DUT1 is a time correction equal to the difference between
Universal Time
Universal Time (UT or UT1) is a time standard based on Earth's rotation. While originally it was mean solar time at 0° longitude, precise measurements of the Sun are difficult. Therefore, UT1 is computed from a measure of the Earth's angle wi ...
(
UT1), which is defined by
Earth's rotation
Earth's rotation or Earth's spin is the rotation of planet Earth around its own Rotation around a fixed axis, axis, as well as changes in the orientation (geometry), orientation of the rotation axis in space. Earth rotates eastward, in progra ...
, and
Coordinated Universal Time
Coordinated Universal Time (UTC) is the primary time standard globally used to regulate clocks and time. It establishes a reference for the current time, forming the basis for civil time and time zones. UTC facilitates international communicat ...
(UTC), which is defined by a network of precision
atomic clock
An atomic clock is a clock that measures time by monitoring the resonant frequency of atoms. It is based on atoms having different energy levels. Electron states in an atom are associated with different energy levels, and in transitions betwee ...
s, with a precision of +/- 0.1s.
:DUT1 = UT1 − UTC (with a precision of +/- 0.1s)
UTC is maintained via
leap second
A leap second is a one-second adjustment that is occasionally applied to Coordinated Universal Time (UTC), to accommodate the difference between precise time (International Atomic Time (TAI), as measured by atomic clocks) and imprecise solar tim ...
s, such that DUT1 remains within the range −0.9 s < DUT1 < +0.9 s. The reason for this correction is partly that the rate of rotation of the Earth is not constant, due to
tidal braking
Tidal acceleration is an effect of the tidal forces between an orbiting natural satellite (e.g. the Moon) and the primary planet that it orbits (e.g. Earth). The acceleration causes a gradual recession of a satellite in a prograde orbit (satelli ...
and the redistribution of mass within the Earth, including its oceans and atmosphere, and partly because the
SI second
The second (symbol: s) is a unit of time derived from the division of the day first into 24 hours, then to 60 minutes, and finally to 60 seconds each (24 × 60 × 60 = 86400). The current and formal definition in the International System of Un ...
(as now used for
UTC
Coordinated Universal Time (UTC) is the primary time standard globally used to regulate clocks and time. It establishes a reference for the current time, forming the basis for civil time and time zones. UTC facilitates international communica ...
) was, when adopted, a little shorter than the current value of the second of mean solar time.
Daily observed values of UT1 - UTC for the past week, and daily forecast values for the coming year, are published by
IERS
The International Earth Rotation and Reference Systems Service (IERS), formerly the International Earth Rotation Service, is the body responsible for maintaining global time and reference frame standards, notably through its Earth Orientation P ...
br>
Bulletin A with more digits than for DUT1. Final values are published in the monthly IER
Bulletin B DUT1 forecasts are published in IER
Bulletin D
Several
time signal service
A radio clock or radio-controlled clock (RCC), and often colloquially (and incorrectly) referred to as an "atomic clock", is a type of quartz clock or watch that is automatically synchronized to a time code transmitted by a radio transmitter co ...
s broadcast values of DUT1.
CHU
Chu or CHU may refer to:
Chinese history
* Chu (state) (c. 1030 BC–223 BC), a state during the Zhou dynasty
* Western Chu (206 BC–202 BC), a state founded and ruled by Xiang Yu
* Chu Kingdom (Han dynasty) (201 BC–70 AD), a kingdom of the H ...
(Canada),
HLA (South Korea),
MSF (United Kingdom), and
WWV (United States) transmit DUT1 with 0.1 s precision. In Russia,
RWM,
RTZ and the longwave
RBU transmit DUT1 with 0.1 s precision and an additional correction dUT1 in 0.02 s increments.
See also
*
Notes
References
* ITU-R Recommendation TF.460-4: Standard-frequency and time-signal emissions.
International Telecommunication Union
The International Telecommunication Union (ITU)In the other common languages of the ITU:
*
* is a list of specialized agencies of the United Nations, specialized agency of the United Nations responsible for many matters related to information ...
.
External links
IERS Bulletins including Bulletin A – latest and past versions, and graphical plots
* Latest version o
Bulletin AIERS Rapid Service/Prediction Center via the US Naval ObservatoryArchived version
available for non-US readers)
* http://scienceworld.wolfram.com/astronomy/Time.html
{{Time measurement and standards
Time scales