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Deep Space Atomic Clock Mission Will Improve Navigation Technology

by Nancy Atkinson on April 11, 2012 from UniverseToday.com

Precise radio navigation — using radio frequencies to determine position — is vital to the success of all deep-space exploration missions. To improve navigation technology, a small demonstration mission called the Deep Space Atomic Clock (DSAC) will fly as part of a future NASA mission in order to validate a miniaturized, ultra-precise mercury-ion atomic clock that is 100 times more stable than today’s best navigation clocks.

The mission is now being readied for its preliminary design review in 2013, and is scheduled to fly as a hosted payload on an Iridium NEXT spacecraft.

Launch is set for 2015.

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NOW A COMPLETE NASA PAGE CONCERNING THE EFFECT OF THE FEB 2010 CHILEAN EARTHQUAKE

ON THE EARTH'S ROTATION

The Science@NASA site works best with JavaScript enabled in your browser. For instructions, click here

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INTERNATIONAL EARTH ROTATION AND REFERENCE SYSTEMS SERVICE (IERS) SERVICE INTERNATIONAL DE LA ROTATION TERRESTRE ET DES SYSTEMES DE REFERENCE SERVICE DE LA ROTATION TERRESTRE OBSERVATOIRE DE PARIS 61, Av. de l'Observatoire 75014 PARIS (France) Tel. : 33 (0) 1 40 51 22 26 FAX : 33 (0) 1 40 51 22 91 e-mail : services.iers@obspm.fr http://hpiers.obspm.fr/eop-pc Paris, 5 January 2012 Bulletin C 43 To authorities responsible for the measurement and distribution of time UTC TIME STEP on the 1st of July 2012 A positive leap second will be introduced at the end of June 2012. The sequence of dates of the UTC second markers will be: 2012 June 30, 23h 59m 59s 2012 June 30, 23h 59m 60s 2012 July 1, 0h 0m 0s The difference between UTC and the International Atomic Time TAI is: from 2009 January 1, 0h UTC, to 2012 July 1 0h UTC : UTC-TAI = - 34s from 2012 July 1, 0h UTC, until further notice : UTC-TAI = - 35s Leap seconds can be introduced in UTC at the end of the months of December or June, depending on the evolution of UT1-TAI. Bulletin C is mailed every six months, either to announce a time step in UTC or to confirm that there will be no time step at the next possible date. Daniel GAMBIS Head Earth Orientation Center of IERS Observatoire de Paris, France

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A LEAP SECOND WAS ADDED TO OUR CLOCKS AT 7PM EST ON NEW YEAR'S EVE - 2008 DEC 31

SEE ALL THE ARTICLES BELOW FOR DETAILS AS TO WHY THIS WAS DONE:

About Leap Seconds – 2008 DECEMBER 31st Leap Second

Leap seconds are added to keep the clocks synchronized with the Earth's rotation.

Basic details

The second is the base unit for modern time keeping. The second was previously defined based on the Earth's rotation, but because modern atomic clocks are more accurate than the Earth's rotation the definition was changed in 1967. A second is currently defined as being the duration of 9,192,631,770 periods/oscillations of radiation from a Cesium-133 atom at the ground state (near 0 Kelvin - coldest possible).

The Earth is rotating slower and slower over time, while the atomic clocks are not slowing down. On one average day the difference is around 0.002 seconds, which means around 1 second in 500 days. In order to synchronize the atomic clocks with the Earth's observed rotation, the atomic clocks are occasionally instructed to add an extra second – the leap second. Leap seconds are inserted so that the difference between the UTC (Coordinated Universal Time) and UT1 (mean solar time - observed Earth rotation) is kept below 0.9 seconds in theory, but only 0.7 sec in practice.

The leap second is added in the end of June or December. It is also possible to have a negative leap second, where one second is removed, in a case where the Earth is rotating faster, but such a negative second has never been used, and is rather unlikely to be used in the future.

How are leap seconds declared?

The International Earth Rotation and Reference System Service (IERS) observes the Earth's rotation and nearly 6 months in advance (January and July) a "Bulletin C" message is sent out, which reports whether or not to add a leap second in the end of June and December.

How leap seconds are inserted

Leap seconds are inserted at the end of June or December as an additional second after 23:59:59 UTC (Universal Time Coordinated). The additional second is the 61st second of the last minute of the month, and it is written as 23:59:60 (or 11:59:60 PM in 12-hour format).

