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The infrastructure of society depends critically on time and frequency services. Requirements for time and frequency exist with widely varying precision and accuracy. The applications include positioning and navigation services, time domain astronomy, intelligent transportation systems, communications, power grid, banking and finance, emergency services, water flow, science, religion, and general public needs.
Historically the second was considered a 60th of a minute, which was one 60th of an hour. A second based on the Earth’s variable rotation was impractical for modern timekeeping requirements. With the introduction of Ephemeris Time, the ephemeris second was defined in 1954 and revised in 1956 as 1/31 556 925.9747 of the length of the tropical year for 1900.0. The availability of atomic clocks made a more accurate and available second possible. So in 1968, the Système International (SI) second was defined as the duration of 9 192 631 770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the Caesium 133 atom.
With such an accurate measure of the second, the meter was defined as the length of path traveled by light in vacuum during a time interval of 1/299 792 458 of a second. Other SI units are defined in terms of the second and meter. With improved accuracies of timekeeping based on optical frequency standards, a redefinition of the second is under consideration.
Apparent solar time is the time given by sundials and varies by location and time of year. Mean solar time provides a more uniform version of solar time based on solar theories or tables. The difference between them is the equation of time. Sidereal time is the hour angle of the equinox of the celestial frame, so it measures the rotation of the Earth with respect to the stars. In 1884, the Greenwich meridian was established as the international prime meridian, and Greenwich Mean Time (GMT) was defined to be mean solar time measured from noon on the Greenwich meridian. In 1935, Universal Time was defined to be the mean solar time on the meridian of Greenwich reckoned from midnight.
UT1, the description of the rotation angle of the Earth, agrees with mean solar time within 0.2 seconds. The current international standard Coordinated Universal Time (UTC) is based on International Atomic Time (TAI) and kept within 0.9 second of UT1 by leap seconds. Time zones and Daylight Savings Time provide local times. In 2000, the concept of the Earth rotation angle (ERA) was used to develop a new definition of UT1.
Ephemerides, the plural of ephemeris, are tables of positions of moving celestial bodies. By entering tables for the Sun, Moon, or planets with a specific date and performing arithmetical operations, the location of the body for the date can be determined. The timescales for the tables are the independent variables and should be uniform in rate. There is a long history of tables and theories for the solar system bodies. The motion of the Moon is the most complicated, due to its rapid motion and closeness to the Earth. Punched card equipment made it possible to compute ephemerides from the tables by machine, and computers made numerical integration of ephemerides more rapid and accurate.
Historically, optical observations were used to produce ephemerides, but now radar, laser-ranging data, and spacecraft observations provide more accuracy. Reference systems have progressed from catalogs of nearby moving optical stars to the International Celestial Reference System (ICRS), which is based on observations of distant radio sources. Astronomical constants have been updated as observational accuracy improved and general relativistic concepts were developed.
As atomic clocks became available in laboratories and observatories around the world atomic timescales were developed. The Bureau International de l'Heure began an atomic time scale in July 1955 based on clock comparisons at other locations. The IAU recommended a unification of an atomic timescale in 1967, and in 1971, the Conférence Générale des Poids et Mesures (CGPM) requested the definition of International Atomic Time (TAI), and the methods and weighting for the determination of TAI have evolved over the years along with the organizations participating in the process. Coordinated Universal Time (UTC) differs from TAI by integral seconds, so that TAI can be accessed through the international standard UTC. TAI can be considered as a realization of Terrestrial Time (TT).
From earliest times, observed astronomical cycles were used to mark the passage of time. Calendars were established based on lunar and solar cycles, growing seasons, and religious holidays. Systems of timekeeping were developed based on the apparent motion of the Sun in the sky, but early astronomers were already able to note the variations of apparent solar time, and mean solar time was established around AD 150.
Timekeeping developed using sundials, water clocks, mechanical clocks, pendulum clocks, and chronometers. Conventions for the hour marking the beginning of the day evolved from noon to midnight and hours of different lengths evolved to those of uniform length. Time transfer techniques changed from bells, to time balls, and to the telegraph. Until the mid-20th century, the rotation of the Earth was the basis of timekeeping. At the beginning of the 20th century, official time was based on mean solar time from Newcomb’s Theory of the Sun and kept pendulum clocks set by astronomical observations. There was no international timescale, and the variation in the Earth's rotational speed was suspected, but not proven.
The solar system is composed of bodies whose positions are observed and analyzed in a four-dimensional relativistic reference system. A uniform and accurate timescale is a critical part of the description of solar system phenomena. Eclipses, occultations, transits, Sunrises and Sunsets, and Moonrises and Moonsets are observed and predicted based on solar system dynamics. The intervals between equinoxes and solstices have different periods, whose average is the length of the tropical year, which is the time for the Sun’s mean longitude to increase by 360 degrees. The accurate knowledge of the SI second led to the definition of the meter based on the speed of light and the second, thus providing the distance scale for the solar system. Radar and laser ranging systems use timing to measure precise distances. Global Navigation Satellite Systems (GNSS) use time signals to determine positions and distribute time. Space missions use the Doppler effect and measured time delays to determine satellite and planetary probe positions. Proper time on solar system objects can be modeled based on dynamics. Possible future use of pulsars and white dwarfs for independent timekeeping requires very accurate knowledge of the motion of the Earth with respect to distant objects.
