Time

Atomic Clocks

The oldest clock is the Earth. We know its morning when the Sun rises, noon when the Sun is overhead, and evening when the Sun sets. The Earths accuracy as a clock is about one thousandth of a second per day. Man-made atomic clocks, by measuring the resonant frequency of a given atom (currently Cesium, Hydrogen or Mercury), are accurate to more than a billionth of a second per day.

Time measured by the rotation of the Earth is not uniform when compared to the time kept by atomic clocks. Such irregularities in the earths rotation are determined by scientists using radio telescopes to observe quasars, the most distant objects in the universe.

In 1972, by international agreement, it was decided to let atomic clocks run independently of the Earth and then coordinate the two. To keep the difference between Earth time and atomic time within nine tenths of a second, as the two times get out of sync, leap seconds are added to the atomic time scale.

The International Earth Rotation Service is the organization that monitors the difference between the two time scales and calls for leap seconds to be inserted when necessary. Leap seconds are added because the Earths rotation tends to slow down. If the Earth were to speed up, a leap second would be removed.

The U. S. Naval Observatory provides the Master Clock for the Department of Defense and the entire nation. Modern electronic systems, such as electronic navigation or communication systems depend increasingly on precise time and time interval (PTTI). Examples are the ground-based LORAN-C navigation system and the satellite based Global Positioning System (GPS).

These systems are all based on the travel time of the electromagnetic signals: an accuracy of 10 nanoseconds (ten one-billionths of a second) corresponding to a positional accuracy of about 10 feet. In fast communications, time synchronization is equally important. All of these systems are referenced to the U. S. Naval Observatory Master Clock.

The present Master Clock is based on a system of 59 atomic clocks: 10 hydrogen masers and 49 HP-5071 cesiums. These clocks are distributed over 12 environmentally controlled clock vaults, to insure their stability.

The U. S. Naval Observatory is the largest single contributor to the international time scale (UTC), which is computed in Paris, France, at the International Bureau of Weights and Measures.

Events, like astronomical and weather phenomena, are often given in Universal Time (UT), sometimes colloquially referred to as Greenwich Mean Time (GMT). In 1884, under international agreement, the prime meridian was established as running through the Royal Observatory in Greenwich, England, setting the standard of Greenwich Mean Time (GMT).

The two termsUT and GMTare often used loosely to refer to time kept on the Greenwich meridian (longitude zero), five hours ahead of Eastern Standard Time. In keeping with tradition, the start of a solar day occurred at noon. In 1925 the numbering system for GMT was changed so that the day began at midnight to make it consistent with the civil day. Some confusion in terminology resulted, however, and in 1928 the International Astronomical Union (IAU) changed the designation of the standard time of the prime meridian to universal time. Greenwich Mean Time is a widely used historical term, but one that has been used in several ways. Because of ambiguity, it is no longer used in technical contexts.

Times given in UT are almost always given in terms of a 24-hour clock. Thus, 14:42 (often written simply 1442) is 2:42 p.m., and 21:17 (2117) is 9:17 p.m. Sometimes a Z is appended to a time to indicate UT, as in 0935Z.

In 1955 the IAU defined several kinds of UT. The initial values of universal time obtained at 75 observatories, denoted UT0, differ slightly because of polar motion. By adding a correction each observatory converts UT0 into UT1, which gives the Earths rotational position in space. An empirical correction to take account of annual changes in the speed of rotation is then added to convert UT1 to UT2. However, UT2 has since been superseded by atomic time Universal time is also called world time, Z time, and Zulu time.

In the most common civil usage, UT refers to a time scale called Coordinated Universal Time (UTC). UTC is the basis for the worldwide system of civil time and is determined by atomic clocks. The International Bureau of Weights and Measures makes use of data from the clocks to provide the international standard UTCaccurate to nearly one nanosecond (billionth of a second) per day. The length of a UTC second is defined in terms of an atomic transition of the element cesium under specific conditions, and is not directly related to any astronomical phenomena.

UTC is the time distributed by standard radio stations that broadcast time, such as WWV and WWVH. It is also obtained from the Global Positioning System (GPS) satellites. The difference between UTC and UT1 is made available electronically and broadcast so that navigators can obtain UT1.

Standard time within U.S. time zones is established by a certain number of hours offset from UTC. Since a day is 24 hours long, the world may be split into 15-degree wide longitudinal bands (360 degrees/24 hours). Each band represents one hour. As an example, Huntsville, Alabama is located at approximately 90 degrees west longitude; hence, local time lags UTC time by 6 hours (90/15, assuming Central Standard Time, 5 hours in Central Daylight Time). So, if the universal time is 14:30 UTC, United States Central Standard Time would be 8:30 am CST.

