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International Atomic Time (TAI, from the French name Temps Atomique International) is a high-precision atomic time standard that tracks proper time on Earth's geoid. It is the principal realisation of Terrestrial Time, and the basis for Coordinated Universal Time which is used for civil timekeeping all over the Earth's surface. A time scale specifies divisions of time. ...
Proper time is time as measured by the clock for an observer who is traveling through spacetime. ...
Earth (often referred to as The Earth) is the third planet in the solar system in terms of distance from the Sun, and the fifth in order of size. ...
The GOCE project will measure high-accuracy gravity gradients and provide an accurate geoid model based on the Earths gravity field. ...
Terrestrial Time (TT) is the modern time standard for time on the surface of the Earth. ...
It has been suggested that leap second be merged into this article or section. ...
Time coordinates on the TAI scales are conventionally specified using traditional means of specifying days, carried over from non-uniform time standards based on the rotation of the Earth. Specifically, both Julian Dates and the Gregorian calendar are used. TAI in this form was synchronised with Universal Time at the beginning of 1958, and the two have drifted apart ever since. As of 2006 TAI is about 33 s ahead of Universal Time. The Julian day or Julian day number (JDN) is the number of days that have elapsed since 12 noon Greenwich Mean Time (UT or TT) on Monday, January 1, 4713 BC (in the proleptic Julian calendar; or November 24, 4714 BC in the proleptic Gregorian calendar). ...
This article or section does not cite its references or sources. ...
Universal Time (UT) is a timescale based on the rotation of the Earth. ...
Look up second in Wiktionary, the free dictionary. ...
Operation TAI as a frequency standard is a weighted average of the time kept by about 300 atomic clocks in over 50 national laboratories worldwide. Many of these are caesium atomic clocks, which are the standard by which the SI second is defined. Due to the averaging it is far more stable than any clock would be alone. Sine waves of various frequencies; the lower waves have higher frequencies than those above. ...
Atomic clock Chip-Scale Atomic Clock Unveiled by NIST An atomic clock is a type of clock that uses an atomic resonance frequency standard as its counter. ...
General Name, Symbol, Number caesium, Cs, 55 Chemical series alkali metals Group, Period, Block 1, 6, s Appearance silvery gold Atomic mass 132. ...
Cover of brochure The International System of Units. ...
The participating institutions each broadcast in real time a frequency signal with time codes, which is their estimate of TAI. (Actually the time codes are usually published in the form of UTC.) The better laboratories' signals are mutually synchronised to within less than 10-7 s, but there are outliers up to 10-5 s out. These time scales are denoted in the form "TAI(NPL)" ("UTC(NPL)" for the UTC form), where "NPL" in this case identifies the National Physical Laboratory, UK. Some laboratories also publish their own atomic time scale, denoted in the form "TA(USNO)" ("USNO" identifies the United States Naval Observatory). It has been suggested that Real-time computing be merged into this article or section. ...
Timecode is also the title of a 2000 film directed by Mike Figgis which was shot in one continuous take. ...
The National Physical Laboratory (NPL) is the national measurement standards laboratory for the United Kingdom, based at Bushy Park in Teddington in the London Borough of Richmond upon Thames. ...
Aerial view of USNO. The United States Naval Observatory (USNO) is one of the oldest scientific agencies in the United States. ...
The clocks at different institutions are regularly compared against each other. The BIPM combines these measurements to retrospectively calculate the weighted average that forms the most stable time scale possible. This combined time scale is published monthly in Circular T, and is the canonical TAI. This time scale is expressed in the form of tables of differences UTC-UTC(x) and TAI-TA(x), for each participating institution x. The Bureau International des Poids et Mesures (International Bureau of Weights and Measures, or BIPM) is a standards organization, one of the three organizations established to maintain the SI system under the terms of the Metre Convention. ...
Canonical is an adjective derived from canon. ...
Errors in publication may be corrected by issuing a revision of the faulty Circular T or by errata in a subsequent Circular T. Aside from this, once published in Circular T the TAI scale is not revised. In hindsight it is possible to discover errors in TAI, and to make better estimates of the true proper time scale. Doing so does not create another version of TAI; it is instead considered to be creating a better realisation of Terrestrial Time (TT). See the article on TT for more information.
History Atomic timekeeping services started experimentally in 1955, using the first caesium atomic clock at the National Physical Laboratory, UK (NPL). The first formalised atomic time scale was the A.1 scale defined by the United States Naval Observatory (USNO) in 1959. A.1 was defined by an epoch at the beginning of 1958: it was set to read Julian Date 2436204.5 (1958-01-01T00:00:00) at the UT2 instant JD 2436204.5 (1958-01-01T00:00:00) as calculated at USNO. This synchronisation was inevitably imperfect, depending as it did on the astronomical realisation of UT2. At the time, UT2 as published by various observatories differed by several centiseconds. A.1 was extrapolated backwards to 1956. In chronology, an epoch is an instant chosen as the origin of a particular time scale. ...
