Хэрэглэгч:Timur/Ноорог/Цагийн стандартууд

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.

Дэлхийн хоногийн эргэлт дээр тулгуурласан цагийн стандартууд

Нарны цаг

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 a day's variation depends on the observer's latitude, solar time varies as much as 15 minutes from mean solar time. There are also other perturbations such as Earth's wobble, but these are less than a second per year.

Solar time is based on the idea that when the sun reaches its highest point in the sky, it is noon.

Apparent solar time is based on the apparent solar day, which is the interval between two successive returns of the Sun to the local meridian. Solar time can be measured by a sundial.

The length of a solar day varies throughout the year for two reasons. First, Earth's orbit is an ellipse, not a circle, so the Earth moves faster when it is nearest the Sun (perihelion) and slower when it is farthest from the Sun (aphelion) (see Kepler's laws of planetary motion). Second, due to Earth's axial tilt, the Sun moves along a great circle (the ecliptic) that is tilted to Earth's celestial equator. When the Sun crosses the equator at both equinoxes, it is moving at an angle to it, so the projection of this tilted motion onto the equator is slower than its mean motion; when the Sun is farthest from the equator at both solstices, it moves parallel to it, so the projection of this parallel motion onto the equator is faster than its mean motion (see tropical year). Consequently, apparent solar days are shorter in March (26–27) and September (12–13) than they are in June (18–19) or December (20–21). These dates are shifted from those of the equinoxes and solstices by the fast/slow Sun at Earth's perihelion/aphelion.

Одны цаг

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.

Sidereal time vs solar time. Above left, a distant star (the small red circle) and the Sun are at culmination, on the local meridian. Centre: only the distant star is at culmination (a mean sidereal day). Right: few minutes later the Sun is on the local meridian again. A solar day is complete

Sidereal time is time measured by the apparent diurnal motion of the vernal equinox, which is very close to, but not identical with, the motion of stars. They differ by the precession of the vernal equinox relative to the stars.

Solar time is measured by the apparent diurnal motion of the sun, and local noon in solar time is defined as the moment when the sun is at its highest point in the sky (exactly due south in the northern hemisphere and due north in the southern hemisphere). The average time taken for the sun to return to its highest point is 24 hours.

However, the stars appear to move in a slightly different way. During the course of one day, the earth has moved a short distance along its orbit around the sun, and so must rotate a small extra angular distance before the sun reaches its highest point. The stars, however, are so far away that the earth's movement along its orbit makes a generally negligible difference to their apparent direction (see, however parallax), and so they return to their highest point in slightly less than 24 hours. A mean sidereal day is about 23h 56m 4.1s in length. However, due to variations in the rotation rate of the Earth the rate of an ideal sidereal clock deviates from any simple multiple of a civil clock. In practice, the difference is kept track of by the difference UTC–UT1, which is measured by radio telescopes and kept on file and available to the public at the IERS and at the United States Naval Observatory.

Sidereal time is defined as the hour angle of the vernal equinox. When the meridian of the vernal equinox is directly overhead, local sidereal time is 00:00. Greenwich Sidereal time is the hour angle of the vernal equinox at the prime meridian at Greenwich, England; local values differ according to longitude. When one moves eastward 15° in longitude, sidereal time is larger by one hour (note that it wraps around at 24 hours). Unlike computing local solar time, differences are counted to the accuracy of measurement, not just in whole hours. Greenwich sidereal time and UT1 differ from each other by a constant rate (1.00273790935). Sidereal time is used at astronomical observatories because sidereal time makes it very easy to work out which astronomical objects will be observable at a given time. Objects are located in the night sky using right ascension and declination relative to the celestial equator (analogous to longitude and latitude on Earth), and when sidereal time is equal to an object's right ascension, the object will be at its highest point in the sky, or culmination, at which time it is best placed for observation, as atmospheric extinction is minimised.

Дундаж нарны цаг

Mean solar time is artificial clock time adjusted via observations of the diurnal rotation of the fixed stars to agree with average apparent solar time. The length of a mean solar day is a constant 24 hours throughout the year even though the amount of daylight within it may vary. An apparent solar day may differ from a mean solar day (of 86,400 seconds) by as much as nearly 22 seconds shorter to nearly 29 seconds longer. Because many of these long or short days occur in succession, the difference builds up to as much as nearly 17 minutes early or a little over 14 minutes late. Since these periods are cyclical, they do not accumulate from year to year. The difference between apparent solar time and mean solar time is called the equation of time.