The second is inserted at the same time all over the world - the actual local time will therefore depend on the time zone. Only regions in the UTC time zone will add the second just before midnight, for time zones east of UTC, the second will be added the next day (first day in January or July), for time zones west of UTC, the second will be added earlier on the same day as for UTC.

FROM WESTCHESTER CHANNEL 12 METEOROLOGIST AND

HAYDEN PLANETARIUM LECTURER JOE RAO:

For all those who missed the insertion of the Leap Second on New Year's Eve,

here is a chance to see it and hear it again:

You can HEAR this exciting moment at:

http://www.heliotown.com/Leap_Second.html

The leap second appears just before the top of the hour beep

on WWV or in this case the first second of 2009. Normally there is a

one-second pause between the announcement and the tone that marks

the top of the hour. But tonight, there was a two-second pause.

And you can also SEE it here as depicted on the US Naval Observatory's Master Clock:

http://www.totalitysoftware.com/downloads/leapsecond2008.jpg

Clear skies to all and I hope you are now all in sync with Astronomical Time!

THE WEBSITE LISTED JUST ABOVE IS DOWNLOADED FOR YOU JUST BELOW:

Leap second on 2008-12-31 23:59:60 UTC

The next leap second will be inserted like this, in the UTC time scale, and corresponding times elsewhere in the world. (2008-12-31 means December 31, 2008, and 2009-01-01 means January 1, 2009).

Leap Second Announcement of July 2008

The latest leap second bulletin sent by IERS on 4 July 2008 is displayed here:

UTC TIME STEP

on the 1st of January 2009

A positive leap second will be introduced at the end of December 2008.

The sequence of dates of the UTC second markers will be:

2008 December 31, 23h 59m 59s

2008 December 31, 23h 59m 60s

2009 January 1, 0h 0m 0s

The difference between UTC and the International Atomic Time TAI is:

from 2006 January 1, 0h UTC, to 2009 January 1 0h UTC : UTC-TAI = - 33s

from 2009 January 1, 0h UTC, until further notice : UTC-TAI = - 34s

Leap seconds can be introduced in UTC at the end of the months of December

or June, depending on the evolution of UT1-TAI. Bulletin C is mailed every

six months, either to announce a time step in UTC or to confirm that there

will be no time step at the next possible date.

Daniel GAMBIS

Head

Earth Orientation Center of IERS

Observatoire de Paris, France

The next message will be announced in January 2009 and will tell if there will be a leap second in June 2009 (earliest possiblity, but unlikely).

Historic Leap Seconds

The following table shows all leap seconds that have been added so far.

UTC–TAI means the difference between the civil time (UTC) which is kept within 0.9 seconds from Earth's rotation and the International Atomic Time (TAI) which does not care about the Earth's rotation, but rather observations of the Cesium-133 atom. A difference of 33 seconds means that the Earth has slowed by 33 seconds compared with TAI since 1958 (when TAI and UTC were the same). The difference between UTC and TAI was defined as 10 seconds from January 1972 and the first leap second was added in June 1972.

Future for Leap Seconds

There have been proposals for changing the current time scale, so that UTC is no longer tied so closely with the earth's rotation. Over years, this will lead to minutes and eventually hours of difference, so maybe something like a leap hour will be needed to maintain some synchronization between the day and night and the clock.

It is not yet decided what will happen. SEE QUESTIONNAIRE BELOW AND PLEASE VOTE TO KEEP UTC AND LEAP SECONDS ALONE - WE MUST HAVE THEM TO KEEP ACCURATE TIME

TO VOTE AND SUMMIT THE FORM BELOW WHICH IS JUST A COPY GO TO:

http://hpiers.obspm.fr/eop-pc/index.php?index=questionnaire

BACK

HOME SHOW THIS PAGE

• • THEORY AND MODELLING

  • EARTH ORIENTATION DATA

  • GEOPHYSICAL EXCITATION

  • LINKS

Leap Second Support on timeanddate.com

Currently leap seconds are not supported on timeanddate.com, but such support is likely to be added in the future.

More information

• Why and When Leap Years are Used

• Time Zone News

• List of countries that observe Daylight Saving Time in 2008

• Time Zone Abbreviations

Related links

• The World Clock – Current time all over the world

• Personal World Clock

• Meeting Planner

• Time Zone Converter

• Fixed Time Calculator – If it's 3 pm in New York, what time is it in the rest of the world?

