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Encyclopedia > Terrestrial Time

Terrestrial Time (TT) is the modern time standard for time on the surface of the Earth. It is the proper time experienced by a clock located on the geoid. In astronomy it is used as the time coordinate for apparent ephemerides for an Earthbound viewer. It is directly related to Geocentric Coordinate Time (TCG), which is the astronomical time standard for the Earth system. TT ticks slower than TCG by a constant rate, due to gravitational time dilation. A time scale specifies divisions of time. ... Earth is the third planet in the solar system. ... Proper time is time as measured by the clock for an observer who is traveling through spacetime. ... The GOCE project will measure high-accuracy gravity gradients and provide an accurate geoid model based on the Earths gravity field. ... Astronomy (Greek: αστρονομία = άστρον + 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, aurora, galaxies, and the cosmic background radiation. ... An ephemeris (plural: ephemerides) (from the Greek word ephemeros= daily) was, traditionally, a table providing the positions (given in a Cartesian coordinate system, or in right ascension and declination or, for astrologers, in longitude along the zodiacal ecliptic), of the Sun, the Moon, and the planets in the sky at... 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. ... Astronomy (Greek: αστρονομία = άστρον + 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, aurora, galaxies, and the cosmic background radiation. ... 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. ...

Contents


History

The approximate concept of TT was standardised by the International Astronomical Union (IAU) in 1976 at its XVIth General Assembly, under the name Terrestrial Dynamical Time (TDT). It was the counterpart to Barycentric Dynamical Time (TDB), which was a time standard for Solar system ephemerides. Both of these time standards turned out to be poorly defined, and TDT was also misnamed, having nothing dynamical about it. Logo of the IAU The International Astronomical Union (French: Union astronomique internationale) unites national astronomical societies from around the world. ... 1976 (MCMLXXVI) was a leap year starting on Thursday (the link is to a full 1976 calendar). ... Barycentric Dynamical Time (TDB) was defined by the International Astronomical Union (IAU) in 1976 to be used as the relativistic replacement for the non-relativistic Ephemeris Time which had been used in the ephemerides starting in 1960. ... An ephemeris (plural: ephemerides) (from the Greek word ephemeros= daily) was, traditionally, a table providing the positions (given in a Cartesian coordinate system, or in right ascension and declination or, for astrologers, in longitude along the zodiacal ecliptic), of the Sun, the Moon, and the planets in the sky at...


In 1991, in Recommendation IV of the XXIst General Assembly, the IAU redefined TDT more precisely, renaming it to "Terrestrial Time". TT was defined in terms of Geocentric Coordinate Time (TCG), which was defined by the same General Assembly. TT was defined to be a linear transformation of TCG, such that TT agrees with proper time on the geoid. This left the exact ratio between TT time and TCG time as something to be determined by experiment. The determination of the gravitational potential at the geoid is a task in physical geodesy. 1991 (MCMXCI) was a common year starting on Tuesday of the Gregorian calendar. ... 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. ... The word linear comes from the Latin word linearis, which means created by lines. ... Proper time is time as measured by the clock for an observer who is traveling through spacetime. ... The GOCE project will measure high-accuracy gravity gradients and provide an accurate geoid model based on the Earths gravity field. ... In physics, gravitational potential is the measure of potential energy an object possesses due to its position in a gravitational field. ... Definition Physical geodesy is the study of the physical properties of the gravity field of the Earth, the geopotential, with a view to their application in geodesy. ...


In 2000, in Resolution B1.9 of the XXIVth General Assembly, the IAU refined the definition of TT by specifying the exact ratio between TT and TCG time as 1 - 6.969290134 × 10-10. This has the effect of redefining the geoid in terms of a precise gravitational potential, thus removing the need for horologists to study sea levels. This article is about the year 2000. ... The GOCE project will measure high-accuracy gravity gradients and provide an accurate geoid model based on the Earths gravity field. ... In physics, gravitational potential is the measure of potential energy an object possesses due to its position in a gravitational field. ...


Definition

TT differs from TCG by a constant rate. Formally it is defined by the equation

TT = (1 - LG) TCG + E

where TT and TCG are linear counts of SI seconds in Terrestrial Time and Geocentric Coordinate Time respectively, LG is the constant difference in the rates of the two time scales, and E is a constant to resolve the epochs (see below). LG is defined as exactly 6.969290134 × 10-10. (In 1991 when TT was first defined, LG was to be determined by experiment, and the best available estimate was 6.969291 × 10-10.) Cover of brochure The International System of Units. ... Look up second in Wiktionary, the free dictionary. ... In chronology, an epoch is an instant chosen as the origin of a particular time scale. ... 1991 (MCMXCI) was a common year starting on Tuesday of the Gregorian calendar. ...