Many methods have been used to simulate mean solar time throughout history. The earliest were clepsydras or water clocks, used for almost four millennia from as early as the middle of the second millennium BC until the early second millennium. Before the middle of the first millennium BC, the water clocks were only adjusted to agree with the apparent solar day, thus were no better than the shadow cast by a gnomon (a vertical pole), except that they could be used at night.

On a prograde planet like the Earth, the sidereal day is shorter than the solar day. At time 1, the sun and a certain distant star are both overhead. At time 2, the planet has rotated 360° and the distant star is overhead again (1→2 = one sidereal day). But it is not until a little later, at time 3, that the sun is overhead again (1→3 = one solar day).

Nevertheless, it has long been known that the sun moves eastward relative to the fixed stars along the ecliptic. Thus since the middle of the first millennium BC, the diurnal rotation of the fixed stars has been used to determine mean solar time, against which clocks were compared to determine their error rate. Babylonian astronomers knew of the equation of time and were correcting for it as well as the different rotation rate of stars, sidereal time, to obtain a mean solar time much more accurate than their water clocks. This ideal mean solar time has been used ever since then to describe the motions of the planets, Moon, and Sun.

Mechanical clocks did not achieve the accuracy of Earth's 'star clock' until the beginning of the twentieth century. Even though today's atomic clocks have a much more constant rate than the Earth, its star clock is still used to determine mean solar time. Since sometime in the late 1900s, Earth's rotation has been defined relative to an ensemble of extra-galactic radio sources and then converted to mean solar time by an adopted ratio. The difference between this calculated mean solar time and Coordinated Universal Time (UTC) is used to determine whether a leap second is needed.

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 precice 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.

Дэлхий дахины цаг

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. Universal Time (UT) is a timescale based on the rotation of the Earth. It is a modern continuation of the Greenwich Mean Time (GMT), i.e., the mean solar time on the meridian of Greenwich, England, which is the conventional 0-meridian for geographic longitude. GMT is sometimes used, incorrectly, as a synonym for UTC. The old GMT has been split, in effect, into UTC and UT1.

Хэмжилт

One can measure time based on the rotation of the Earth by observing celestial bodies crossing the meridian every day. Astronomers have preferred observing meridian crossings of stars over observations of the Sun, because these are more accurate. Nowadays, UT in relation to International Atomic Time (TAI) is determined by Very Long Baseline Interferometry (VLBI) observations of distant quasars, a method which has an accuracy of micro-seconds. Most sources of time and celestial coordinate system standards use UT1 as the default meaning of UT, though occasionally UTC may be implied.

The rotation of the Earth and UT are monitored by the International Earth Rotation and Reference Systems Service (IERS). The International Astronomical Union is also involved in setting standards, but the final arbiter of broadcast standards is the International Telecommunication Union or "ITU."

The rotation of the Earth is somewhat irregular; also the length of the day very gradually increases due to tidal acceleration. Furthermore, the length of the second is based on its conventional length as determined from observations of the Moon between 1750 and 1890. This also causes the mean solar day, on the average, to now extend longer than the nominal 86,400 SI seconds. As UT is slightly irregular in its rate, astronomers introduced Ephemeris Time, which has since been replaced by Terrestrial Time (TT). However, because Universal Time is synchronous with night and day, and more precise atomic-frequency standards drift away from this, UT is still used to produce a correction called leap seconds to atomic time to obtain a broadcast form of civil time that carries atomic frequency. Thus, civil broadcast standards for time and frequency are a compromise that usually follows, with an offset found from the total of all leap seconds, International Atomic Time (TAI), but occasionally jumps in order to prevent it from drifting too far from mean solar time. Terrestrial Time is TAI + 32.184 s.

Barycentric Dynamical Time (TDB), a form of atomic time, is now used in the construction of the ephemerides of the planets and other solar system objects, for two main reasons. For one thing, these ephemerides are tied to optical and radar observations of planetary motion, and the TDB time scale is fitted so that Newton's laws of motion, with corrections for general relativity, are followed. For another, the time scales based on Earth's rotation are not uniform, so are not suitable for predicting the motion of solar system objects.

