International Atomic Time

International Atomic Time (TAI, from the French name temps atomique international[1]) is a high-precision atomic coordinate time standard based on the notional passage of proper time on Earth's geoid.[2] It is the principal realisation of Terrestrial Time (with a fixed offset of epoch). It is also the basis for Coordinated Universal Time (UTC), which is used for civil timekeeping all over the Earth's surface. As of 31 December 2016, when another leap second was added,[3] TAI is exactly 37 seconds ahead of UTC. The 37 seconds results from the initial difference of 10 seconds at the start of 1972, plus 27 leap seconds in UTC since 1972.

TAI may be reported 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, due to the changing motion of the Earth.

Operation

TAI is a weighted average of the time kept by over 400 atomic clocks[4] in over 50 national laboratories worldwide.[5] The majority of the clocks involved are caesium clocks; the International System of Units (SI) definition of the second is based on caesium.[6] The clocks are compared using GPS signals and two-way satellite time and frequency transfer.[7] Due to the signal averaging TAI is an order of magnitude more stable than its best constituent clock.

The participating institutions each broadcast, in real time, a frequency signal with timecodes, which is their estimate of TAI. Time codes are usually published in the form of UTC, which differs from TAI by a well-known integer number of seconds. These time scales are denoted in the form UTC(NPL) in the UTC form, where NPL in this case identifies the National Physical Laboratory, UK. The TAI form may be denoted TAI(NPL). The latter is not to be confused with TA(NPL), which denotes an independent atomic time scale, not synchronised to TAI or to anything else.

The clocks at different institutions are regularly compared against each other. The International Bureau of Weights and Measures (BIPM, France), combines these measurements to retrospectively calculate the weighted average that forms the most stable time scale possible.[5] This combined time scale is published monthly in "Circular T",[7] and is the canonical TAI. This time scale is expressed in the form of tables of differences UTC − UTC(k) (equivalent to TAI − TAI(k)) for each participating institution k. The same circular also gives tables of TAI − TA(k), for the various unsynchronised atomic time scales.

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. Since the published circulars are definitive, better estimates do not create another version of TAI; it is instead considered to be creating a better realisation of Terrestrial Time (TT).

History

Early atomic time scales consisted of quartz clocks with frequencies calibrated by a single atomic clock; the atomic clocks were not operated continuously. Atomic timekeeping services started experimentally in 1955, using the first caesium atomic clock at the National Physical Laboratory, UK (NPL). It was used as a basis for calibrating the quartz clocks at the Royal Greenwich Observatory and to establish a time scale, called Greenwich Atomic (GA). The United States Naval Observatory began the A.1 scale on 13 September 1956, using an Atomichron commercial atomic clock, followed by the NBS-A scale at the National Bureau of Standards, Boulder, Colorado on 9 October 1957.[8]

The International Time Bureau (BIH) began a time scale, Tm or AM, in July 1955, using both local caesium clocks and comparisons to distant clocks using the phase of VLF radio signals. The BIH scale, A.1, and NBS-A were defined by an epoch at the beginning of 1958[a] The procedures used by the BIH evolved, and the name for the time scale changed: "A3" in 1964[10] and "TA(BIH)" in 1969.[11]

The SI second was defined in terms of the caesium atom in 1967. From 1971 to 1975 the General Conference on Weights and Measures and the International Committee for Weights and Measures made a series of decisions which designated the BIPM time scale International Atomic Time (TAI).[12]

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 (1 January 1977 00:00:00), 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 were, on average, well above sea level, this meant that TAI slowed down, by about one part in a trillion. The former uncorrected time scale continues to be published, under the name EAL (Echelle Atomique Libre, meaning Free Atomic Scale).[13]

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), which represent three fundamental time scales in the solar system.[14] All three of these time scales were defined to read JD 2443144.5003725 (1 January 1977 00:00:32.184) exactly at that instant.[b] TAI was henceforth a realisation of TT, with the equation TT(TAI) = TAI + 32.184 s.[15]

The continued existence of TAI was questioned in a 2007 letter from the BIPM to the ITU-R which stated, "In the case of a redefinition of UTC without leap seconds, the CCTF would consider discussing the possibility of suppressing TAI, as it would remain parallel to the continuous UTC."[16]