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The Astronomical Almanac Online

• • Contents

  • Errata

• Constants

• Glossary

  • Utilities

  • Additional Info

Delta T : Past, Present and Future

Delta T is the difference between Terrestrial Time (TT) and Universal Time (UT1) i.e. Delta T = TT – UT1. It is a measure of the difference between a time scale based on the rotation of the Earth (UT1) and an idealised uniform timescale at the surface of the Earth (TT). TT is realised in practice by TAI, International Atomic Time, where TT = TAI + 32.184 seconds. In order to predict the circumstances of an event on the surface of the Earth such as a solar eclipse, a prediction of Delta T must be made for that instant of TT.

Telescopic Era Values of Delta T:

The diagram above displays the values of Delta T for the telescopic era (1620 to 2010) as tabulated on pages K8 and K9 of the current edition of The Astronomical Almanac. Data are given for the beginning of each year. A simple parabolic function used to estimate Delta T is also plotted for comparison purposes. It takes the form Delta T = – 20 + 32T² (Morrison & Stephenson, 2004) where T is the number of centuries since 1820. This function is based on the assumption that the length of the mean solar day has been increasing by about 1.7 milliseconds per century.

Current Values and Short Term Predictions of Delta T

The diagram above shows the values of Delta T tabulated daily for the interval 2000 January 1 to the present as derived from the (UT1 - UTC) smoothed data provided in Bulletin B of the IERS, the International Earth Rotation and Reference Systems Service. It also shows the daily Delta T data and predictions provided with the USNO MICA v2.2.1 software package. The predictions used by MICA are occasionally updated; the data used here are those whose predictions of Delta T start on Julian Date 2454863.5. The current observed Delta T trend is increasing less rapidly than the MICA v2.2.1 predictions but more rapidly than the data derived from the (UT1 - TAI) predictions of the IERS Sub-bureau for Rapid Service and Predictions. The annual rate of change of Delta T over the last 5 years or so is approximately 0.30 seconds / year.

Current Values and Longer Term Predictions of Delta T

The diagram above again shows the values of Delta T derived from Bulletin B of the IERS. Two sets of predictions are also provided for comparison purposes. The daily Delta T data and predictions are plotted for the interval 2000 to 2050 from MICA v2.2.1. The predictions derived from the IERS Sub-bureau for Rapid Service and Predictions are also plotted for the period 2006 April 1 to 2027 October 1 along with their uncertainties (vertical error bars). The current trend of observed Delta T data lies between the two sets of predictions. This diagram illustrates the problem faced by almanac producers when trying to estimate suitable values of Delta T for future almanacs.

The Article above is from > The Astronomical Almanac Online! 2012

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'Quantum Logic Clock' Rivals Mercury Ion As World's Most Accurate Clock (Below the Introduction)

THE FOLLOWING INTRODUCTION IS BY LARRY GERSTMAN:

Before you read this article, have you ever thought about exactly what does it mean to have an accurate clock? They do keep better time, but what does that really mean? And how do we know what the right time is and how is it measured? How do we judge which time piece is best?

THE KEY ANSWER IS THAT THE BEST CLOCKS KEEP STRICT UNIFORM TIME. THAT IS EVERY INTERVAL OF TIME SUCH AS AN HOUR FOR EXAMPLE IS THE SAME SIZE AS EVERY OTHER HOUR WITHIN THE SMALLEST OF ERRORS. INFERIOR TIME PIECES GAIN OR LOSE SOME TIME TO A GREATER EXTENT AND AT A SOMEWHAT IRREGULAR RATE. THE ARTICLE BELOW DISCUSSES THE MOST ACCURATE CLOCK EVER DEVELOPED WHICH KEEPS STRICT UNIFORM TIME TO AN UNBELIEVABLE ACCURACY WITH A DEVIATION FROM PERFECT UNIFORM TIME OF ONLY ONE SECOND IN ABOUT A BILLION YEARS!