In 1928 the term Universal Time was adopted internationally as a more precise term than Greenwich Mean Time, because the GMT could refer to either an astronomical day starting at noon or a civil day starting at midnight. However, the term Greenwich Mean Time persists in common usage to this day in reference to civil timekeeping.

Хувилбарууд

There are several versions of Universal Time:

• UT0 is the rotational time of a particular place of observation. It is observed as the diurnal motion of stars or extraterrestrial radio sources. UT0 is Universal Time determined at an observatory by observing the diurnal motion of stars or extragalactic radio sources, and also from ranging observations of the Moon and artificial Earth satellites. It is uncorrected for the displacement of Earth's geographic pole from its rotational pole. This displacement, called polar motion, causes the geographic position of any place on Earth to vary by several metres, and different observatories will find a different value for UT0 at the same moment. It is thus not, strictly speaking, Universal.

• 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. UT1 is the principal form of Universal Time. It is computed from the raw observed UT0 by correcting UT0 for the effect of polar motion on the longitude of the observing site. UT1 is the same everywhere on Earth, and is proportional to the true rotation angle of the Earth with respect to a fixed frame of reference. Since the rotational speed of the earth is not uniform, UT1 has an uncertainty of plus or minus 3 milliseconds per day. The ratio of UT1 to mean sidereal time is defined to be 0.997269566329084 − 5.8684×10⁻¹¹T + 5.9×10⁻¹⁵T², where T is the number of Julian centuries of 36525 days each that have elapsed since JD 2451545.0 (J2000).

• UT1R is a smoothed version of UT1, filtering out periodic variations due to tides. It includes 62 smoothing terms, with periods ranging from 5.6 days to 18.6 years.

• UT2 is a smoothed version of UT1, filtering out periodic seasonal variations. It is mostly of historic interest and rarely used anymore. It is defined by the equation:

$UT2=UT1+0.0220\cdot \sin(2\pi t)-0.0120\cdot \cos(2\pi t)-0.0060\cdot \sin(4\pi t)+0.0070\cdot \cos(4\pi t)\;{\mbox{seconds}}$ where t is the time as fraction of the Besselian year.

• UT2R is a smoothed version of UT1, incorporating both the seasonal corrections of UT2 and the tidal corrections of UT1R. It is the most smoothed form of Universal Time. Its non-uniformities reveal the unpredictable components of Earth rotation, due to atmospheric weather, plate tectonics, and currents in the interior of the Earth.

• UTC (Coordinated Universal Time) is an atomic timescale that approximates UT1. It is the international standard on which civil time is based. It ticks SI seconds, in step with TAI. It usually has 86400 SI seconds per day, but is kept within 0.9 seconds of UT1 by the introduction of occasional intercalary leap seconds. As of 2007 these leaps have always been positive, with a day of 86401 seconds. When an accuracy better than one second is not required, UTC can be used as an approximation of UT1. The difference between UT1 and UTC is known as DUT1.

• UTC-SLS (UTC with Smoothed Leap Seconds) is a modified form of UTC that avoids unequal day lengths. It usually ticks the same as UTC, but modifies the length of the second for the last 1000 UTC seconds of a day containing a leap second so that there are always 86400 seconds in the UTC-SLS day.

• UTS (Smoothed Universal Time) is an obscure form of UT used internally at IERS. The same abbreviation was for a time used to refer to UTC-SLS.

Хүний бүтээсэн цагийн стандартууд

Олон улсын атомын цаг

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.

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. It is the principal realisation of Terrestrial Time, and the basis for Coordinated Universal Time (UTC) which is used for civil timekeeping all over the Earth's surface. As of 2006 TAI is exactly 33 seconds ahead of UTC: 10 seconds initial difference at the start of 1972, plus 23 leap seconds in UTC since 1972.[1]

Time coordinates on the TAI 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. TAI in this form was synchronised with Universal Time at the beginning of 1958, and the two have drifted apart ever since.

Ажиллагаа

TAI as a frequency standard is a weighted average of the time kept by about 300 atomic clocks in over 50 national laboratories worldwide. Many of these are caesium atomic clocks, which are the standard by which the SI second is defined. Due to the averaging it is far more stable than any clock would be alone.

