- Time standard
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**time standard**is any officially-recognized specification for measuring time: either therate at whichtime passes; or points in time; or both. For example, the standard for civil time specifies both time intervals and time-of-day. Atime scale specifies divisions of time.Standardized time measurements are done using a

clock by counting the periods of some cyclic change, which may be either the changes of a natural phenomenon or of an artificial machine.Historically, time standards were based on Earth's rotational period, because it was believed that the rotational speed of Earth was constant. However, analyses of eclipse records made in the 19th century revealed that the rate at which Earth rotates is gradually slowing, and measurements made with quartz clocks at the beginning of the 20th century made clear that the speed varies seasonally. Earth rotational standards were first replaced by ones based on the period of Earth's orbit and the motion of other solar system bodies, but these cannot be measured to fractions of a second. Relatively recently, time interval standards based on very accurate and stable

atomic clock s have replaced the previous standards based on Earth's rotational and orbital speeds.Various types of

second andday are used as the basic time interval for most time scales. Other intervals of time (minute s,hour s, andyear s) are usually defined in terms of these two.**Time standards based on Earth's rotation****True solar time**is based on the solar day, which is the period between one solar noon and the next. A solar day is approximately 24 hours of mean time. Because Earth's orbit around the sun is elliptical, and because of the earth axis tilt, the true solar day varies a few dozen seconds above or below the mean value of 24 hours. As this variations accumulates over a few weeks, there are differences as large as 15 minutes between the true solar time and the mean solar time. However, these variations cancel out completely over a year. There are also other perturbations such as Earth's wobble, but these are less than a second per year.is time by the stars. A sidereal day is the time it takes Earth to make one revolution with respect to the stars. A sidereal day is approximately 23 hours 56 minutes 4 seconds. It cannot be used as a time standard because stars have a slight proper motion, so the exact period depends on which star are we observing.Sidereal time (Greenwich Mean Time **GMT**) is mean time on thePrime Meridian . Mean time was derived by observing the true solar time and then adding to it a calculated correction, the**equation of time**, which smoothed the known irregularities caused by the ellipticity of Earth's orbit and the non-perpendicularity of Earth's axis to the plane of Earth's orbit around the sun. GMT used to be an international time standard before the advent of precise atomic clocks. GMT no longer exists as a time standard, although the name GMT is often incorrectly used to denote Universal Time. Greenwich Mean Time also used to be the international standard for civil time. In that sense as well, GMT technically no longer exists, although GMT is still often used as asynonym for UTC, which is the current international standard. The only sense in which Greenwich Mean Time officially still exists is as the name of a time zone.(Universal Time **UT**) is a time scale based on the mean solar day, defined to be as uniform as possible despite variations in Earth's rotation.

***UT0**is the rotational time of a particular place of observation. It is observed as the diurnal motion of stars or extraterrestrial radio sources.

***UT1**is computed by correcting UT0 for the effect of polar motion on the longitude of the observing site. It varies from uniformity because of the irregularities in Earth's rotation.**Time standards for planetary motion calculations**Ephemeris time, dynamical time and

coordinate time are all intended to provide a uniform time for planetary motion calculations.

***Ephemeris Time**(**ET**) is an obsolete time standard based on the ephemeris second, which was a fraction of the tropical year. The ephemeris second was the standard for theSI second from 1956 to 1967. Ephemeris Time was discontinued in 1984. For applications on Earth's surface, ET was replaced by TDT, which has since been redefined as TT. For the calculation of ephemerides, ET was replaced by TDB, but deficiencies in the definition of TDB led to its replacement by TCB for use in the solar system as a whole, and by TCG for use in the vicinity of Earth. In actual practice, ephemerides are calculated using T_{eph}, which is linearly related to TCB but not officially defined.

***Terrestrial Dynamic Time**(**TDT**) replaced Ephemeris Time and maintained continuity with it. TDT is a uniform atomic time scale, whose unit is the SI second. TDT is tied to International Atomic Time (TAI) but, because the zero point of TAI was somewhat arbitrarily defined, TT was offset from TAI by a constant 32.184 seconds. The offset provided a continuity with Ephemeris Time. Terrestrial Dynamic Time has been redefined as Terrestrial Time.