THE STRANGE CONCEPT YOU MUST UNDERSTAND IS THAT WHEN WE ADD A LEAP SECOND WE ARE ADDING A SECOND OF ERROR INTO OUR CLOCKS BECAUSE THE EARTH WHICH DOES NOT KEEP PERFECT UNIFORM TIME IS THE CLOCK WE LIVE ON (The Title of a Great Isaac Asimov Book on this subject). THE ACCUMULATION OF THIS ERROR IS CALLED DELTA T WHICH MEANS THE CHANGE IN TIME AND HAS BEEN MEASURED TO BE 66.769 SECONDS AS OF 2012 JUNE 21 OF ACCUMULATED ERROR IN THE EARTH'S ROTATION RATE SINCE THE ZERO POINT IN THE YEAR 1902. THE EARTH'S ROTATION IS CURRENTLY SLOWING DOWN AT A RATE OF ABOUT 3 TENTHS OF A SECOND PER YEAR. THEREFORE, THE NEXT FOLLOWING LEAP SECOND WILL PROBABLY NOT OCCUR FOR ABOUT ANOTHER THREE YEARS. BUT THIS IS DEFINITELY NOT A UNIFORM CHANGE. WHY THE YEAR 1902 FOR A DELTA T OF ZERO? ACTUALLY THE ZERO POINT WAS SUPPOSED TO BE IN 1900 WHEN THIS SYSTEM WAS DEVELOPED AROUND 1950, BUT FURTHER MORE ACCURATE RESEARCH DONE AFTER 1950 SHOWED A SMALL ERROR LEADING TO 1902 BEING THE ZERO POINT NOT 1900. THIS IS SOMEWHAT ANALOGOUS TO THE CREATION OF THE FAHRENHEIT TEMPERATURE SCALE. DANIEL GABRIEL FAHRENHEIT WAS TRYING TO FORM A SCALE WHICH HAD A SATURATED SALINE WATER SOLUTION's FREEZING POINT BE THE ZERO POINT AND THE AVERAGE HUMAN BODY TEMPERATURE (WHICH IS ABOUT 98.5 DEGREES) TO BE THE 100 DEGREE POINT.

REGARDING HOW THE CORRECT TIME IS MEASURED - THERE ARE SEVERAL DIFFERENT METHODS SUCH AS SATELLITE RANGING, GPS, TRANSIT TELESCOPES AND MORE WHICH IS COMPILED AND AVERAGED BY THE INTERNATIONAL EARTH ROTATION SERVICE (IERS). OUR CLOCKS ARE BASED ON COORDINATED UNIVERSAL TIME WHOSE TIME SIGNALS ARE BROADCAST OVER SHORT WAVE RADIO. THEN TV AND RADIO STATIONS PICK UP THE SIGNAL WHICH THEY REBROADCAST TO YOU ON THE HOUR. THAT IS HOW THEY KNOW WHAT TIME IT IS. YOU CAN DO THE SAME BY PICKING UP THE SIGNAL YOURSELF ON 2.5, 6, 10, 15 AND 20 MEGAHERTZ. THIS SIGNAL FOR THE UNITED STATES IS LOCATED IN FORT COLLINS, COLORADO. A PICTURE OF THIS STATION IS BELOW THE FOLLOWING TWO ARTICLE:

'Quantum Logic Clock' Rivals Mercury Ion As World's Most Accurate Clock:

ScienceDaily (Mar. 10, 2008) — An atomic clock that uses an aluminum atom to apply the logic of computers to the peculiarities of the quantum world now rivals the world's most accurate clock, based on a single mercury atom. Both clocks are at least 10 times more accurate than the current U.S. time standard.

The measurements were made in a yearlong comparison of the two next-generation clocks, both designed and built at the Commerce Department's National Institute of Standards and Technology (NIST). The clocks were compared with record precision, allowing scientists to measure the relative frequencies of the two clocks to 17 digits-the most accurate measurement of this type ever made. The comparison produced the most precise results yet in the worldwide quest to determine whether some of the fundamental constants that describe the universe are changing slightly over time, a hot research question that may alter basic models of the cosmos.

The research is described in the March 6 issue of Science Express.* The aluminum and mercury clocks are both based on natural vibrations in ions (electrically charged atoms) and would neither gain nor lose one second in over 1 billion years-if they could run for such a long time-compared to about 80 million years for NIST-F1, the U.S. time standard based on neutral cesium atoms.