Relation to UTC

UTC is a discontinuous time scale. It is regularly adjusted by leap seconds. Between these adjustments it is composed from segments that are linear transformations of atomic time. From its beginning in 1961 through December 1971 the adjustments were made regularly in fractional leap seconds so that UTC approximated UT2. Afterwards these adjustments were made only in whole seconds to approximate UT1. This was a compromise arrangement in order to enable a publicly broadcast time scale; the post-1971 more linear transformation of the BIH's atomic time meant that the time scale would be more stable and easier to synchronize internationally. The fact that it continues to approximate UT1 means that tasks such as navigation which require a source of Universal Time continue to be well served by the public broadcast of UTC.[17]

See also

Notes

  1. ^ They were set to read Julian Date 2436204.5 (1 January 1958 00:00:00) at the corresponding UT2 instant. However, each observatory used its own value of UT2.[9]
  2. ^ The 32.184 second offset is to provide continuity with the older ephemeris time.

References

  • "History of TAI−UTC". Time Service Dept., United States Naval Observatory. 2009. Retrieved 4 January 2010.
  • "International Atomic Time". International Bureau of Weights and Measures. Retrieved 22 February 2013.

Footnotes

  1. ^ Temps atomique 1975
  2. ^ Guinot, B. (1986). "Is the International Atomic Time TAI a coordinate time or a proper time?". Celestial Mechanics. 38 (2): 155–161. Bibcode:1986CeMec..38..155G. doi:10.1007/BF01230427.
  3. ^ Bizouard, Christian (6 July 2016). "Bulletin C 52". Paris: IERS. Retrieved 31 December 2016.
  4. ^ "Bureau International des Poids et Mesures (BIPM) Time Department" (PDF). Report of the International Association of Geodesy 2011-2013. Retrieved 11 April 2017.
  5. ^ a b "Time". International Bureau of Weights and Measures. Retrieved 22 May 2013.
  6. ^ McCarthy & Seidelmann 2009, p. 207, 214.
  7. ^ a b Circular T, International Bureau of Weights and Measures, retrieved 5 September 2017
  8. ^ McCarthy & Seidelmann 2009, pp. 199–200.
  9. ^ Guinot 2000, p. 181.
  10. ^ Allen, Steve. "The epoch of TAI is 1961-01-01T20:00:00 UT2". UCO/Lick Observatory. By 1964 BIH realized that some atomic chronometers were much better than others, and they constructed A3 based on the best 3
  11. ^ McCarthy & Seidelmann 2009, pp. 200–201.
  12. ^ McCarthy & Seidelmann 2009, pp. 203–204.
  13. ^ McCarthy & Seidelmann 2009, p. 215.
  14. ^ Brumberg, V.A.; Kopeikin, S.M. (March 1990). "Relativistic time scales in the solar system". Celestial Mechanics and Dynamical Astronomy. 48 (1): 23–44. Bibcode:1990CeMDA..48...23B. doi:10.1007/BF00050674. ISSN 0923-2958.
  15. ^ McCarthy & Seidelmann 2009, p. 218–219.
  16. ^ "CCTF 09-27" (PDF). International Bureau of Weights and Measures. 3 September 2007. Archived (PDF) from the original on 16 March 2012. Retrieved 24 September 2016.
  17. ^ McCarthy & Seidelmann 2009, p. 227–229.

Bibliography

External links

Atomic clock

An atomic clock is a clock device that uses an electron transition frequency in the microwave, optical, or ultraviolet region of the electromagnetic spectrum of atoms as a frequency standard for its timekeeping element. Atomic clocks are the most accurate time and frequency standards known, and are used as primary standards for international time distribution services, to control the wave frequency of television broadcasts, and in global navigation satellite systems such as GPS.

The principle of operation of an atomic clock is based on atomic physics; it measures the electromagnetic signal that electrons in atoms emit when they change energy levels. Early atomic clocks were based on masers at room temperature. Since 2004, more accurate atomic clocks first cool the atoms to near absolute zero temperature by slowing them with lasers and probing them in atomic fountains in a microwave-filled cavity. An example of this is the NIST-F1 atomic clock, one of the national primary time and frequency standards of the United States.

The accuracy of an atomic clock depends on two factors. The first factor is temperature of the sample atoms—colder atoms move much more slowly, allowing longer probe times. The second factor is the frequency and intrinsic width of the electronic transition. Higher frequencies and narrow lines increase the precision.