What Is An Atomic Clock?
An atomic clock is an electronic timekeeping device controlled by atomic or molecular oscillations. A timekeeping device must contain or be connected to some apparatus that oscillates at a uniform rate to control the rate of movement of its hands or the rate of change of its digits. Mechanical clocks and watches use oscillating balance wheels, pendulums, and tuning forks.

Because the frequency of such oscillations is so high, it is not possible to use them as a direct means of controlling a clock. Instead, a highly stable crystal oscillator whose output is automatically multiplied and compared with the frequency of the atomic system controls the clock. Errors in the oscillator frequency are then automatically corrected. Time is usually displayed by an atomic clock with digital or other sophisticated readout devices.

The first atomic clock, invented in 1948, utilized the vibrations of ammonia molecules. The error rate was typically about one second in three thousand years. In 1955 the first cesium-beam clock was placed in operation at the National Physical Laboratory at Teddington, England. The cesium-beam clock is the most accurate standard of atomic time currently in use; it is estimated that such a clock would gain or lose less than a second in three million years.

Many of the worlds nations maintain cesium clocks at standards laboratories, the time kept by these clocks being averaged to produce a standard called international atomic time (TAI).

Highly accurate time signals from these standards laboratories are broadcast around the globe by shortwave-radio broadcast stations or by artificial satellites, the signals being used for such things as tracking space vehicles, electronic navigation systems, and studying the motions of the earths crust.

As of January, 2002, NIST's latest primary cesium standard was capable of keeping time to about 30 billionths of a second per year. Called NIST-F1, it is the 8th of a series of cesium clocks built by NIST and NIST's first to operate on the "fountain" principle.

Prototypes of atomic clocks using other kinds of atoms, such as hydrogen or beryllium, could be thousands of times more accurate even than todays cesium clocks. Current atomic clocks using the hydrogen atom maser can attain an error rate of about one part in 2 quadrillion. Such an extremely low error rate allowed their use in an experiment confirming an important prediction of Einsteins theory of relativity.

National Institute of Standards and Technology, the U.S. Naval Observatory, and the International Bureau of Weights and Measures in Paris assist the world in maintaining a single, uniform time system.

NIST Time and Frequency Services
NOTE: The following is reprinted from the NIST Time and Frequency Division Web site at: http://tf.nist.gov

Since 1923, NIST radio station WWV has provided round-the-clock shortwave broadcasts of time and frequency signals. WWV's audio signal is also offered by telephone: dial (303) 499-7111 (not toll-free). A sister station, WWVH, was established in 1948 in Hawaii, and its signal can be heard by dialing (808) 335-4363 in Hawaii.

Broadcast frequencies are 2.5 MHz (megahertz), 5 MHz, 10 MHz, and 15 MHz for both stations, plus 20 MHz on WWV. The signal includes UTC time in both voice and coded form; standard carrier frequencies, time intervals and audio tones; information about Atlantic or Pacific storms; geophysical alert data related to radio propagation conditions; and other public service announcements. Accuracies of one millisecond (one thousandth of a second) can be obtained from these broadcasts if one corrects for the distance from the stations (near Ft. Collins, Colorado, and Kauai, Hawaii) to the receiver. The telephone services provide time signals accurate to 30 milliseconds or better, which is the maximum delay in cross-country telephone lines.

In 1956, low-frequency station WWVB, which offers greater accuracy than WWV or WWVH, began broadcasting at 60 kilohertz. The broadcast power for WWVB was increased in 1999 from about 10 kilowatts to 50 kilowatts, providing much improved signal strength and coverage to most of the North American continent. This has stimulated commercial development of a wide range of inexpensive radio-controlled clocks and watches for general consumer use.

Time signals are an important byproduct of the Global Positioning System (GPS), and indeed this has become the premier satellite source for time signals. The time scale operated by the USNO serves as reference for GPS, but it is important to note that the time scales of NIST and USNO are highly coordinated (that is, synchronized to well within 100 nanoseconds, or 100 billionths of a second). Thus, signals provided by either NIST or USNO can be considered as traceable to both institutions. The agreements and coordination of time between these two institutions are important to the country, since they simplify the process of achieving legal traceability when regulations require it.

Official U.S. Government time, as provided by NIST and USNO, is available on the Internet at http://www.time.gov. NIST also offers an Internet Time Service (ITS) and an Automated Computer Time Service (ACTS) that allow setting of computer and other clocks through the Internet or over standard commercial telephone lines. Free software for using these services on several types of popular computers can be downloaded there.

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