The participating institutions each broadcast in real time a frequency signal with time codes, which is their estimate of TAI. (Actually the time codes are usually published in the form of UTC.) The better laboratories' signals are mutually synchronised to within less than 10^-7 s, but there are outliers up to 10^-5 s out. These time scales are denoted in the form "TAI(NPL)" ("UTC(NPL)" for the UTC form), where "NPL" in this case identifies the National Physical Laboratory, UK. Some laboratories also publish their own atomic time scale, denoted in the form "TA(USNO)" ("USNO" identifies the United States Naval Observatory).

The clocks at different institutions are regularly compared against each other. The International Bureau of Weights and Measures (BIPM) combines these measurements to retrospectively calculate the weighted average that forms the most stable time scale possible. This combined time scale is published monthly in Circular T, and is the canonical TAI. This time scale is expressed in the form of tables of differences UTC-UTC(x) and TAI-TA(x), for each participating institution x.

Errors in publication may be corrected by issuing a revision of the faulty Circular T or by errata in a subsequent Circular T. Aside from this, once published in Circular T the TAI scale is not revised. In hindsight it is possible to discover errors in TAI, and to make better estimates of the true proper time scale. Doing so does not create another version of TAI; it is instead considered to be creating a better realisation of Terrestrial Time (TT). See the article on TT for more information.

Түүх

Atomic timekeeping services started experimentally in 1955, using the first caesium atomic clock at the National Physical Laboratory, UK (NPL). The first formalised atomic time scale was the A.1 scale defined by the United States Naval Observatory (USNO) in 1959. A.1 was defined by an epoch at the beginning of 1958: it was set to read Julian Date 2436204.5 (1958-01-01T00:00:00) at the UT2 instant JD 2436204.5 (1958-01-01T00:00:00) as calculated at USNO. This synchronisation was inevitably imperfect, depending as it did on the astronomical realisation of UT2. At the time, UT2 as published by various observatories differed by several centiseconds. A.1 was extrapolated backwards to 1956.

In 1961 the Bureau International de l'Heure (BIH) (later superseded by the BIPM and the IERS) constructed an atomic time scale named AM based on three atomic clocks. The clocks were compared by listening to radio time signals based on them. The BIH's time scale was synchronised with A.1's epoch, and extrapolated back to 1955 using time signals from the first caesium clock at NPL. This time scale was soon renamed from AM to A3.

Also in 1961, UTC began. UTC is a discontinuous time scale composed from segments that are linear transformations of atomic time, the discontinuities being arranged so that UTC approximates UT1. This was a compromise arrangement for a broadcast time scale: a linear transformation of the BIH's atomic time meant that the time scale was stable and internationally synchronised, while approximating UT1 means that tasks such as navigation which require a source of Universal Time continue to be well served by public time broadcasts.

In 1967 the SI second was redefined in terms of the frequency supplied by a caesium atomic clock.

More clocks were added to the A3 time scale from 1967, and it was renamed to TA. Finally in 1971 it was renamed TAI.

In the 1970s it became clear that the clocks participating in TAI were ticking at different rates due to gravitational time dilation, and the combined TAI scale therefore corresponded to an average of the altitudes of the various clocks. Starting from Julian Date 2443144.5 (1977-01-01T00:00:00), corrections were applied to the output of all participating clocks, so that TAI would correspond to proper time at mean sea level (the geoid). Because the clocks had been on average well above sea level, this meant that TAI slowed down, by about 10^-12. The former uncorrected time scale continues to be published, under the name "EAL" ("Echelle Atomique Libre", meaning "Free Atomic Scale").

The instant that the gravitational correction started to be applied serves as the epoch for Barycentric Coordinate Time (TCB), Geocentric Coordinate Time (TCG), and Terrestrial Time (TT). All three of these time scales were defined to read JD 2443144.5003725 (1977-01-01T00:00:32.184) exactly at that instant. (The 32.184 s offset is to provide continuity with the older Ephemeris Time.) TAI is henceforth a realisation of TT, with the equation TT(TAI) = TAI + 32.184 s.

In the 1990s annual periodic variations in the rate of some clocks were traced to blackbody radiation that varies with the ambient temperature. It became clear that a correction for this was required. Accordingly, in 1997 the BIPM declared that the definition of the SI second referred to a caesium atom at rest and at absolute zero temperature. Temperature corrections were implemented in TAI from 1995 to 1998, speeding TAI up by about 10^-14.3.

