History of UTC

The history of UTC can be traced back to 1847 with the first development of British Grenwich Mean Time (GMT). In 1884 at the International Meridian Conference in Washington D.C., GMT was first adopted as an international standard. GMT was based purely on astronomical observations; however, due to the steady slow-down of the Earth's revolutions due to tidal frictions, and also due to variations in the many competing gravitational pulls over time within the solar system, the Earth's solar year was found to vary slightly from year to year, thus creating the need for greater accuracy than could be found via purely astronomical calculations of the solar-year.

Nearly a century later in 1967, soon after the invention of the atomic clock, a new and more highly refined time standard was agreed upon based on a combination of the newly discovered and quite invariable frequency of cesium 133 vibration, combined with the length of the slightly varying solar year, thus creating UTC. The new UTC time standard has since replaced GMT as the most consistent, accurate, and easily derived internationally agreed upon method for the determination of Earth-time.

British development of GMT

With the advent of the industrial revolution, a greater understanding and agreement on the nature of time itself became increasingly necessary and helpful. In 1847 in Britain, Greenwich Mean Time (GMT) was first created for use by the British railways, navy, and shipping industries. Using telescopes, GMT was calibrated to the mean solar time at the Royal Observatory, Greenwich in the UK.

The International Meridian Conference

As international commerce continued to increase throughout Europe, in order to achieve a more efficiently functioning modern society, an agreed upon, and highly accurate international standard of time measurement became necessary. In order to find or determine such a time-standard, three steps had to be followed:

1. An internationally agreed upon time-standard had to be defined.
2. This new time-standard then had to be consistently and accurately measured.
3. The new time-standard then had to be freely shared and distributed around the world.

The development of what is now known as UTC time first came about historically as an agreement between 41 nations, signed at the 1884 International Meridian Conference held in Washington, D.C. At this conference, the local mean solar time at the Royal Observatory, Greenwich in England was chosen to define the Universal day, counted from 0 hours at mean midnight. This agreed with civil Greenwich Mean Time (GMT), used on the island of Great Britain since 1847. In contrast, astronomical GMT began at mean noon, 12 hours after mean midnight of the same date until 1 January 1925, whereas nautical GMT began at mean noon, 12 hours before mean midnight of the same date, at least until 1805 in the Royal Navy, but persisted much later elsewhere because it was mentioned at the 1884 conference. In 1884, the Greenwich Meridian was used for two-thirds of all charts and maps as their Prime Meridian.

GMT redefined as UT

In 1928 the modern day descendant of GMT (though slightly less accurate than UTC) was defined by the International Astronomical Union as Universal Time (UT). Even to the present day, UT is still a purely "astronomical time" being based strictly on reports from a certain network of international telescopes and observers. Observations at the Greenwich Observatory itself ceased in 1954, though the location is still used as the basis for the coordinate system. Because the rotational period of Earth is not perfectly constant, the duration of a second would vary if calibrated to a telescope-based standard like GMT or UT—in which a second was defined as a fraction of a day or year. Until the 1950s, broadcast time signals were based on UT, and hence on the rotation of the Earth. The terms "GMT" and "Greenwich Mean Time" are sometimes used correctly and interchangeably with the term: UT. However, the terms GMT and UTC are not the same thing, and accordingly are not properly interchangeable terms.

The ephemeris-second

For the better part of the first century following the "International Meridian Conference," until 1960, the methods and definitions of time-keeping that had been laid out at the conference proved to be adequate to meet time tracking needs of society. Still, with the advent of the "electronic revolution" in the latter half of the 20th century, the technologies that had been available at the time of the International Meridian Conference proved to be in need of further refinement in order to meet the needs of the ever increasing precision that the "electronic revolution" had begun to require. Therefore in 1960, due to irregularities that had been found in the length of a solar year over time, it was agreed upon that the solar year of 1900 would thenceforth serve as the "reference year" for all future computations and definitions of the exact length of a standardized one-year time interval, and by inference, of a standardized one second time interval as well. This new definition of a second-of-time, based on the solar year that fell in the calendar year of 1900, came to be known as the ephemeris-second.

The si-second

Once a more exact and measurable definition of a second-of-time had been agreed upon, known as the ephemeris-second, in 1967 at the 13th General Conference on Weights and Measures, the new and more easily measured technology of the atomic clock resulted in the agreed upon definition of the si-second, now based directly on the atomic clock equivalent of the ephemeris-second. The si-second (Standard Internationale second) has stood since 1967 as the internationally recognized fundamental building-block to be used for the computation and measurement of time, and is based directly on the measurement of the atomic-clock observation of the frequency oscillation of cesium atoms.

Contemporary implimentations of UTC

International distribution channels of UTC

Some of the main distribution channels for UTC include:

• Various national radio timing signals such as the US Bureau of Standards signal broadcast from Ft. Collins, Colorado, USA.
• GPS satellite broadcasts available globally and based directly on UTC.
• Cell phone network timing signals which are typically synchronized with the phones connected to the various cell phone networks, and which are also most typically synchronized with UTC.
• Internet broadcasts of atomic clock signals, which can be found on the websites of some UTC timing centers.

Accuracy of various modern timing devices

The most accurate modern timing devices are cesium/ atomic clocks, which if properly maintained, are generally believed to be accurate to within 1 second per 1.4 million years or better. Next come GPS satellites. GPS satellites both "inherit" their accuracy from those certain groups of cesium/ atomic clocks stationed on Earth with which they are synchronized, and between synchronizations, rely on their own onboard atomic clocks.

The belief that cell phone times derive from GPS signals is a common misconception, as these devices most typically derive their times from, and synchronize with, the time signals of the cell phone provider networks to which they are attached. Most cell phone provider networks in turn synchronize directly with national UTC atomic clock services.

Amongst watches, the most accurate watches are those which regularly synchronize with national UTC atomic clock services. By synchronizing with these services, such watches also naturally "inherit" the same level of accuracy as the services with which they synchronize. Non-synchronizing watches vary from 10 seconds deviation per year, to several minutes of deviation per year.

Mechanics of atomic clocks

It is a common misconception that atomic clocks measure nuclear decay rates. Much to the contrary, atomic clocks have nothing to do with nuclear energy, but instead concern the physics of the cesium atom, and merely measure a certain normal vibrational frequency of Cesium-133, a naturally occurring and non-radioactive metal.

See also

• UTC
• Time
• List of UTC timing centers

References

This article "History of UTC" is from Wikipedia. The list of its authors can be seen in its historical. Articles copied from Draft Namespace on Wikipedia could be seen on the Draft Namespace of Wikipedia and not main one.

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