National standards agencies in many countries maintain a network of atomic clocks which are intercompared and kept synchronized to an accuracy of 10−9 seconds per day (approximately 1 part in 1014). These clocks collectively define a continuous and stable time scale, the International Atomic Time (TAI). For civil time, another time scale is disseminated, Coordinated Universal Time (UTC). UTC is derived from TAI, but has added leap seconds from UT1, to account for variations in the rotation of the Earth with respect to the solar time.

Barycentric Coordinate Time

Barycentric Coordinate Time (TCB, from the French Temps-coordonnée barycentrique) 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 problematic 1976 definition of 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 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 1.550505 × 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 an ongoing 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 International Atomic Time (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.

Coordinated Universal Time

Coordinated Universal Time (abbreviated to UTC) is the primary time standard by which the world regulates clocks and time. It is within about 1 second of mean solar time at 0° longitude, and is not adjusted for daylight saving time. In some countries where English is spoken, the term Greenwich Mean Time (GMT) is often used as a synonym for UTC and predates UTC by nearly 300 years.The first Coordinated Universal Time was informally adopted on 1 January 1960 and was first officially adopted as CCIR Recommendation 374, Standard-Frequency and Time-Signal Emissions, in 1963, but the official abbreviation of UTC and the official English name of Coordinated Universal Time (along with the French equivalent) were not adopted until 1967.The system has been adjusted several times, including a brief period where time coordination radio signals broadcast both UTC and "Stepped Atomic Time (SAT)" before a new UTC was adopted in 1970 and implemented in 1972. This change also adopted leap seconds to simplify future adjustments. This CCIR Recommendation 460 "stated that (a) carrier frequencies and time intervals should be maintained constant and should correspond to the definition of the SI second; (b) step adjustments, when necessary, should be exactly 1 s to maintain approximate agreement with Universal Time (UT); and (c) standard signals should contain information on the difference between UTC and UT."A number of proposals have been made to replace UTC with a new system that would eliminate leap seconds. A decision whether to remove them altogether has been deferred until 2023.The current version of UTC is defined by International Telecommunications Union Recommendation (ITU-R TF.460-6), Standard-frequency and time-signal emissions, and is based on International Atomic Time (TAI) with leap seconds added at irregular intervals to compensate for the slowing of the Earth's rotation. Leap seconds are inserted as necessary to keep UTC within 0.9 seconds of the UT1 variant of universal time. See the "Current number of leap seconds" section for the number of leap seconds inserted to date.

Indian Standard Time

Indian Standard Time (IST) is the time observed throughout India, with a time offset of UTC+05:30. India does not observe daylight saving time (DST) or other seasonal adjustments. In military and aviation time IST is designated E* ("Echo-Star").Indian Standard Time is calculated on the basis of 82.5' E longitude, in Mirzapur (Amravati Chauraha), Uttar Pradesh, which is nearly on the corresponding longitude reference line.

International Earth Rotation and Reference Systems Service

The International Earth Rotation and Reference Systems Service (IERS), formerly the International Earth Rotation Service, is the body responsible for maintaining global time and reference frame standards, notably through its Earth Orientation Parameter (EOP) and International Celestial Reference System (ICRS) groups.

Leap second

A leap second is a one-second adjustment that is occasionally applied to civil time Coordinated Universal Time (UTC) to keep it close to the mean solar time at Greenwich, in spite of the Earth's rotation slowdown and irregularities. UTC was introduced on January 1, 1972, initially with a 10 second lag behind International Atomic Time (TAI). Since that date, 27 leap seconds have been inserted, the most recent on December 31, 2016 at 23:59:60 UTC, so in 2018, UTC lags behind TAI by an offset of 37 seconds.The UTC time standard, which is widely used for international timekeeping and as the reference for civil time in most countries, uses the international system (SI) definition of the second. The UTC second has been calibrated with atomic clock on the duration of the Earth's mean day of the astronomical year 1900. Because the rotation of the Earth has since further slowed down, the duration of today's mean solar day is longer (by roughly 0.001 seconds) than 24 SI hours (86,400 SI seconds). UTC would step ahead of solar time and need adjustment even if the Earth's rotation remained constant in the future. Therefore, if the UTC day were defined as precisely 86,400 SI seconds, the UTC time-of-day would slowly drift apart from that of solar-based standards, such as Greenwich Mean Time (GMT) and its successor UT1. The point on the Earth's equator where the sun culminates at 12:00:00 UTC would wander to the East by some 300 m each year. The leap second compensates for this drift, by occasionally scheduling a UTC day with 86,401 or (in principle) 86,399 SI seconds.