Дэлхий дахины координатчлагдсан цаг

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.

Coordinated Universal Time (UTC) is a high-precision atomic time standard. UTC has uniform seconds defined by International Atomic Time (TAI), with leap seconds announced at irregular intervals to compensate for the earth's slowing rotation and other discrepancies. Leap seconds allow UTC to closely track Universal Time (UT), a time standard based not on the uniform passage of seconds, but on the Earth's angular rotation.

Time zones around the world are expressed as positive or negative offsets from UTC. Local time is UTC plus the time zone offset for that location, plus an offset (typically +1) for daylight savings, if in effect.

As the zero-point reference, UTC is also referred to as Zulu time (Z).

Механизм

As a time scale, UTC divides up time into days, hours, minutes, and seconds. Days are conventionally identified using the Gregorian calendar, but Julian Day Numbers can also be used. Each day contains 24 hours and each hour contains 60 minutes, but the number of seconds in a minute is slightly variable.

Most UTC days contain exactly 86,400 seconds, with exactly 60 seconds in each minute. Occasionally the last minute of a day has 59 or 61 seconds. So these irregular days have 86,399 seconds or 86,401 seconds. The irregular day lengths mean that fractional Julian days don't work properly with UTC. The intercalary seconds are known as "leap seconds".

UTC is derived from International Atomic Time (TAI), which is a time scale tracking proper time on the surface of the Earth with no reference to the rotation of the Earth. At any particular time, UTC proceeds as a linear function of TAI. From 1972 onwards UTC ticks at the same rate as TAI, but earlier (back to the 1961 start of UTC) UTC ticked at a different rate from TAI. In order to remain a close approximation of UT1 (equivalent to GMT before 1960), UTC occasionally has discontinuities where it changes from one linear function of TAI to another. These discontinuities take the form of leaps implemented by a UTC day of irregular length, and (prior to 1972) changes to the rate at which UTC ticks relative to TAI. Discontinuities in UTC have only ever occurred at the end of a Gregorian month.

The International Earth Rotation and Reference Systems Service (IERS) tracks and publishes the difference between UTC and Universal Time, DUT1 = UT1 - UTC, and introduces discontinuities into UTC to keep DUT1 in the range -0.9 s < DUT1 < +0.9 s. Since 1972 the discontinuities have consisted only of a leap of one second at the end of June 30 or December 31. The IERS publishes its decision on whether to have a leap second on each of these dates a few months in advance, in Bulletin C. In principle leap seconds can also occur on March 31 or September 30, but the IERS has never found this necessary.

The complete definition of UTC so far, in terms of TAI, is published in the file tai-utc.dat.

As with TAI, UTC is only known with the highest precision in retrospect. The International Bureau of Weights and Measures (BIPM) publishes monthly tables of differences between canonical TAI/UTC and TAI/UTC as estimated in real time by participating laboratories. See the article on TAI for more details.

Иргэний стандарт цаг

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.

Standard time is the result of synchronizing clocks in different geographical locations within a time zone to the same time rather than using the local meridian as in local mean time or solar time. The time so set has come to be defined in terms of offsets from Universal Time. (See more about standard time.)

Where daylight saving time is used, standard time may refer to the time without daylight saving time.

Хугацааны нэгжүүд

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.

Юлианский день (JD) — астрономический способ измерения времени, число дней с полудня 1 января 4713 до н. э. по юлианскому календарю.

Юлианский день был впервые предложен английским астрономом Джоном Гершлем в 1849 году. 4713 до н. э. является начальным годом юлианского цикла, введённого Иосифом Скалигером в 1583 для целей хронологии. Началом дня Гершель выбрал полдень по меридиану Александрии, так как именно так отсчитывались дни в классическом Альмагесте Клавдия Птолемея. Кроме того, использование дня, начинающегося в полдень, удобнее для астрономических наблюдений, обычно проводящихся ночью и, таким образом, попадающих в один юлианский день.

Во второй половине XIX века получил широкое распространение. После 1884 года, когда международный меридиан был перенесён в Гринвич, начало юлианского дня тоже начало отсчитывается по гринвическому меридиану.

