Universal Time
Encyclopedia
Universal Time is a time scale
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 Prime Meridian
at Greenwich
, and GMT is sometimes used loosely as a synonym for UTC. In fact, the expression "Universal Time" is ambiguous, as there are several versions of it, the most commonly used being UTC and UT1 (see below). All of these versions of UT are based on the rotation of the earth in relation to distant celestial objects (star
s and quasar
s), but with a scaling factor and other adjustments to make them closer to solar time
.
, each municipality around the civilized world set its official clock, if it had one, according to the local position of the Sun (see solar time
). This served adequately until the introduction of the steam engine
, the telegraph, and rail travel, which made it possible to travel fast enough over long distances to require almost constant re-setting of timepiece
s as a train
progressed in its daily run through several towns. Standard time, where all clocks in a large region are set to the same time, was established to solve this problem. Chronometer
s or telegraphy
were used to synchronize these clocks.
Standard time, as originally proposed by Sir Sandford Fleming
in 1879, divided the world into twenty-four time zone
s, each one covering 15 degrees of longitude. All clocks within each zone would be set to the same time as the others, but differed by one hour from those in the neighboring zones. The local time at the Royal Greenwich Observatory in Greenwich, England was chosen as standard at the 1884 International Meridian Conference
, leading to the widespread use of Greenwich Mean Time to set local clocks. This location was chosen because by 1884 two-thirds of all chart
s and map
s already used it as their prime meridian
. The conference did not adopt Fleming's time zones because they were outside the purpose for which it was called, which was to choose a prime meridian. Nevertheless, by 1929 all major countries had adopted standard time zones. Political considerations have now increased the number of standard time zones to 40.
In 1928 the term Universal Time was adopted internationally as a more precise term than Greenwich Mean Time
, because 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 time
keeping.
s as they crossed a meridian rather than by observing the position of the Sun
in the sky. Nowadays, UT in relation to International Atomic Time
(TAI) is determined by Very Long Baseline Interferometry
(VLBI) observations of distant quasar
s, a method which has an accuracy of microseconds. 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, and is very gradually slowing due to tidal acceleration
. Furthermore, the length of the second was determined from observations of the Moon
between 1750 and 1890. All of these factors cause the mean solar day, on the average, to be slightly longer than the nominal 86,400 SI
seconds, the traditional number of seconds per day. 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 a leap second
) to atomic time, in order to obtain a broadcast form of civil time
that carries atomic frequency. Thus, civil broadcast standards for time and frequency usually follow TAI closely, but occasionally change discontinuously (or "leap") in order to prevent them from drifting too far from mean solar time.
Barycentric Dynamical Time
(TDB), a form of atomic time, is now used in the construction of the ephemerides of the planet
s and other solar system objects, for two main reasons. First, 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. Next, the time scales based on Earth's rotation are not uniform and therefore are not suitable for predicting the motion of bodies in our solar system.
Time scale
A time scale specifies divisions of time.*Time standard, a specification of either the rate at which time passes, or points in time, or both*Duration , a quantity of time...
based on the rotation of the Earth. It is a modern continuation of Greenwich Mean Time
Greenwich Mean Time
Greenwich Mean Time is a term originally referring to mean solar time at the Royal Observatory in Greenwich, London. It is arguably the same as Coordinated Universal Time and when this is viewed as a time zone the name Greenwich Mean Time is especially used by bodies connected with the United...
(GMT), i.e., the mean solar time on the Prime Meridian
Prime Meridian
The Prime Meridian is the meridian at which the longitude is defined to be 0°.The Prime Meridian and its opposite the 180th meridian , which the International Date Line generally follows, form a great circle that divides the Earth into the Eastern and Western Hemispheres.An international...
at Greenwich
Royal Observatory, Greenwich
The Royal Observatory, Greenwich , in London, England played a major role in the history of astronomy and navigation, and is best known as the location of the prime meridian...
, and GMT is sometimes used loosely as a synonym for UTC. In fact, the expression "Universal Time" is ambiguous, as there are several versions of it, the most commonly used being UTC and UT1 (see below). All of these versions of UT are based on the rotation of the earth in relation to distant celestial objects (star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
s and quasar
Quasar
A quasi-stellar radio source is a very energetic and distant active galactic nucleus. Quasars are extremely luminous and were first identified as being high redshift sources of electromagnetic energy, including radio waves and visible light, that were point-like, similar to stars, rather than...
s), but with a scaling factor and other adjustments to make them closer to solar time
Solar time
Solar time is a reckoning of the passage of time based on the Sun's position in the sky. The fundamental unit of solar time is the day. Two types of solar time are apparent solar time and mean solar time .-Introduction:...