*(Barycentric Dynamical Time **TDB**) is similar to TDT but includes relativistic corrections that move the origin to the barycenter. TDB differs from TT only in periodic terms. The difference is at most 10 milliseconds, which is negligible for many applications.In 1991, in order to clarify the relationships between space-time coordinates, new time scales were introduced, each with a different frame of reference. Terrestrial Time is time at Earth's surface. Geocentric Coordinate Time is a coordinate time scale at Earth's center. Barycentric Coordinate Time is a coordinate time scale at the center of mass of the solar system, which is called the barycenter. Barycentric Dynamical Time is a dynamical time at the barycenter.

*(Terrestrial Time **TT**) is the time scale which had formerly been called Terrestrial Dynamical Time. It is now defined as a coordinate time scale at Earth's surface.

*(Geocentric Coordinate Time **TCG**) is a coordinate time having its spatial origin at the center of Earth's mass. TCG is linearly related to TT as: TCG - TT =`L`* (JD -2443144.5) * 86400 seconds, with the scale difference_{G}`L`defined as 6.969290134e-10 exactly._{G}

*(Barycentric Coordinate Time **TCB**) is a coordinate time having its spatial origin at the solar system barycenter. TCB differs from TT in rate and other mostly periodic terms. Neglecting the periodic terms, in the sense of an average over a long period of time the two are related by: TCB - TT =`L`* (JD -2443144.5) * 86400 seconds. According to IAU the best estimate of the scale difference_{B}`L`is 1.55051976772e-08._{B}**Constructed time standards**(International Atomic Time **[**) is the primary international time standard from which other time standards, including UTC, are calculated. TAI is kept by the BIPM (International Bureau of Weights and Measures), and is based on the combined input of many*http://www.bipm.org/en/scientific/tai/tai.html TAI*]atomic clock s [*http://www.bipm.org/en/scientific/tai/clock_comparisons.html around the world*] , each corrected for environmental and relativistic effects. It is the primary realisation ofTerrestrial Time .(Coordinated Universal Time **UTC**) is an atomic time scale designed to approximateUniversal Time . UTC differs from TAI by an integral number of seconds. UTC is kept within 0.9 seconds of UT1 by the introduction of one-second steps to UTC, the "leap second ". To date these steps have always been positive.**Standard time**or**civil time**in a region deviates a fixed, round amount, usually a whole number of hours, from some form ofUniversal Time , now usually UTC. The offset is chosen such that a new day starts approximately while the sun is at thenadir . SeeTime zone . Alternatively the difference is not really fixed, but it changes twice a year a round amount, usually one hour, seeDaylight saving time .**Other time scales**Julian day number is a count of days elapsed since Greenwich mean noon on 1 January 4713 B.C., Julian proleptic calendar. The Julian Date is the Julian day number followed by the fraction of the day elapsed since the preceding noon. Conveniently for astronomers, this avoids the date skip during an observation night.

Modified Julian day (MJD) is defined as MJD = JD - 2400000.5. An MJD day thus begins at midnight, civil date. Julian dates can be expressed in UT, TAI, TDT, etc. and so for precise applications the timescale should be specified, e.g. MJD 49135.3824 TAI.

**ee also***

Radio clock

*ml|Orbital_period|Small_body_orbiting_a_central_body|Orbital period as unit of time

*Systems of measurement for units of time other than the modern international one**Further reading*** "Explanatory Supplement to the Astronomical Almanac," P. K. Seidelmann, ed., University Science Books, 1992, ISBN 0-935702-68-7

**External links*** [

*http://tycho.usno.navy.mil/systime.html Systems of Time*] by Dr. Demetrios Matsakis, Director, Time Service Dept.,United States Naval Observatory

* [*http://tycho.usno.navy.mil/leapsec.html USNO article on the definition of seconds and leap seconds*]

* [*http://www.ucolick.org/~sla/leapsecs/timescales.html A history of astronomical time scales*] by Steve Allen

* [*http://www.cv.nrao.edu/~rfisher/Ephemerides/times.html Astronomical times*]

* [*http://www.sciam.com/article.cfm?id=experts-time-division-days-hours-minutes Why is a minute divided into 60 seconds, an hour into 60 minutes, yet there are only 24 hours in a day?*] Ask the Experts - March 5, 2007. SCIENTIFIC AMERICAN

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