When it occurs, a positive leap second is inserted between second 23:59:59 of a chosen UTC calendar date and second 00:00:00 of the following date. The definition of UTC states that the last day of December and June are preferred, with the last day of March or September as second preference, and the last day of any other month as third preference. All leap seconds (as of 2017) have been scheduled for either June 30 or December 31. The extra second is displayed on UTC clocks as 23:59:60. On clocks that display local time tied to UTC, the leap second may be inserted at the end of some other hour (or half-hour or quarter-hour), depending on the local time zone. A negative leap second would suppress second 23:59:59 of the last day of a chosen month, so that second 23:59:58 of that date would be followed immediately by second 00:00:00 of the following date. Since the introduction of leap seconds, the mean solar day has outpaced UTC only for very brief periods, and has not triggered a negative leap second.

Because the Earth's rotation speed varies in response to climatic and geological events, UTC leap seconds are irregularly spaced and unpredictable. Insertion of each UTC leap second is usually decided about six months in advance by the International Earth Rotation and Reference Systems Service (IERS), when needed to ensure that the difference between the UTC and UT1 readings will never exceed 0.9 seconds.

Lilian date

A Lilian date is the number of days since the beginning of the Gregorian Calendar on October 15, 1582, regarded as Lilian date 1. It was invented by Bruce G. Ohms of IBM in 1986 and is named for Aloysius Lilius, who devised the Gregorian Calendar. Lilian dates can be used to calculate the number of days between any two dates occurring since the beginning of the Gregorian calendar. It is currently used by date conversion routines that are part of IBM Language Environment (LE) software.The Lilian date is only a date format: it is not tied to any particular time standard. Another, better known, date notation that is used for similar purposes is the Julian date, which is tied to Universal time (or some other closely related time scale, such as International Atomic Time). The Julian date always begins at noon, Universal time, and a decimal fraction may be used to represent the time of day. In contrast, Ohms did not make any mention of time zones or time of day in his paper.If the Lilian date was to be reckoned in Universal Time, and if the Lilian date is taken to begin at midnight, the Lilian date can be obtained from the Julian date by subtracting 2,299,159.5 from the Julian date, and ignoring the decimal fraction in the result.

Lod (disambiguation)

Lod is a city in Israel.

Lod, LOD and LoD may also refer to:

Law of Demeter, a design guideline for developing software

Legal Operations Detachment; see United States Army Reserve Legal Command

The Legend of Dragoon, a PlayStation role-playing game

Legacy of Darkness (disambiguation), multiple uses

Legion of Doom (disambiguation), multiple uses

Length on deck, a measurement of a ship over the deck from forward deck to the transom

Length of day

LOD, the length of a mean solar day in a uniform time scale such as International Atomic Time; see ΔT

Letter of demand (of payment), an instrument in debt collection

Level of detail, a computer graphics technique to adapt the detail of the displayed 3D object to the user needs

Limit of detection

Line of dance in Ballroom

Line of Duty, British police detective television series

Linked Open Data, using web technologies to link open data

Lod airport, a former name of Ben Gurion Airport, Lod, Israel

LOD score, logarithm of odds

Diablo II: Lord of Destruction, an official expansion to the computer game Diablo II

Loss on Drying

Lodi, Lombardy

Province of Lodi, in Italy

Last order date a milestone in Product Life Cycle

Lod (crater) a crater in the Oxia Palus quadrangle of Mars

L.O.D. (EP), an EP by the American rapper Desiigner

Metre Convention

The Metre Convention (French: Convention du Mètre), also known as the Treaty of the Metre, is an international treaty that was signed in Paris on 20 May 1875 by representatives of 17 nations (Argentina, Austria-Hungary, Belgium, Brazil, Denmark, France, Germany, Italy, Peru, Portugal, Russia, Spain, Sweden and Norway, Switzerland, Ottoman Empire, United States of America, and Venezuela). The treaty created the International Bureau of Weights and Measures (BIPM), an intergovernmental organization under the authority of the General Conference on Weights and Measures (CGPM) and the supervision of the International Committee for Weights and Measures (CIPM), that coordinates international metrology and the development of the metric system.