Universal Time (UT) is a timescale based on the rotation of the Earth. ...
Radio telescopes are among many different tools used by astronomers Astronomy (Greek: αστÏονομία = άστÏον + νόμος, astronomia = astron + nomos, literally, law of the stars) is the science of celestial objects and phenomena that originate outside the Earths atmosphere, such as stars, planets, comets, auroras, galaxies, and the cosmic background radiation. ...
Look up centisecond in Wiktionary, the free dictionary. ...
In 1961 the Bureau International de l'Heure (BIH) (later superseded by the BIPM and the IERS) constructed an atomic time scale named AM based on three atomic clocks. The clocks were compared by listening to radio time signals based on them. The BIH's time scale was synchronised with A.1's epoch, and extrapolated back to 1955 using time signals from the first caesium clock at NPL. This time scale was soon renamed from AM to A3. A time signal is a visible, audible, mechanical, or electronic signal used as a reference to determine the time of day. ...
Also in 1961, UTC began. UTC is a discontinuous time scale composed from segments that are linear transformations of atomic time, the discontinuities being arranged so that UTC approximates UT1. This was a compromise arrangement for a broadcast time scale: a linear transformation of the BIH's atomic time meant that the time scale was stable and internationally synchronised, while approximating UT1 means that tasks such as navigation which require a source of Universal Time continue to be well served by public time broadcasts. Universal Time (UT) is a timescale based on the rotation of the Earth. ...
This article needs to be cleaned up to conform to a higher standard of quality. ...
Table of geography, hydrography, and navigation, from the 1728 Cyclopaedia. ...
In 1967 the SI second was redefined in terms of the frequency supplied by a caesium atomic clock.
More clocks were added to the A3 time scale from 1967, and it was renamed to TA. Finally in 1971 it was renamed TAI.
In the 1970s it became clear that the clocks participating in TAI were ticking at different rates due to gravitational time dilation, and the combined TAI scale therefore corresponded to an average of the altitudes of the various clocks. Starting from Julian Date 2443144.5 (1977-01-01T00:00:00), corrections were applied to the output of all participating clocks, so that TAI would correspond to proper time at mean sea level (the geoid). Because the clocks had been on average well above sea level, this meant that TAI slowed down, by about 10-12. The former uncorrected time scale continues to be published, under the name "EAL" ("Echelle Atomique Libre", meaning "Free Atomic Scale"). Gravitational time dilation is a consequence of Albert Einsteins theories of relativity and related theories under which a clock at a different gravitational potential is found to tick at a different rate than ones own clock. ...
Altitude is the elevation of an object from a known level or datum. ...
For considerations of sea level change, in particular rise associated with possible global warming, see sea level rise. ...
The instant that the gravitational correction started to be applied serves as the epoch for Barycentric Coordinate Time (TCB), Geocentric Coordinate Time (TCG), and Terrestrial Time (TT). All three of these time scales were defined to read JD 2443144.5003725 (1977-01-01T00:00:32.184) exactly at that instant. (The 32.184 s offset is to provide continuity with the older Ephemeris Time.) TAI is henceforth a realisation of TT, with the equation TT(TAI) = TAI + 32.184 s. Barycentric Coordinate Time (TCB) was defined in 1991 by the International Astronomical Union as one of the replacements for the ill-defined Barycentric Dynamical Time (TDB). ...
Geocentric Coordinate Time (TCG) is a coordinate time standard intended to be used as the independent variable of time for all calculations pertaining to precession, nutation, the Moon, and artificial satellites of the Earth. ...
Ephemeris Time (ET) is the time scale used in ephemerides of celestial bodies, in particular the Sun (as observed from the Earth), Moon, planets, and other members of the solar system. ...
In the 1990s annual periodic variations in the rate of some clocks were traced to blackbody radiation that varies with the ambient temperature. It became clear that a correction for this was required. Accordingly, in 1997 the BIPM declared that the definition of the SI second referred to a caesium atom at rest and at absolute zero temperature. Temperature corrections were implemented in TAI from 1995 to 1998, speeding TAI up by about 10-14.3. As the temperature decreases, the peak of the black body radiation curve moves to lower intensities and longer wavelengths. ...
Temperature is also the name of a song by Sean Paul. ...
Absolute zero is a fundamental lower bound on the temperature of any macroscopic system. ...
See also Time and frequency transfer describes mechanisms for comparing measurements of time and frequency from one location to another. ...
Clock drift Clock synchronization is a problem from computer science which deals with the idea that internal clocks of several computers may differ. ...
The Network Time Protocol (NTP) is a protocol for synchronising the clocks of computer systems over packet-switched, variable-latency data networks. ...
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