Information about time standard

A time standard is any officially-recognized specification for measuring time: either the rate at which time passes; or points in time; or both. For example, the standard for civil time specifies both time intervals and time-of-day. A time 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 a 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 clocks have replaced the previous standards based on Earth's rotational and orbital speeds.

Various types of second and day are used as the basic time interval for most time scales. Other intervals of time (minutes, hours, and years) 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 hour. 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 variation 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.

Sidereal time 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.

Greenwich Mean Time (GMT) is mean time on the Prime 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 a synonym 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 the SI 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_G * (JD -2443144.5) * 86400 seconds, with the scale difference L_G 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 = L_B * (JD -2443144.5) * 86400 seconds. According to IAU the best estimate of the scale difference L_B is 1.55051976772e-08.

Constructed time standards

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. It is the primary realisation of Terrestrial Time.

Coordinated Universal Time (UTC) is an atomic time scale designed to approximate 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.

Standard time or civil time in a region deviates a fixed, round amount, usually a whole number of hours, from some form of Universal Time, now usually UTC. The offset is chosen such that a new day starts approximately while the sun is at the nadir. 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.

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.

External links

Systems of Time by Dr. Demetrios Matsakis, Director, Time Service Dept., United States Naval Observatory
USNO article on the definition of seconds and leap seconds
A history of astronomical time scales by Steve Allen
Astronomical times

Example

When dealing with rates, the division operator is sometimes expressed as per.
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time.

One view is that time is part of the fundamental structure of the universe, a dimension in which events occur in sequence, and time itself is something that can be measured.
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A spatial point is a concept used to define an exact location in space. It has no volume, area or length, making it a zero dimensional object. Points are used in the basic language of geometry, physics, vector graphics (both 2D and 3D), and many other fields.
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standardization or standardisation can have several meanings depending on its context. Common use of the word standard implies that it is a universally agreed-upon set of guidelines for interoperability.
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A time scale specifies divisions (scale) of time.

A time standard is a specification of either the rate at which time passes, or points in time, or both.
A duration is a quantity of time.

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Circadian Locomotor Output Cycles Kaput, or Clock is a gene which encodes proteins regulating circadian rhythm. The CLOCK protein seems to affect both the persistence and length of the circadian cycle. CLOCK forms part of a basic-helix-loop-helix transcription factor.
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Atomic Clock is a type of clock that uses an atomic resonance frequency standard to feed its counter. Early atomic clocks were masers with attached equipment. Today's best atomic frequency standards (or clocks) are based on absorption spectroscopy of cold atoms in atomic fountains.
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second (SI symbol: s), sometimes abbreviated sec., is the name of a unit of time, and is the International System of Units (SI) base unit of time.

SI prefixes are frequently combined with the word second to denote subdivisions of the second, e.g.
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day (symbol: d) is a unit of time equivalent to 24 hours. It is not an SI unit but it is accepted for use with SI.^[1] The SI unit of time is the second. The term comes from the Old English dÃ¦g.

Definitions

The day has several definitions.
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minute is a unit of time equal to 1/60th of an hour and to 60 seconds. (Some rare minutes have 59 or 61 seconds; see leap second.)

The minute is not a SI unit, however it is accepted for use with SI units. The correct abbreviation for minute or minutes is "min".
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The hour (symbol: h, or occasionally hr; via Latin from Greek ὥρα "season, time span", ultimately cognate to English ) is a unit of time. It is not an SI unit but is accepted for use with the SI.
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A year (from Old English gēr) is the time between two recurrences of an event related to the orbit of the Earth around the Sun. By extension, this can be applied to any planet: for example, a "Martian year" is the time in which Mars completes its own orbit.
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Solar times are measures of the apparent position of the Sun on the celestial sphere. They are not actually the physical time, but rather hour angles, that is, angles expressed in time units.
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For the novel Sidereal Time see Christopher Meredith.

Sidereal time is a measure of the position of the Earth in its rotation around its axis.
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Time zones of Europe:

blue Western European Time (UTC+0)
Western European Summer Time (UTC+1)
red Central European Time (UTC+1)
Central European Summer Time (UTC+2)
yellow Eastern European Time (UTC+2)
Eastern European Summer Time (UTC+3)
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Prime Meridian, also known as the International Meridian or Greenwich Meridian, is the meridian (line of longitude) passing through the Royal Greenwich Observatory in London — it is the meridian at which longitude is defined to be 0 degrees.
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For the taxonomical term, see .

Synonyms (in ancient Greek, συν ("syn") = plus and όνομα ("onoma") = name
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Universal Time (UT) is a timescale based on the rotation of the Earth. It is a modern continuation of Greenwich Mean Time (GMT), i.e., the mean solar time on the meridian of Greenwich, England, which is the conventional zero meridian for geographic longitude.
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The dynamical time scale is in general, the length of time over which changes in one part of a body can be communicated to the rest of that body. This is often related to the time taken for a system to move from one equilibrium state to another after a sudden change.
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coordinate time to distinguish it from proper time.

In special relativity, the coordinate time (relative to an inertial observer) at an event is the proper time measured by a clock that is at the same location as the event, that is stationary relative to the observer and
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Ephemeris Time (ET) is a now obsolete 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.
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Si, si, or SI may refer to (all SI unless otherwise stated):

In language:

One of two Italian words:
sÃ¬ (accented) for "yes"
si

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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.
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Barycentric Dynamical Time (TDB) was a time standard used to take account of time dilation when calculating orbits of planets, asteroids, comets and interplanetary spacecraft in the Solar system.
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See also:

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
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Barycentric Coordinate Time (TCB) is a coordinate time standard intended to be used as the independent variable of time for all calculations pertaining to orbits of planets, asteroids, comets, and interplanetary spacecraft in the Solar system.
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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.
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Time Standard