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A time scale specifies divisions of time. A time standard is a specification of either the rate at which time passes, or points in time, or both. For example, the standard for civil time specifies both standard time intervals and time-of-day.

Historically, time standards were based on the Earth's rotational period. However, the rate at which the Earth rotates is not constant. Earth rotational standards were first replaced by ones based on the period of Earth's orbit but, because it's orbit is elliptical , the Earth moves faster when it is closer to the sun, so the orbital period is not constant, either. Relatively recently, time interval standards based based on very accurate and stable atomic clocks have replace the previous standards base on the Earth's rotational and orbital speeds.

The internationally recognized time interval is the second. The second is used as the basic time interval for many time scales. Other intervals of time (minutes, hours, days, and years) are usually defined in terms of the second.

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Some time standards

Solar time is based on the solar day, which is the period of time between one solar noon and the next. A solar day is approximately 24 hours, on average. However, because of the irregular rate of the Earth's rotation, solar time varies as much as 15 minutes from mean solar time.

Sidereal time is time by the stars. A sidereal day is the time it takes the Earth to make one revolution with respect to the stars. A sidereal day is approximately 23 hours 56 minutes 4 seconds.

Greenwich Mean Time (GMT) is time on the prime meridian. GMT used to be an international time standard. In that sense, technically, GMT no longer exists, although Universal Time is essentially what GMT used to be. 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 a synonym for UTC, which is the international standard. The only sense in which Greenwich Mean Time technically still exists is as the name of a time zone.

International Atomic Time (TAI) 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 atomic clocks around the world, each corrected for environmental and relativistic effects.

Universal Time (UT) is a time scale based the mean solar day, defined to be as uniform as possible despite variations in the rotation of the Earth.

  • 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 the Earth's rotation.
  • Coordinated Universal 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.

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 the SI second from 1956 to 1967. Ephemeris Time was discontinued in 1984, and replaced by TDT, which has since been replaced by TDB.
  • 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 renamed Terrestrial Time and also replaced by Barycentric Dynamical Time.
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 the surface of the Earth. Geocentric Coordinate Time is a proper time scale at the Earth's center. Barycentric Coordinate Time is a proper time scale at the center of gravity 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 the surface of the Earth.
  • Barycentric Dynamical Time[?] (TDB) is similar to TT 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.
  • Geocentric Coordinate Time (TCG) is a coordinate time having its spatial origin at the center of mass of the Earth. TCG differs from TT as: TCG - TT = Lg x (JD -2443144.5) x 86400 seconds, with Lg = 6.969291e-10.
  • Barycentric Coordinate Time (TCB) is a coordinate time having its spatial origin at the solar system barycenter. TCB differs from TDB in rate. The two are related by: TCB - TDB = iLb x (JD -2443144.5) x 86400 seconds, with Lb = 1.550505e-08.

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 date (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.

See also: time, clock, calendar, era, epoch, period

For more information, see:

  • Explanatory Supplement to the Astronomical Almanac, P. K. Seidelmann, ed., University Science Books, 1992, ISBN 0-935702-68-7

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