The equation linking TT and TCG is more commonly seen in the form

TT = TCG - LG x (JDTCG - 2443144.5003725) x 86400

where JDTCG is the TCG time expressed as a Julian Date. This is just a transformation of the raw count of seconds represented by the variable TCG, so this form of the equation is needlessly complex. The use of a Julian Date does specify the epoch fully, however (see next paragraph). The above equation is often given with the Julian Date 2443144.5 for the epoch, but that is wrong, the value given above is exactly correct. 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). ...


Time coordinates on the TT and TCG 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. For continuity with their predecessor Ephemeris Time, TT and TCG were set to match ET at around Julian Date 2443144.5 (1977-01-01T00Z). More precisely, it was defined that TT instant 1977-01-01T00:00:32.184 exactly and TCG instant 1977-01-01T00:00:32.184 exactly correspond to TAI instant 1977-01-01T00:00:00.000 exactly. This is also the instant at which TAI introduced corrections for gravitational time dilation. Inscription on the tomb of Pope Gregory XIII celebrating the introduction of the Gregorian Calendar The Gregorian calendar is the calendar that is used nearly everywhere in the world. ... 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. ... For the album by Ash, see 1977 (album). ... International Atomic Time (TAI, from the French name Temps Atomique International) is a high-precision atomic time standard that tracks proper time on Earths geoid. ... 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. ...


TT and TCG expressed as Julian Dates can be related precisely and most simply by the equation

JDTT = EJD + (JDTCG - EJD) (1 - LG)

where EJD is 2443144.5003725 exactly.


Realisation

TT is a Platonic time scale: a theoretical ideal, not dependent on a particular realisation. For practical purposes, TT must be realised by actual clocks in the Earth system. Platonic idealism is the theory that the substantive reality around us is only a reflection of a higher truth. ...


The main realisation of TT is supplied by International Atomic Time (TAI). The TAI service, running since 1958, attempts to match the rate of proper time on the geoid, using an ensemble of atomic clocks spread over the surface and low orbital space of the Earth. TAI is canonically defined retrospectively, in monthly bulletins, in relation to the readings that particular groups of atomic clocks showed at the time. Estimates of TAI are also provided in real time by the institutions that operate the participating clocks. Because of the historical difference between TAI and ET when TT was introduced, the TAI realisation of TT is defined thus: International Atomic Time (TAI, from the French name Temps Atomique International) is a high-precision atomic time standard that tracks proper time on Earths geoid. ... 1958 (MCMLVIII) was a common year starting on Wednesday of the Gregorian calendar. ... Proper time is time as measured by the clock for an observer who is traveling through spacetime. ... The GOCE project will measure high-accuracy gravity gradients and provide an accurate geoid model based on the Earths gravity field. ... 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. ... Earth is the third planet in the solar system. ... It has been suggested that Real-time computing be merged into this article or section. ...

TT(TAI) = TAI + 32.184 s

Because TAI is never revised once published, it is possible for errors in it to become known and remain uncorrected. It is thus possible to produce a better realisation of TT based on reanalysis of historical TAI data. The BIPM has done this approximately annually since 1992. These realisations of TT are named in the form "TT(BIPM05)", with the digits indicate the year of publication. They are published in the form of table of differences from TT(TAI). The latest as of April 2006 is TT(BIPM05). 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. ... 1992 (MCMXCII) was a leap year starting on Wednesday. ... To suggest a relevant news story for the main page, refer to the criteria then add your suggestion at the candidates page. ...


The international communities of precision timekeeping, astronomy, and radio broadcasts are preparing to create a new precision time scale based on observations of an ensemble of pulsars. This new pulsar time scale will serve as an independent means of computing TT, and it may eventually be useful to identify defects in TAI. Astronomy (Greek: αστρονομία = άστρον + 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, aurora, galaxies, and the cosmic background radiation. ... Composite Optical/X-ray image of the Crab Nebula pulsar, showing surrounding nebular gases stirred by the pulsars magnetic field and radiation. ...


See also


  Results from FactBites:
 
Terrestrial Time - Wikipedia, the free encyclopedia (886 words)
Terrestrial Time (TT) is the modern time standard for time on the surface of the Earth.
In astronomy it is used as the time coordinate for apparent ephemerides for an Earthbound viewer.
TT is a Platonic time scale: a theoretical ideal, not dependent on a particular realisation.
Time standard - Wikipedia, the free encyclopedia (1242 words)
A time standard is a specification or standard of either the rate at which time passes, or points in time, or both.
Solar time is based on the solar day, which is the period of time between one solar noon and the next.
Terrestrial Time is time at the surface of the Earth.
  More results at FactBites »

 
 

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