The mercury clock was first demonstrated in 2000 and is now four times better than its last published evaluation in 2006, thanks to ongoing improvements in the clock design and operation. The mercury clock continues its reign as the world's most accurate for now, by a margin of 20 percent over the aluminum clock, but the designers say both experimental clocks could be improved further.

"The aluminum clock is very accurate because it is insensitive to background magnetic and electric fields, and also to temperature," says Till Rosenband, the NIST physicist who built the clock and is the first author of the new paper. "It has the lowest known sensitivity of any atomic clock to temperature, which is one of the most difficult uncertainties to calibrate."

Both the aluminum clock and the mercury clock are based on ions vibrating at optical frequencies, which are 100,000 times higher than microwave frequencies used in NIST-F1 and other similar time standards around the world. Because optical clocks divide time into smaller units, they can be far more precise than microwave standards. NIST scientists have several other optical atomic clocks in development, including one based on thousands of neutral strontium atoms. The strontium clock recently achieved twice the accuracy of NIST-F1, but still trails the mercury and aluminum clocks.

Highly accurate clocks are used to synchronize telecommunications networks and deep-space communications, and for satellite navigation and positioning. Next-generation clocks may also lead to new types of gravity sensors, which have potential applications in exploration for underground natural resources and fundamental studies of the Earth.

Laboratories around the world are developing optical clocks based on a variety of different designs and atoms; it is not yet clear which design will emerge as the best candidate for the next international standard.

The new paper provides the first published evaluation of the operational quantum logic clock, so-named because it is based on the logical reasoning process used in quantum computers (see sidebar for details). The clock is a spin-off of NIST research on quantum computers, which grew out of earlier atomic clock research. Quantum computers, if they can be built, will be capable of solving certain types of complex problems that are impossible or prohibitively costly or time consuming to solve with today's technologies.

The NIST quantum logic clock uses two different kinds of ions, aluminum and beryllium, confined closely together in an electromagnetic trap and slowed by lasers to nearly "absolute zero" temperatures. Aluminum is a stable source of clock ticks, but its properties cannot be detected easily with lasers. The NIST scientists applied quantum computing methods to share information from the aluminum ion with the beryllium ion, a workhorse of their quantum computing research. The scientists can detect the aluminum clock's ticks by observing light signals from the beryllium ion.

NIST's tandem ion approach is unique among the world's atomic clocks and has a key advantage: "You can pick from a bigger selection of atoms," explains NIST physicist Jim Bergquist, who built the mercury clock. "And aluminum has a lot of good qualities-better than mercury's."

An optical clock can be evaluated precisely only by comparison to another clock of similar accuracy serving as a "ruler." NIST scientists used the quantum logic clock to measure the mercury clock, and vice versa. In addition, based on fluctuations in the frequencies of the two clocks relative to each other over time, NIST scientists were able to search for a possible change over time in a fundamental quantity called the fine-structure constant. This quantity measures the strength of electromagnetic interactions in many areas of physics, from studies of atoms and molecules to astronomy. Some evidence from astronomy has suggested the fine-structure constant may be changing very slowly over billions of years. If such changes are real, scientists would have to dramatically change their theories of the fundamental nature of the universe.

The NIST measurements indicate that the value of the fine-structure constant is not changing by more than 1.6 quadrillionths of 1 percent per year, with an uncertainty of 2.3 quadrillionths of 1 percent per year (a quadrillionth is a millionth of a billionth). The result is small enough to be "consistent with no change," according to the paper. However, it is still possible that the fine-structure constant is changing at a rate smaller than anyone can yet detect. The new NIST limit is approximately 10 times smaller than the best previous measurement of the possible present-day rate of change in the fine-structure constant. The mercury clock is an especially useful tool for such tests because its frequency fluctuations are magnified by any changes in this constant.

The work described in the new Science paper was supported in part by the Office of Naval Research and Disruptive Technology Office.

As a non-regulatory agency of the Commerce Department, NIST promotes U.S. innovation and industrial competitiveness by advancing measurement science, standards and technology in ways that enhance economic security and improve our quality of life.

*Journal reference: T. Rosenband, D.B. Hume, P.O. Schmidt, C.W. Chou, A. Brusch, L. Lorini, W.H. Oskay, R.E. Drullinger, T.M. Fortier, J.E. Stalnaker, S.A. Diddams, W.C. Swann, N.R. Newbury, W.M. Itano, D.J. Wineland, and J.C. Bergquist. 2008. Frequency ratio of Al+ and Hg+ single-ion optical clocks; metrology at the 17th decimal place. Science Express. Published online March 6.