.
Universal Time and standard time
Prior to the introduction of standard timeStandard 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. Historically, this helped in the process of weather forecasting and train travel. The concept...
, each municipality around the civilized world set its official clock, if it had one, according to the local position of the Sun (see solar time
Solar time
Solar time is a reckoning of the passage of time based on the Sun's position in the sky. The fundamental unit of solar time is the day. Two types of solar time are apparent solar time and mean solar time .-Introduction:...
). This served adequately until the introduction of the steam engine
Steam engine
A steam engine is a heat engine that performs mechanical work using steam as its working fluid.Steam engines are external combustion engines, where the working fluid is separate from the combustion products. Non-combustion heat sources such as solar power, nuclear power or geothermal energy may be...
, the telegraph, and rail travel, which made it possible to travel fast enough over long distances to require almost constant re-setting of timepiece
Clock
A clock is an instrument used to indicate, keep, and co-ordinate time. The word clock is derived ultimately from the Celtic words clagan and clocca meaning "bell". A silent instrument missing such a mechanism has traditionally been known as a timepiece...
s as a train
Train
A train is a connected series of vehicles for rail transport that move along a track to transport cargo or passengers from one place to another place. The track usually consists of two rails, but might also be a monorail or maglev guideway.Propulsion for the train is provided by a separate...
progressed in its daily run through several towns. Standard time, where all clocks in a large region are set to the same time, was established to solve this problem. Chronometer
Marine chronometer
A marine chronometer is a clock that is precise and accurate enough to be used as a portable time standard; it can therefore be used to determine longitude by means of celestial navigation...
s or telegraphy
Telegraphy
Telegraphy is the long-distance transmission of messages via some form of signalling technology. Telegraphy requires messages to be converted to a code which is known to both sender and receiver...
were used to synchronize these clocks.
Standard time, as originally proposed by Sir Sandford Fleming
Sandford Fleming
Sir Sandford Fleming, was a Scottish-born Canadian engineer and inventor, proposed worldwide standard time zones, designed Canada's first postage stamp, a huge body of surveying and map making, engineering much of the Intercolonial Railway and the Canadian Pacific Railway, and was a founding...
in 1879, divided the world into twenty-four time zone
Time zone
A time zone is a region on Earth that has a uniform standard time for legal, commercial, and social purposes. In order for the same clock time to always correspond to the same portion of the day as the Earth rotates , different places on the Earth need to have different clock times...
s, each one covering 15 degrees of longitude. All clocks within each zone would be set to the same time as the others, but differed by one hour from those in the neighboring zones. The local time at the Royal Greenwich Observatory in Greenwich, England was chosen as standard at the 1884 International Meridian Conference
International Meridian Conference
The International Meridian Conference was a conference held in October 1884 in Washington, D.C., in the United States to determine the Prime Meridian of the world. The conference was held at the request of U.S. President Chester A...
, leading to the widespread use of Greenwich Mean Time to set local clocks. This location was chosen because by 1884 two-thirds of all chart
Nautical chart
A nautical chart is a graphic representation of a maritime area and adjacent coastal regions. Depending on the scale of the chart, it may show depths of water and heights of land , natural features of the seabed, details of the coastline, navigational hazards, locations of natural and man-made aids...
s and map
Map
A map is a visual representation of an area—a symbolic depiction highlighting relationships between elements of that space such as objects, regions, and themes....
s already used it as their prime meridian
Prime Meridian
The Prime Meridian is the meridian at which the longitude is defined to be 0°.The Prime Meridian and its opposite the 180th meridian , which the International Date Line generally follows, form a great circle that divides the Earth into the Eastern and Western Hemispheres.An international...
. The conference did not adopt Fleming's time zones because they were outside the purpose for which it was called, which was to choose a prime meridian. Nevertheless, by 1929 all major countries had adopted standard time zones. Political considerations have now increased the number of standard time zones to 40.
