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 its 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 on very accurate and stable atomic clocks have replaced 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.
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 the Earth's orbit around the sun is elliptical, and the day's variation depends on the observer's lattitude, solar time varies as much as 15 minutes from mean solar time. There are also other perturbations such as the Earth's wobble, but these are less than a second per year.
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 on 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.
Local time or civil time in a region deviates a fixed, round amount, usually a whole number of hours, from UTC, such that a new day starts approximately in the middle of the night ("night" considered in terms of the sun being down). See Time zone. Alternatively the difference is not really fixed, but it changes twice a year a round amount, usually one hour, see Daylight saving time.
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. For applications on the surface of the earth, 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 the earth. In actual practice, ephemerides are calculated using Teph, 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 the surface of the Earth. Geocentric Coordinate Time is a coordinate time scale at the 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 the surface of the Earth.
- Geocentric Coordinate Time (TCG) is a coordinate time having its spatial origin at the center of mass of the Earth. TCG is linearly related to TT as: TCG - TT = LG * (JD -2443144.5) * 86400 seconds, with the scale difference LG defined as 6.969290134e-10 exactly.
- 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 = LB * (JD -2443144.5) * 86400 seconds. According to IAU the best estimate of the scale difference LB is 1.55051976772e-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 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.
- Explanatory Supplement to the Astronomical Almanac, P. K. Seidelmann, ed., University Science Books, 1992, ISBN 0-935702-68-7