Background: Where the 'Quantum Logic Clock' Gets Its Name

The NIST quantum logic clock is so named because it borrows techniques that are key to quantum computers, which would solve problems using quantum mechanics, nature's instruction book for the smallest particles of matter and light. Logic is reasoning that determines an action or result based on which one of different possible options is received as input. In the NIST clock, the input options are two different quantum states, or internal energy levels, of an aluminum ion. Information about this state is transferred to a beryllium ion, which, depending on the input, produces different signals that are easily detected.

NIST scientists use lasers to cool the two ions which are held 4 thousandths of a millimeter apart in an electromagnetic trap. Aluminum is the larger of the two ions, while the beryllium emits light under the conditions of this experiment. Scientists hit the ions with pulses from a "clock laser" within a narrow frequency range. If the laser frequency is at the center of the frequency range, the precise "resonance frequency" of aluminum, this ion jumps to a higher energy level, or 1 in the binary language of computers. Otherwise, the ion remains in the lower energy state, or 0.

If there is no change in the aluminum ion, then another laser pulse causes both ions to begin rocking side to side in unison because of their physical proximity and the interaction of their electrical charges. An additional laser pulse converts this motion into a change in the internal energy level of the beryllium ion. This pulse reverses the direction of the ion's magnetic "spin," and the beryllium goes dark, a signal that the aluminum remained in the 0 state.

On the other hand, if the aluminum ion jumps to the higher energy level, then the additional laser pulses fail to stimulate a shared rocking motion and have no effect on the beryllium ion, which keeps emitting light. Scientists detect this light as a signal that the aluminum ion jumped from 0 to 1.

The goal is to tune the clock laser to the exact frequency that prompts the aluminum to jump from 0 to 1. The actual measurement of the ticking of the clock is provided not by the ions but rather by the clock laser's precisely tuned center frequency, which is measured with a "frequency comb," a tool for measuring very high optical frequencies, or colors of light.

UPDATE BELOW - AN EVEN MORE ACCURATE CLOCK - ERROR IS WITHIN ONE SECOND IN 3.7 BILLION YEARS - LIFE BEGAN ON EARTH ABOUT THAT LENGTH OF TIME AGO! >

The World's Most Accurate Clock is Unveiled

Posted on February 5, 2010 - 09:01 by Emma Woollacott

From http://www.tgdaily.com/clock/48319-worlds-most-accurate-clock-unveiled

Physicists at the National Institute of Standards and Technology (NIST) have built the world's most accurate clock.

Noting that "aluminum is a better timekeeper than mercury" - thanks, we'll bear that in mind when making appointments - the team based the clock on a single atom of aluminum.

Universal Time (UT) is a timescale 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 meridian of 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 sidereal time, but with a scaling factor and other adjustments to make them closer to solar time.

Universal Time and Standard Time

Prior to the introduction of standard time, every 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 timepieces, 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. Chronometers 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 zones, each one covering exactly 15 degrees of longitude. All clocks within each of these zones would be set to the same time as the others, but differed by one hour from those in the neighbouring 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 in order to set local clocks. This location was chosen because by 1884 two-thirds of all charts and maps 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, 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.

Charles F. Dowd proposed in 1870 (after consulting railroad officials in 1869) that American railroads adopt four standard time zones. After further discussion among themselves, American and Canadian railroads adopted five standard time zones on November 18, 1883. Newspapers referred to that day as "the Day of Two Noons." There was no legislative enactment or ruling: the railroads simply adopted a five zone system encompassing North America from Nova Scotia to California, and assumed the public would follow. The American Railway Association, an organization of railroad managers, had noticed growing scientific interest in standardizing time. The ARA devised their own system, which had irregular zone boundaries which followed then-existing boundaries of different lines, partly in order to head off government action which might have been inconvenient to their operations. Most people simply accepted the new time, but a number of cities and counties refused to accept "railroad time", which, after all, had not been made law. In, for example, the expiration of a contract, what does "midnight" mean? In one Iowa Supreme Court case, the owner of a saloon argued that he operated by local (sun) time, not "railroad time," and so he had not violated laws about closing time.^{[citation needed]} Standard time remained a local matter until 1918, when it was made law as part of the introduction of daylight saving.