In 1928 the term Universal Time was adopted internationally as a more precise term than Greenwich Mean Time
Greenwich Mean Time
Greenwich Mean Time is a term originally referring to mean solar time at the Royal Observatory in Greenwich, London. It is arguably the same as Coordinated Universal Time and when this is viewed as a time zone the name Greenwich Mean Time is especially used by bodies connected with the United...
, because 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 time
Civil time
In modern usage, civil time refers to statutory time scales designated by civilian authorities, or to local time indicated by clocks. Modern civil time is generally standard time in a time zone at a fixed offset from Coordinated Universal Time or from Greenwich Mean Time , possibly adjusted by...
keeping.
Measurement
Time can be measured based on the rotation of the Earth by observing celestial bodies crossing the meridian every day. Astronomers found that it was more accurate to establish time by observing starStar
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
s as they crossed a meridian rather than by observing the position of the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
in the sky. Nowadays, UT in relation to International Atomic Time
International Atomic Time
International Atomic Time is a high-precision atomic coordinate time standard based on the notional passage of proper time on Earth's geoid...
(TAI) is determined by Very Long Baseline Interferometry
Very Long Baseline Interferometry
Very Long Baseline Interferometry is a type of astronomical interferometry used in radio astronomy. It allows observations of an object that are made simultaneously by many telescopes to be combined, emulating a telescope with a size equal to the maximum separation between the telescopes.Data...
(VLBI) observations of distant quasar
Quasar
A quasi-stellar radio source is a very energetic and distant active galactic nucleus. Quasars are extremely luminous and were first identified as being high redshift sources of electromagnetic energy, including radio waves and visible light, that were point-like, similar to stars, rather than...
s, a method which has an accuracy of microseconds. 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
International Earth Rotation and Reference Systems Service
The International Earth Rotation and Reference Systems Service , formerly the International Earth Rotation Service, is the body responsible for maintaining global time and reference frame standards, notably through its Earth Orientation Parameter and International Celestial Reference System ...
(IERS). The International Astronomical Union
International Astronomical Union
The International Astronomical Union IAU is a collection of professional astronomers, at the Ph.D. level and beyond, active in professional research and education in astronomy...
is also involved in setting standards, but the final arbiter of broadcast standards is the International Telecommunication Union
International Telecommunication Union
The International Telecommunication Union is the specialized agency of the United Nations which is responsible for information and communication technologies...
or ITU.
The rotation of the Earth is somewhat irregular, and is very gradually slowing due to tidal acceleration
Tidal acceleration
Tidal acceleration is an effect of the tidal forces between an orbiting natural satellite , and the primary planet that it orbits . The "acceleration" is usually negative, as it causes a gradual slowing and recession of a satellite in a prograde orbit away from the primary, and a corresponding...
. Furthermore, the length of the second was determined from observations of the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
between 1750 and 1890. All of these factors cause the mean solar day, on the average, to be slightly longer than the nominal 86,400 SI
Si
Si, si, or SI may refer to :- Measurement, mathematics and science :* International System of Units , the modern international standard version of the metric system...
seconds, the traditional number of seconds per day. As UT is slightly irregular in its rate, astronomers introduced Ephemeris Time
Ephemeris time
The term ephemeris time can in principle refer to time in connection with any astronomical ephemeris. In practice it has been used more specifically to refer to:...
, which has since been replaced by Terrestrial Time
Terrestrial Time
Terrestrial Time is a modern astronomical time standard defined by the International Astronomical Union, primarily for time-measurements of astronomical observations made from the surface of the Earth....
(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 a leap second
Leap second
A leap second is a positive or negative one-second adjustment to the Coordinated Universal Time time scale that keeps it close to mean solar time. UTC, which is used as the basis for official time-of-day radio broadcasts for civil time, is maintained using extremely precise atomic clocks...
) to atomic time, in order to obtain a broadcast form of civil time
Civil time
In modern usage, civil time refers to statutory time scales designated by civilian authorities, or to local time indicated by clocks. Modern civil time is generally standard time in a time zone at a fixed offset from Coordinated Universal Time or from Greenwich Mean Time , possibly adjusted by...
that carries atomic frequency. Thus, civil broadcast standards for time and frequency usually follow TAI closely, but occasionally change discontinuously (or "leap") in order to prevent them from drifting too far from mean solar time.