Сейчас для простоты расчётов используется модифицированный юлианский день (MJD = JD −2400000,5). Точкой отсчёта является полночь UTC 17 ноября 1858.

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.

Гаригийн хөдөлгөөний тооцоонд зориулагдсан цагийн стандартууд

Ephemeris time, dynamical time and coordinate time are all intended to provide a uniform time for planetary motion calculations. 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.

Эфемеридийн цаг

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.

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. This is distinct from Universal Time (UT): the time scale based on the rotation of the Earth around its axis. ET was replaced with the two time scales Terrestrial Dynamical Time (TDT) and Barycentric Dynamical Time (TDB) by the International Astronomical Union (IAU) in 1976. TDT was renamed Terrestrial Time (TT) in 1991.

In the late nineteenth century it was found that the rotation of the Earth (i.e. the length of the day) was both irregular on short time scales, and was slowing down on longer time scales. In fact, observing the position of the Moon, Sun and planets and comparing this with their ephemerides was a better way to determine the time.

Using the ephemerides based on the theory of the apparent motion of the Sun by Simon Newcomb (1898), the SI second was defined in 1960 as:

the fraction 1/31,556,925.9747 of the tropical year for 1900 January 0 at 12 hours ephemeris time.

Caesium atomic clocks became operational in 1955, and quickly made it evident that the rotation of the earth fluctuated randomly. This confirmed the utter unsuitability of the mean solar second of Universal Time as a measure of time interval. After three years of comparisons with lunar observations it was determined that the ephemeris second corresponded to 9,192,631,770 cycles of the caesium resonance. Between 1960 and 1984 the length of the SI second was defined to be equal to the ephemeris second.

In 1976 the IAU resolved that the theoretical basis for Ephemeris Time is wholly non-relativistic, and threfore, beginning in 1984 Ephemeris Time would be replaced by the two relativistic timescales based on Dynamical time scale, the Barycentric Dynamical Time (TDB) and Terrestrial Dynamical Time (TDT). For practical purposes the length of the ephemeris second can be taken as equal to the length of the TDB or TDT second.

The difference between ET and UT is called ΔT; it changes irregularly, but the long-term trend is parabolic, decreasing from ancient times until the nineteenth century, and has been increasing at about 0.7 seconds per year since (see leap seconds). International Atomic Time (TAI) was set equal to UT2 at 1 January 1958 0:00:00 . At that time, ΔT was already about 32.18 seconds. The difference between Terrestrial Time (TT) (the successor to ephemeris time) and atomic time was later defined as follows:

1977 January 1.0003725 TT = 1977 January 1.0000000 TAI, i.e.

ET - TAI = 32.184 seconds

This difference may be assumed constant—the rates of TT and TAI are designed to be identical.

Газрын динамик цаг

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.

Хүндийн төвийн динамик цаг

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.

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. It was based on a Dynamical time scale but was not well defined and not rigorously correct as a relativistic time scale. It was subsequently deprecated in favour of Barycentric Coordinate Time (TCB), but at the 2006 General Assembly of the International Astronomical Union TDB was rehabilitated by making it a specific fixed linear transformation of TCB.

Since ancient times, planetary ephemerides were calculated using a time scale based on the Earth's rotation: Universal Time (UT). In the late nineteenth century it was realised that this time scale was not uniform: the observed timing of planetary positions did not match up correctly with UT. After atomic clocks were invented, they were used from 1960 to realise a new uniform time scale, Ephemeris Time (ET). ET was henceforth used for the time variable in planetary ephemeris calculations.

ET itself was not entirely satisfactory either. Although it was a uniform time scale within its reference frame, it was subject to time dilation when compared with the proper time experienced by other bodies in the Solar system. Because ET was based on subjective Earth time, the Earth's orbit introduced periodic non-uniformities when comparing ET against the true independent time variable of the ephemerides. Earth time ticks slower when it is near perihelion and speeds up in its orbit (in January) and faster near aphelion (in July).

In 1976 two time scales were defined to replace ET in 1984 ephemerides to take account of relativity. ET's direct successor in counting subjective Earth time was Terrestrial Dynamical Time (TDT). The new time scale to be used for planetary ephemerides was Barycentric Dynamical Time (TDB). TDB was to tick uniformly in a reference frame comoving with the barycentre of the Solar system, but over the long term tick at the same rate as TDT. TDT and TDB were defined in a series of resolutions at the same meeting of the International Astronomical Union.