On November 2, 1868 New Zealand officially adopted a standard time to be observed nationally, and was perhaps the first country to do so. It was based on the longitude 172° 30' East of Greenwich, that is 11 hours 30 minutes ahead of Greenwich Mean Time. This standard was known as New Zealand Mean Time.

Measurement

One can measure time based on the rotation of the Earth by observing celestial bodies crossing the meridian every day. Astronomers have preferred observing meridian crossings of stars over observations of the Sun, because these are more accurate. Nowadays, UT in relation to International Atomic Time (TAI) is determined by Very Long Baseline Interferometry (VLBI) observations of distant quasars, a method which has an accuracy of micro-seconds. 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; also the length of the day very gradually increases due to tidal acceleration. Furthermore, the length of the second is based on its conventional length as determined from observations of the Moon between 1750 and 1890. This also causes the mean solar day, on the average, to now extend longer than the nominal 86,400 SI seconds. 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 leap seconds to atomic time to obtain a broadcast form of civil time that carries atomic frequency. Thus, civil broadcast standards for time and frequency are a compromise that usually follows, with an offset found from the total of all leap seconds, International Atomic Time (TAI), but occasionally jumps in order to prevent it from drifting too far from mean solar time. Terrestrial Time is TAI + 32.184 s.

Barycentric Dynamical Time (TDB), a form of atomic time, is now used in the construction of the ephemerides of the planets and other solar system objects, for two main reasons. For one thing, 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. For another, the time scales based on Earth's rotation are not uniform, so are not suitable for predicting the motion of solar system objects.

In 1928, the term Universal Time was adopted internationally as a more precise term than Greenwich Mean Time, because the 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 timekeeping.

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 motion, 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. It is computed from the raw observed UT0 by correcting UT0 for the effect of polar motion on the longitude of the observing site. UT1 is the same everywhere on Earth, and is proportional to the true rotation angle of the Earth with respect to a fixed frame of reference. Since the rotational speed of the earth is not uniform, UT1 has an uncertainty of plus or minus 3 milliseconds per day. The ratio of UT1 to mean sidereal time is defined to be 0.997269566329084 − 5.8684×10⁻¹¹T + 5.9×10⁻¹⁵T², where T is the number of Julian centuries of 36525 days each that have elapsed since JD 2451545.0 (J2000).^[1]

• 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.^[2]

• 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 tectonics, and currents in the interior of the Earth.

  • UTC (Coordinated Universal Time) is an atomic timescale that approximates UT1. It is the international standard on which civil time is based. It ticks SI seconds, in step with TAI. It usually has 86400 SI seconds per day, but is kept within 0.9 seconds of UT1 by the introduction of occasional intercalary leap seconds. As of 2007 these leaps have always been positive, with a day of 86401 seconds. When an accuracyDUT1.^[update] 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

• UTC-SLS (UTC with Smoothed Leap Seconds) is a proposed modification of UTC that avoids unequal day lengths. It usually ticks the same as UTC, but modifies the length of the second for the last 1000 UTC seconds of a day containing a leap second so that there are always 86400 seconds in the UTC-SLS day.^[3]

• UTS (Smoothed Universal Time) is an obscure form of UT used internally at IERS. The same abbreviation was for a time used to refer to UTC-SLS.^[3]

See also

• Greenwich Mean Time (GMT)

• Sir Sandford Fleming

Notes

1. ^ Seidelmann, p.52

2. ^ Earth rotation variations due to zonal tides

3. ^ ^{a b} UTC with Smoothed Leap Seconds (UTC-SLS)

References

• • Galison, Peter (2003). Einstein's clocks, Poincaré's maps: Empires of time. New York: W.W. Norton & Company. ISBN 0-393-02001-0. Discusses the history of time standardization.

• O'Malley, Michael (1996). Keeping watch: A history of American time. Washington: Smithsonian. ISBN 1-56098-672-7.

• Seidelmann, P. Kenneth (1992). Explanatory supplement to the Astronomical Almanac. Mill Valley, California: University Science Books. ISBN 0-935702-68-7.

• McCarthy, Dennis D. (July 1991). "Astronomical Time". Proceedings of the IEEE 79 (7): 915–920. http://www.cl.cam.ac.uk/~mgk25/volatile/astronomical-time.pdf.