Barycentric Dynamical Time
Barycentric Dynamical Time
Barycentric Dynamical Time is a relativistic coordinate time scale, intended for astronomical use as a time standard to take account of time dilation when calculating orbits and astronomical ephemerides of planets, asteroids, comets and interplanetary spacecraft in the Solar system...
(TDB), a form of atomic time, is now used in the construction of the ephemerides of the planet
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...
s and other solar system objects, for two main reasons. First, these ephemerides are tied to optical and radar
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...
observations of planetary motion, and the TDB time scale is fitted so that Newton's laws of motion
Newton's laws of motion
Newton's laws of motion are three physical laws that form the basis for classical mechanics. They describe the relationship between the forces acting on a body and its motion due to those forces...
, with corrections for general relativity
General relativity
General relativity or the general theory of relativity is the geometric theory of gravitation published by Albert Einstein in 1916. It is the current description of gravitation in modern physics...
, are followed. Next, the time scales based on Earth's rotation are not uniform and therefore are not suitable for predicting the motion of bodies in our solar system.
Versions
There are several versions of Universal Time:- 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 motionPolar motionPolar motion of the earth is the movement of Earth's rotational axis across its surface. This is measured with respect to a reference frame in which the solid Earth is fixed...
, 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 the principal form of Universal Time. While conceptually it is mean solar time at 0° longitude, precise measurements of the Sun are difficult. Hence, it is computed from observations of distant quasarQuasarA quasi-stellar radio source is a very energetic and distant active galactic nucleus. Quasars are extremely luminous and were first identified as being high redshift sources of electromagnetic energy, including radio waves and visible light, that were point-like, similar to stars, rather than...
s using long baseline interferometry, laser ranging of the MoonMoonThe Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
and artificial satellites, as well as the determination of GPSGlobal Positioning SystemThe Global Positioning System is a space-based global navigation satellite system that provides location and time information in all weather, anywhere on or near the Earth, where there is an unobstructed line of sight to four or more GPS satellites...
satellite orbits. UT1 is the same everywhere on Earth, and is proportional to the rotation angle of the Earth with respect to distant quasars, specifically, the International Celestial Reference FrameInternational Celestial Reference FrameThe International Celestial Reference Frame is a quasi-inertial reference frame centered at the barycenter of the Solar System, defined by the measured positions of 212 extragalactic sources . Although relativity implies that there is no true inertial frame, the extragalactic sources used to...
(ICRF), neglecting some small adjustments. The observations allow the determination of a measure of the Earth's angle with respect to the ICRF, called the Earth Rotation Angle (ERA, which serves as a modern replacement for Greenwich Mean Sidereal Time). UT1 is required to follow the relationship
-
- ERA = 2π(0.7790572732640 + 1.00273781191135448Tu) radians
-
- where Tu = (Julian UT1 date - 2451545.0)
- 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:
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 tectonicsPlate tectonicsPlate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...
, and currents in the interior of the Earth. - UTCCoordinated Universal TimeCoordinated Universal Time is the primary time standard by which the world regulates clocks and time. It is one of several closely related successors to Greenwich Mean Time. Computer servers, online services and other entities that rely on having a universally accepted time use UTC for that purpose...
(Coordinated Universal Time) is an atomic timescale that approximates UT1. It is the international standard on which civil time is based. It ticks SISiSi, si, or SI may refer to :- Measurement, mathematics and science :* International System of Units , the modern international standard version of the metric system...
seconds, in step with TAIInternational Atomic TimeInternational Atomic Time is a high-precision atomic coordinate time standard based on the notional passage of proper time on Earth's geoid...
. It usually has 86,400 SI seconds per day, but is kept within 0.9 seconds of UT1 by the introduction of occasional intercalary leap secondLeap secondA leap second is a positive or negative one-second adjustment to the Coordinated Universal Time time scale that keeps it close to mean solar time. UTC, which is used as the basis for official time-of-day radio broadcasts for civil time, is maintained using extremely precise atomic clocks...
s. 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 DUT1DUT1The time correction DUT1 is the difference between Universal Time , which is defined by Earth's rotation, and Coordinated Universal Time , which is defined by a network of precision clocks....
.
- where Tu = (Julian UT1 date - 2451545.0)
External links
- Time Lord by Clark Blaise: a biography of Stanford Fleming and the idea of standard time