It was soon realized that TDB was not well defined because it was not accompanied by a general relativistic metric and because the exact relationship between TDB and TDT had not been specified. Furthermore, because the length of the TDB second is determined by clocks on Earth (as opposed to the barycentric reference frame itself) it disagrees with the SI second that would be determined by a clock at rest in the frame. As a result, in 1991 the IAU refined the notions of timescales by creating Barycentric Coordinate Time (TCB) and Geocentric Coordinate Time (TCG). TCB is a replacement for TDB, and TCG is its equivalent for use in near-Earth space. TDT was also renamed to Terrestrial Time (TT), because there is nothing dynamical about it.

TDB is the direct successor of Ephemeris Time in that the values of physical constants, notably the Gaussian gravitational constant, match the traditional values from pre-relativistic days.

Despite IAU recommendations that TCB be used for all further calculations of solar system ephemerides, as of 2002 TDB and Ephemeris Time continue to be used, the latter by the producer of the important DE200 ephemeris and its successors at the Jet Propulsion Laboratory. This somewhat controversial approach is taken because the timescale is fitted to observed data for the planets, and to a lesser extent some of their satellites. To adopt TDB or TCB would be to force a timestream based on terrestrial clocks, albeit "corrected" for (some) general relativistic effects, on a data set with which it might not be quite compatible. That said, the differences between Ephemeris Time and TDB appear to be immeasurably small as of 2005.

Nevertheless, as greater accuracy is attained with International Atomic Time and Ephemeris Time differences may appear; thus it seems worthwhile to retain the two timestreams, Ephemeris Time and TDB or TCB, in hopes that we can learn from any measured differences. For practical purposes the only difference between TDB and TCB is that TCB ticks faster. This rate difference means, according to some scientists, that physical constants have different values in TCB than they do in TDB. Changing software from the traditional TDB values to the recommended TCB values would require considerable effort, but please note the considerations in the next paragraph.

Relativists accept Einstein's Principle of Equivalence (see general relativity), so that fundamental physical constants are the same in all inertial coordinate systems, and most do not use alternate definitions of the second as, for example, set up in TDB or TCB, these seeming to be fossils of early attempts to define absolute time. Relativists who are familiar with general relativity insist that there is no unambiguous way to compare the rates of clocks separated from each other in space or in time, or in relative motion to one another, nor to so compare measures of length and so on. Attempts to set up such comparisons are bound to fail when pursued to higher and higher accuracy. These comparisons and equations that model them may, however, be useful in limited contexts, though they are not normally regarded as a basis for defining different units.

Газрын цаг

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.

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.

Түүх

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, based on a Dynamical time scale. Both of these time standards turned out to be poorly defined, and TDT was also misnamed, having nothing dynamical about it.

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, 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.

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.

Тодорхойлолт

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

TT = (1 − L_G) TCG + E

where TT and TCG are linear counts of SI seconds in Terrestrial Time and Geocentric Coordinate Time respectively, L_G is the constant difference in the rates of the two time scales, and E is a constant to resolve the epochs (see below). L_G is defined as exactly 6.969290134 × 10⁻¹⁰. (In 1991 when TT was first defined, L_G was to be determined by experiment, and the best available estimate was 6.969291 × 10⁻¹⁰.)

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

TT = TCG − L_G × (JD_TCG − 2443144.5003725) × 86400

where JD_TCG 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.

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 the International Atomic Time (TAI) instant 1977-01-01T00:00:00.000 exactly. This is also the instant at which TAI introduced corrections for gravitational time dilation.

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

JD_TT = E_JD + (JD_TCG − E_JD) (1 − L_G)

where E_JD is 2443144.5003725 exactly.

Хэрэгжүүлэлт

TT is a theoretical ideal, not dependent on a particular realisation. For practical purposes, TT must be realised by actual clocks in the Earth system.

The main realisation of TT is supplied by 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:

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(BIPM06)", 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 March 2007 is TT(BIPM06).

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.

Дэлхийн төвийн координат цаг

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.

