earth's rotation , it reflects its shortterm fluctuations and longterm deceleration. UT is therefore not a strictly uniform measure of time and is not suitable for some scientific or technical applications. On the other hand, precisely because of this dependency, UT provides information about the current speed of rotation of the earth and the exact angle of rotation, which is essential for numerous applications in astronomy , space travel , surveying , etc.
In addition, the UT, because it takes part in all changes in the earth's rotation due to its definition, always synchronizes with the daynight change of the earth, which z. B. is not the case for atomic time scales in the long term. UT therefore forms the basis for the bourgeois time scales used in everyday life .
Demarcation
While International Atomic Time (TAI) and Coordinated Universal Time (UTC) are physically uniform time scales, UT1 represents the true phase angle of the earth's rotation  i.e. H. the angle between the Greenwich meridian and the astronomical vernal equinox . With this UT1 defines the current "position of the gyro Earth" in the astronomical fundamental system , but has small irregularities of a few tenths of a second per year.
 UT1 has small irregularities, but exactly reflects the rotation of the earth . It is most important to the geoscientists .
 Except for the occasional leap second, UTC is a uniform time scale . Every one to six years it is adjusted to UT1 by means of these leap seconds . It is broadcast by the time services on radio waves.
 International Atomic Time (TAI) is also completely uniform, but "runs ahead of the earth"  currently 37 seconds since January 1st, 2017. It is used in physics .
There is no ideal timescale for all disciplines, but one that is well suited for every purpose.
variants
There are several variants that differ by milliseconds. “UT” without any further specification usually means UT1.
 UT0 : mean local time of the prime meridian derived directly from observations
 UT1R : corrected for the effects of polar fluctuations (periods over 35 days)
 UT1 : corrected for the effects of polar fluctuations (periods over 7 days)
 UT1D : corrected for the effects of polar fluctuations (periods over 12 hours)
 UT2 : The mean annual fluctuation of the earth's rotation was also subtracted here.
UT0
The determination is made by an observer measuring his local sidereal time (or, equivalently, the rotation angle of the earth) and converting it into UT using suitable formulas. He must also take into account the difference in longitude between himself and Greenwich. If it is based on its mean geographic coordinates, it receives UT0.
UT1  Universal Time No.1
UT1 is obtained through astronomical observations  for example with a modern meridian circle  or by means of Very Long Baseline Interferometry . The small differences to UTC are referred to as time correction and, along with the polar coordinates x and y, is one of the three parameters of the earth's rotation.
The difference UT1  UTC tends towards negative values over the course of a few months, because our time system is based on the middle second from 1900 to 1905 and the rotation of the earth has since slowed by around 0.002 seconds per day. Before UT1  UTC achieved an absolute value of 0.9 seconds, a will leap second on June 30 or December 31 inserted.
The exact position of the current axis of rotation of the earth is subject to slight fluctuations in the meter range. Since the position of the observer in relation to the axis of rotation also changes in the course of these pole movements , his geographic coordinates are subject to small fluctuations. The fluctuations in its longitude mean that the conversion of his observations to UT, which only included the mean longitude, is not entirely correct. After a corresponding correction, UT1 results.
The correction is as a first approximation (with the mean longitude , mean latitude and polar coordinates and ):
The pole coordinates are published in arc seconds . (15 arc seconds correspond to one time second .)
UT1R
This timescale is similar to UT1, with the terms with a period that is less than 35 days removed. This time scale is smoothed compared to UT1. The R stands for Removed (in German: removed) or Reduced (in German: reduced).
UT1D
This timescale is similar to UT1, although tidal effects with a halfday or daily period are also taken into account. This time scale is roughened compared to UT1.
The D stands for Daily (in German: daily) or tiDes (in German: tides).
UT2
The UT1 obtained in this way has so far only been subject to the fluctuations resulting from the earth's rotation. The seasonal proportion , on the one hand due to vegetative changes and on the other hand due to water shifts due to snow, can be reproduced reasonably accurately with an empirically obtained schematic formula. In an effort to obtain a time scale that is as uniform as possible, this percentage of fluctuation has been subtracted from UT1:
The point in time of observation is expressed as a fraction of the Bessel year . The radio time signals were derived from UT2 in the 1960s. With the introduction of UTC in 1972, UT2 lost its importance.
UT and UTC
Since the definition of the unit seconds won not by astronomical observations, but from an atomic clock is derived (1968), which is universal time ( UTC ) with the atomic time ( TAI ) synchronized. By switching seconds ensures that the difference between UT1 and UTC is not more than 0.9 seconds.
See also
 Barycenter
 Geodynamics
 longitude
 GPS time
 International Bureau of Weights and Measures
 Time signal transmitter
 Zone time
Web links

Time scales. IERS , accessed April 6, 2019 . Query of UT1, UT1  UTC, TAI, TT and leap seconds for any point in time from 1972
 IERS Rapid Service / Prediction Center . US Naval Observatory .
 The year is getting a second longer . In: press information . PhysikalischTechnische Bundesanstalt . June 15, 2015.