As well as founding the BIPM and laying down the way in which the activities of the BIPM should be financed and managed, the Metre Convention established a permanent organizational structure for member governments to act in common accord on all matters relating to units of measurement.

The three organs of the BIPM are:

The General Conference on Weights and Measures (Conférence générale des poids et mesures or CGPM) – the plenary organ of the BIPM which consists of the delegates of all the contracting Governments;

The International Committee for Weights and Measures (Comité international des poids et mesures or CIPM) – the direction and supervision body of the BIPM that is made of 18 prominent metrologists from 18 different Member States;

The International Bureau of Weights and Measures (Bureau international des poids et mesures or BIPM) – the headquarters of the BIPM located at Sèvres, France that has custody of the International Prototype Kilogram and houses the secretariat for this organization and hosts its formal meetings.Only States can be Members as per the Metre Convention. In addition to Member status, the General Conference on Weights and Measures (CGPM) created in 1999 the status of Associate of the CGPM open to States and Economic Entities to enable them to participate in some activities of the BIPM through their National Metrology Institutes (NMIs). Membership of the convention requires payment of substantial fees. Failure to pay these over a span of years, without any expectation of a payment agreement, has caused a number of nations such as North Korea to be removed from the protocol. As of 7 August 2018, there are 60 Member States and 42 Associate State and Economies.

Initially the Metre Convention was only concerned with the units of mass and length but, in 1921, at the 6th meeting of the General Conference on Weights and Measures (CGPM), it was revised and it extended the scope and responsibilities of the BIPM to other fields in physics. In 1960, at the 11th meetings of the CGPM, the system of units it had established was named the International System of Units, with the abbreviation SI.

OpenNTPD

OpenNTPD is a Unix daemon implementing the Network Time Protocol to synchronize the local clock of a computer system with remote NTP servers. It is also able to act as an NTP server to NTP-compatible clients.

OpenNTPD is primarily developed by Henning Brauer as part of the OpenBSD project. Its design goals include being secure (non-exploitable), easy to configure, and accurate enough for most purposes. Its portable version, like that of OpenSSH, is developed as a child project which adds the portability code to the OpenBSD version and releases it separately. The portable version is developed by Brent Cook. The most recent portable version was released in 2016. The project developers receive some funding from the OpenBSD Foundation.

TAI

TAI may refer to:

IATA airport code for Ta'izz International Airport

ICAO code for TACA International Airlines

Transports Aériens Intercontinentaux, a now defunct French airline

International Atomic Time (French: Temps atomique international)

Thai Airways International

The Academy Is..., a rock band

The Australia Institute, a leftwing think tank

Toshiba America, Inc.

Turkish Aerospace Industries

5-Trifluoromethyl-2-aminoindan, a psychoactive drug and serotonin releasing agent

Technical analysis indicator

Term (time)

A term is a period of duration, time or occurrence, in relation to an event. To differentiate an interval or duration, common phrases are used to distinguish the observance of length are near-term or short-term, medium-term or mid-term and long-term.

It is also used as part of a calendar year, especially one of the three parts of an academic term and working year in the United Kingdom: Michaelmas term, Hilary term / Lent term or Trinity term / Easter term, the equivalent to the American semester. In America there is a midterm election held in the middle of the four-year presidential term, there are also academic midterm exams.

In economics, it is the period required for economic agents to reallocate resources, and generally reestablish equilibrium. The actual length of this period, usually numbered in years or decades, varies widely depending on circumstantial context. During the long term, all factors are variable.

In finance or financial operations of borrowing and investing, what is considered long-term is usually above 3 years, with medium-term usually between 1 and 3 years and short-term usually under 1 year. It is also used in some countries to indicate a fixed term investment such as a term deposit.

In law, the term of a contract is the duration for which it is to remain in effect (not to be confused with the meaning of "term" that denotes any provision of a contract). A fixed-term contract is one concluded for a pre-defined time.

Terrestrial Time

Terrestrial Time (TT) is a modern astronomical time standard defined by the International Astronomical Union, primarily for time-measurements of astronomical observations made from the surface of Earth.