Microwave atomic clocks introduced a new level of timekeeping accuracy. The Caesium atomic frequency standard became available in 1955, but it needed to be calibrated to provide a timescale matching Ephemeris Time. Commercial Caesium tubes made atomic clocks generally available. Caesium fountains have significantly improved the accuracy of atomic clocks. Hydrogen masers have been in use since the 1960s. Rubidium cell atomic clocks are readily available and Rubidium fountains promise to provide frequency standards with unprecedented stability. Stored ion and Mercury ion clocks are other sources of precise time and frequency. Laser-cooled microgravity atomic clocks are being developed for space missions.
The reality of the rotation of the Earth was not generally accepted until the 15th century. The connection between the secular acceleration of the Moon’s motion and the secular retardation of the Earth’s rotation rate was considered in the mid-19th century, but the values did not agree. Newcomb investigated the possibility of variation in the Earth’s rotation rate, but was unable to prove it. R. T. A. Innes, H. Spencer Jones, and W. de Sitter provided evidence of the variations in the 1920s based on observations of the Sun, Moon, Mercury, Venus, and Mars. Various explanations for the variations were proposed, but now low-frequency and higher-frequency variations in the Earth’s rotation are recognized.
Because the Earth's rotation was known to be variable, a uniform time was needed for ephemerides. This led to the introduction of Ephemeris Time and the use of more precise quartz crystal and atomic clocks. Also, observations of the Earth’s rotation led to the field of Earth orientation sciences, combining astronomy, geodesy, and geophysics.
The discovery of the variability of the Earth's rotation rate called for a uniform timescale for ephemerides and other scientific purposes. André Danjon and Gerald Clemence proposed a timescale based on the orbital motion of the Earth that was adopted in 1952, and called Ephemeris Time (ET). The working definition was based on a mathematical expression for the longitude of the Sun as a function of time. However, observations of the Sun are difficult and limited in accuracy, so observations of the Moon were used in practice to determine Ephemeris Time. This time depended on the analysis of astronomical observations. Consequently, it was not available immediately. Fortunately, atomic clocks started to become available in 1956, providing a timescale accurately representing ET. The availability of atomic timescales led to the introduction of International Atomic Time (TAI) and dynamical timescales.
In the 19th century, many local timescales and reference meridians were in practical use. The railroad companies pushed for time standardization and navigators wanted a standard meridian. In 1884, the Greenwich meridian was adopted as the international prime meridian for longitude and time zones. In 1928, Universal Time (UT) was recommended to replace Greenwich Mean Time (GMT) and the BIH was to coordinate transmission of radio time signals. In 1960, the United Kingdom and the United States began to coordinate time transmissions by making the same adjustments to their clocks at the same times. In 1972, the International Radio Consultative Committee (CCIR), a predecessor organization of the International Telecommunications Union, introduced a new definition of UTC, which is based on TAI with leap seconds in order to keep UTC within 0.9s of UT1. UTC is now distributed and used worldwide. Since 2000 there has been discussion of redefining UTC without leap seconds. A number of meetings, proceedings, and papers have been devoted to this possibility. Arguments for and against are presented.
Because of the many physical phenomena that affect the motion of a rotating Earth, e.g., weather systems, glacial isostatic adjustment, tectonic motion, etc., its kinematics are difficult to predict. So observations are necessary to complete the models describing the celestial and terrestrial reference systems and the transformations between them. This information is made available by the International Earth Rotation and Reference Systems Service (IERS). Very long baseline interferometry (VLBI) techniques provide estimates of celestial pole offsets, polar motion, and the Earth's rotation angle. GPS and satellite laser ranging observations can be analyzed for polar motion and length-of-day values. DORIS (Doppler Orbitography and Radiopositioning Integrated by Satellite) observations contribute to the terrestrial reference frame. Meteorological, oceanographic, and geophysical models are used to estimate variations in the angular momentum of the atmosphere and oceans, the core mantle boundary topography, and seasonal station displacements. Jerks of the Earth’s magnetic field seem to have correlations with all Earth orientation parameters.
In the 19th century, time balls were used to distribute accurate time in harbors and cities, and the telegraph was used for distribution over distances. In the 20th century, radio signals became the primary means of time and frequency distribution. One-way and/or two-way methods are used in time and frequency transfer of many types, but in each case, propagation and relativistic effects must be determined and calibrations made. The effects of making measurements in either a rotating or nonrotating reference frame must be determined. Modern methods of time and frequency dissemination include coaxial cable, telephone, optical fibers, microwave links, television broadcasts, internet, high- and low-frequency radio signals, navigation signals, navigation satellite system broadcast signals, navigation satellite carrier phase, two-way satellite radio signals, optical two-way transfer, and Atomic Clock Ensemble in Space (ACES).
The growing demand for improvements in the precision and accuracy of time and frequency will drive the developments in the field for years to come. Anticipated future requirements include improved transportation methods, the ability to navigate within buildings, precise spectrum allocation, and scientific applications. Improvements in Earth orientation modeling will depend on improved observations and understanding of the physics of the Earth, Real-time estimates of the Earth's rotation angle may permit more reliable predictions.
The future of clocks could include chip-scale atomic clocks, optical clocks, and alkali atom fountains. A pulsar-based timescale will be an independent source of time able to identify possible systematic variations in TAI. An ensemble white dwarf timescale may be possible, if additional white dwarfs with accurate rotation rates are found. It is likely that improvements in clock technology will result in the redefinition of the second and a more precise timescale. Dynamical timescales based on higher orders of relativity and post-post-Newtonian parameters could be expected, as well as definitions of proper times for planets to satisfy planetary mission requirements.
Correspondingly accurate time and frequency distribution methods will be required, including optical fibers, quantum entanglement, and a distributed clock.