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. It is equivalent to the proper time experienced by a clock at rest in a coordinate frame co-moving with the center of the Earth: that is, a clock that performs exactly the same movements as the Earth but is outside the Earth's gravity well. It is therefore not influenced by the gravitational time dilation caused by the Earth.

TCG was defined in 1991 by the International Astronomical Union, in Recommendation III of the XXIst General Assembly. It was intended as one of the replacements for the ill-defined Barycentric Dynamical Time (TDB). Unlike former astronomical time scales, TCG is defined in the context of the general theory of relativity. The relationships between TCG and other relativistic time scales are defined with fully general relativistic metrics.

Because the reference frame for TCG is not rotating with the surface of the Earth and not in the gravitational potential of the Earth, TCG ticks faster than clocks on the surface of the Earth by about 7.0 × 10⁻¹⁰ (about 22 milliseconds per year). Consequently, the values of physical constants to be used with calculations using TCG differ from the traditional values of physical constants. (The traditional values were in a sense wrong, incorporating corrections for the difference in time scales.) Adapting the large body of existing software to change from TDB to TCG is a formidable task, and as of 2002 many calculations continue to use TDB in some form.

Time coordinates on the TCG scale 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 its predecessor Ephemeris Time, TCG was set to match ET at around Julian Date 2443144.5 (1977-01-01T00Z). More precisely, it was defined that TCG instant 1977-01-01T00:00:32.184 exactly corresponds to TAI instant 1977-01-01T00:00:00.000 exactly. This is also the instant at which TAI introduced corrections for gravitational time dilation.

TCG is a Platonic time scale: a theoretical ideal, not dependent on a particular realisation. For practical purposes, TCG must be realised by actual clocks in the Earth system. Because of the linear relationship between Terrestrial Time (TT) and TCG, the same clocks that realise TT also serve for TCG. See the article on TT for details of the relationship and how TT is realised.

Barycentric Coordinate Time (TCB) is the equivalent of TCG for calculations relating to the solar system beyond Earth orbit. TCG is defined by a different reference frame from TCB, such that they are not linearly related. Over the long term, TCG ticks more slowly than TCB by about 1.6 × 10⁻⁸. In addition there are periodic variations, as Earth moves within the Solar system. When the Earth is at perihelion in January, TCG ticks even more slowly than it does on average, due to gravitational time dilation from being deeper in the Sun's gravity well and also velocity time dilation from moving faster relative to the Sun. At aphelion in July the opposite holds, with TCG ticking faster than in does on average.

Хүндийн төвийн координат цаг

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.

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. It is equivalent to the proper time experienced by a clock at rest in a coordinate frame co-moving with the barycenter of the Solar system: that is, a clock that performs exactly the same movements as the Solar system but is outside the system's gravity well. It is therefore not influenced by the gravitational time dilation caused by the Sun and the rest of the system.

TCB was defined in 1991 by the International Astronomical Union, in Recommendation III of the XXIst General Assembly. It was intended as one of the replacements for the ill-defined Barycentric Dynamical Time (TDB). Unlike former astronomical time scales, TCB is defined in the context of the general theory of relativity. The relationships between TCB and and other relativistic time scales are defined with fully general relativistic metrics.

Because the reference frame for TCB is not influenced by the gravitational potential caused by the Solar system, TCB ticks faster than clocks on the surface of the Earth by about 1.6 × 10⁻⁸ (about 490 milliseconds per year). Consequently, the values of physical constants to be used with calculations using TCB differ from the traditional values of physical constants. (The traditional values were in a sense wrong, incorporating corrections for the difference in time scales.) Adapting the large body of existing software to change from TDB to TCB is a formidable task, and as of 2002 many calculations continue to use TDB in some form.

Time coordinates on the TCB scale 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 its predecessor Ephemeris Time, TCB was set to match ET at around Julian Date 2443144.5 (1977-01-01T00Z). More precisely, it was defined that TCB instant 1977-01-01T00:00:32.184 exactly corresponds to the TAI instant 1977-01-01T00:00:00.000 exactly, at the geocenter. This is also the instant at which TAI introduced corrections for gravitational time dilation.

Мөн үзэх

• Календарь
• Цаг
• Эрин (цагийн агшин)
• Радио цаг

Гадны линкүүд

• http://tycho.usno.navy.mil/systime.html 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

Ном хэвлэл

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