For example, the Astronomical Almanac uses TT for its tables of positions (ephemerides) of the Sun, Moon and planets as seen from Earth. In this role, TT continues Terrestrial Dynamical Time (TDT or TD), which in turn succeeded ephemeris time (ET). TT shares the original purpose for which ET was designed, to be free of the irregularities in the rotation of Earth.

The unit of TT is the SI second, the definition of which is currently based on the caesium atomic clock, but TT is not itself defined by atomic clocks. It is a theoretical ideal, and real clocks can only approximate it.

TT is distinct from the time scale often used as a basis for civil purposes, Coordinated Universal Time (UTC). TT indirectly underlies UTC, via International Atomic Time (TAI). Because of the historical difference between TAI and ET when TT was introduced, TT is approximately 32.184 s ahead of TAI.

Timation

The Timation satellites were conceived, developed, and launched by the Naval Research Laboratory in Washington, D.C. beginning in 1964. The concept of Timation was to broadcast an accurate time reference for use as a ranging signal to receivers on the ground. On 31 May 1967 the Timation-1 satellite was launched. This was followed by the Timation-2 satellite launch in 1969. The results of this program and Air Force Project 621B formed the basis for the Global Positioning System (GPS). The Navy's contribution to the GPS program continued to be focused on ever more accurate clocks.There is a historical connection between accurate time keeping, navigation, and the Navy. In 1714 the British government passed the Longitude Act (see longitude prize) to create an incentive to solve the problem of navigation at sea. The solution, developed by John Harrison, was an accurate clock which could compare local time to Greenwich, England time. To this day, Coordinated Universal Time (UTC), the successor of Greenwich Mean Time (GMT), is the reference time for the planet, and in the United States, the official time for the Department of Defense is kept by the United States Navy at the U.S. Naval Observatory in Washington, D.C.. This is kept in sync with the official civilian time reference maintained by NIST and contributes to the International Atomic Time.

Time and frequency transfer

Time and frequency transfer describes mechanisms for comparing measurements of time and frequency from one location to another. The technique is commonly used for creating and distributing standard time scales such as International Atomic Time (TAI).

Time standard

A time standard is a specification for measuring time: either the rate at which time passes; or points in time; or both. In modern times, several time specifications have been officially recognized as standards, where formerly they were matters of custom and practice. An example of a kind of time standard can be a time scale, specifying a method for measuring divisions of time. A standard for civil time can specify both time intervals and time-of-day.

Standardized time measurements are made using a clock to count periods of some period changes, which may be either the changes of a natural phenomenon or of an artificial machine.

Historically, time standards were often based on the Earth's rotational period. From the late 18 century to the 19th century it was assumed that the Earth's daily rotational rate was constant. Astronomical observations of several kinds, including eclipse records, studied in the 19th century, raised suspicions that the rate at which Earth rotates is gradually slowing and also shows small-scale irregularities, and this was confirmed in the early twentieth century. Time standards based on Earth rotation were replaced (or initially supplemented) for astronomical use from 1952 onwards by an ephemeris time standard based on the Earth's orbital period and in practice on the motion of the Moon. The invention in 1955 of the caesium atomic clock has led to the replacement of older and purely astronomical time standards, for most practical purposes, by newer time standards based wholly or partly on atomic time.

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.

Two-way satellite time and frequency transfer

Two-way satellite time and frequency transfer (TWSTFT) is a high-precision long distance time and frequency transfer mechanism used between time bureaux to determine and distribute time and frequency standards.

As of 2003 TWSTFT is being evaluated as an alternative to be used by the Bureau International des Poids et Mesures in the determination of International Atomic Time (TAI), as a complement to the current standard method of simultaneous observations of GPS transmissions.

Universal Time

Universal Time (UT) is a time standard based on Earth's rotation. It is a modern continuation of Greenwich Mean Time (GMT), i.e., the mean solar time on the Prime Meridian at Greenwich, England. In fact, the expression "Universal Time" is ambiguous (when accuracy of better than a few seconds is required), as there are several versions of it, the most commonly used being Coordinated Universal Time (UTC) and UT1 (see § Versions). All of these versions of UT, except for UTC, are based on Earth's rotation relative to distant celestial objects (stars and quasars), but with a scaling factor and other adjustments to make them closer to solar time. UTC is based on International Atomic Time, with leap seconds added to keep it within 0.9 second of UT1.

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