Universal Time

The Universal Time (UT ) is the leading of the observatory of Greenwich zero meridian which obtained by astronomical observation mean time. It is a universal time scale, which reflects the measured Earth's rotation. In the geosciences and astrometry it is the time most commonly used system.

History

Today's world time was from 1884 to 1928, the Greenwich Mean Time (GMT). Different handling of GMT confusion had arisen, so that one in 1928 agreed a new world time and she called Universal Time (UT ). In 1968, the Universal Time split into multiple time systems ( UT0, UT1 and UTC) to support small irregularities of the earth's rotation into account.

The UT1 variant is directly the phase angle of the Earth's rotation and serves as a reference time for the Coordinated Universal Time ( UTC). This completely uniform time scale will be adjusted as needed by individual leap seconds to the UT1 and so is the contribution of the Astronomical chronology for the precise determination of the time.

Measurement and meaning

According to its definition as a local mean time UT measures the angle of rotation of the earth relative to the mean sun. In practice, however, determines the angle of rotation of the earth with respect to the vernal equinox (star time in the equatorial system ), with respect to suitable fixed stars ( Galactic System, with astrolabes or Zenit cameras) or with respect to further radio sources ( in the fundamental system, with VLBI ( interferometry ) ) and calculates the resultant angle on appropriate formulas in solar time at ( 86400 solar time - seconds is about 86636.6 sidereal seconds). The advantage is that the positions of stars, and the radio resources can be monitored much more accurately than that of the sun. Radio resources are moreover anmessbar day, so that a continuous determination of UT is possible.

Since UT is obtained through observation of the angle of rotation of the earth and therefore ultimately derived from the earth's rotation, it reflects their short-term fluctuations and long-term slowdown. UT is therefore not suitable not strictly uniform time scale and for some scientific or technical applications. On the other hand, UT delivers precisely because of this dependency information about the instantaneous speed of rotation of the earth and the exact rotation angle, etc. is essential for numerous applications in astronomy, space exploration, surveying.

In addition, UT is because they are all changes in the Earth's rotation takes part due to their definition, the future is always synchronized with the day - night cycle of the earth, which, for example, an atomic time scale long term would not be the case. UT therefore forms the basis for the used in everyday civil time scales.

Demarcation

While the International Atomic Time (TAI ) and Coordinated Universal Time (UTC) are physically uniform time scale, UT1 represents the true phase angle of the Earth's rotation is - that is, the angle between the Greenwich meridian and the astronomical vernal equinox. This defines UT1 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.

There is no ideal time scale for all disciplines, but for any purpose a well-suited.

Variants

There are several different variants of milliseconds. With "UT " without further specification is usually meant UT1.

• UT0: direct observations derived from average of the prime meridian local time
• UT1R: adjusted for influences of Polschwankungen (periods over 35 days)
• UT1: adjusted for influences of Polschwankungen (periods over 7 days)
• UT1D: adjusted for influences of Polschwankungen (periods over 12 hours)
• UT2: Here also the average annual variation of the earth's rotation was removed.

UT0

The determination is done by an observer 's local sidereal time (or, equivalently, the angle of rotation of the earth ) measures and converts on appropriate formulas in UT. He must also take into account the difference in longitude between himself and Greenwich. Sets it for his middle geographic coordinate basis, then he gets UT0.

UT1 - Universal Time No.1

UT1 is obtained by astronomical observations - about a modern meridian circle - or by means of Very Long Baseline Interferometry. The small differences to UTC will be referred to as a time correction and are the polar coordinates x and y are the three parameters of the Earth's rotation.

The time correction DUT1 tends in the course of a few months to negative values ​​, because our time system has the average second from 1900 to 1905 to the base and the Earth's rotation has been pushing about 0.002 seconds per day has slowed. Before DUT1 reached an absolute value of 0.9 seconds, a leap second on 30 June or 31 December is inserted.

However, the exact location of the instantaneous axis of rotation of the earth is subject to minor fluctuations in the meter range. Since also changes the position of the observer relative to the rotational axis in the course of this Polbewegungen, its geographical coordinates are subject to small fluctuations. The fluctuations of its geographical length lead to the conversion of his observations after BDC, which went down in only the middle longitude, is not entirely correct. After appropriate correction results in UT1.

The correction is in the first approximation ( with the average length, the average width and the pole coordinates and ):

The pole coordinates are published in arc seconds. (15 arcsec corresponding to a second of time. )

UT1R

This time scale is similar to UT1, the members having a period that is less than 35 days are weggerechnet. This time scale is so smooth compared to UT1. The R stands for ' Removed ' ( in German: away) or ' Reduced ' ( in German: reduced).

UT1D

This time scale is similar to the UT1, where tidal effects are also taken into account with a half-day or daily period. This time scale is so roughened compared to UT1.

The D stands for ' Daily ' ( in German: daily) or ' Tides ' ( in German: Tides).

UT2

The UT1 thus obtained subject so far only originating from the Earth's rotation fluctuations. The seasonal component, on the one hand by vegetative changes, on the other hand due to water shifts by snow, can be fairly accurately reflect with an empirically derived schematic formula. In an effort to obtain a uniform time scale as possible, you remove this fluctuation component of UT1:

This is expressed as a fraction of the Bessel year time of observation. From UT2 the radio time signal derived in the 60s. With the introduction of UTC in 1972 UT2 lost its importance.

UT and UTC

Since the definition of the second unit not won by astronomical observations, but derived from an atomic clock (1968 ), World Time (UTC ) with this Atomic Time ( TAI ) is synchronized. Due to leap seconds ensures that the difference between UT1 and UTC is not more than 0.9 seconds.