Richard B. Langley
Geodetic Research Laboratory
Dept. of Geodesy and Geomatics Engineering
University of New Brunswick
Fredericton, N.B., Canada E3B 5A3
(original version: 3rd February 1990; this version: 3rd May 2004
This HTML version: 3rd February 2006)
In answer to the question "Does anyone know the exact difference between GMT and UTC?" here are a few facts concerning Greenwich Mean Time, Universal Time, and the Royal Greenwich Observatory. Various versions of this document have been posted to Usenet newsgroups and made available on Web sites over the years under the original title A Few Facts Concerning RGO, GMT, and UT. The current title has been in use since the 23rd December 1995 version.
Prior to 1948, the observatory at Greenwich (located on a hill back from the River Thames with a view of the London Docks) was known as the Royal Observatory.
In 1948, the observatory moved to Herstmonceux Castle in Sussex, becoming the Royal Greenwich Observatory (yes, even though it wasn’t at Greenwich any more!).
The site at Greenwich became known as the Old Greenwich Observatory and the historic buildings and instruments were progressively incorporated into the National Maritime Museum, the main buildings of which are located at the foot of Observatory Hill, close to the river. Highly recommended for a visit if you’re in London!
Following the closing of the RGO in the autumn of 1998, the Old Greenwich Observatory was renamed the Royal Observatory Greenwich (see Where’s the RGO Now? below).
Greenwich Mean Time is a time scale based on the apparent motion of the ‘mean’ sun with respect to the meridian through the Old Greenwich Observatory (zero degrees longitude). The ‘mean’ sun is used because time based on the actual or true apparent motion of the sun doesn’t ‘tick’ at a constant rate. The earth’s orbit is slightly eccentric and the plane of the earth’s orbit is inclined with respect to the equator (about 23.5 degrees) hence at different times of the year the sun appears to move faster or slower in the sky. That’s why an uncorrected sundial can be ‘wrong’ (if it is supposed to be telling mean time) by up to 16 minutes. So if the mean (ie. corrected) sun is directly over the meridian through Greenwich, it is exactly 12 noon GMT or 12:00 GMT (Prior to 1925, astronomers reckoned mean solar time from noon so that when the mean sun was on the meridian, it was actually 00:00 GMT. This practice arose so that astronomers wouldn’t have a change in date during a night’s observing. Some in the astronomical community still use the pre-1925 definition of GMT in the analysis of old data although it is recommended that the term Greenwich Mean Astronomical Time now be used to refer to time reckoned from noon.)
Mean time on selected meridians 15 degrees apart is generally known as standard time. For example, Eastern Standard Time (EST) is the mean solar time of the meridian at 75 degrees W.
In 1928, the International Astronomical Union recommended that the time used in the compilation of astronomical almanacs, essentially GMT, or what was also sometimes called Greenwich Civil Time, be referred to as Universal Time (UT). The terms Universal Time and Universal Day were introduced at the various conferences in the 1800s held to set up the standard time system.
There are actually a couple of variants of UT. UT as determined by actual astronomical observations at a particular observatory is known as UT0 (‘UT-zero’). It is affected by the motion of the earth’s rotation pole with respect to the crust of the earth. If UT0 is corrected for this effect, we get UT1 which is a measure of the true angular orientation of the earth in space. However, because the earth does not spin at exactly a constant rate, UT1 is not a uniform time scale. The variation in UT1 is dominated by seasonal oscillations due primarily to the exchange of angular momentum between the atmosphere and the solid earth and seasonal tides. In an effort to derive a more uniform time scale, scientists established UT2. UT2 is obtained from UT1 by applying an adopted formula that approximates the seasonal oscillations in the earth’s rotation. However, due to other variations including those associated with the secular effects of tidal friction (the earth’s spin is continually but gradually slowing down), high frequency tides and winds, and the exchange of angular momentum between the earth’s core and its shell, UT2 is also not a uniform time scale.
So rather than base our civil time keeping on the rotation of the earth we now use Atomic Time, time based on the extremely constant frequency of a radio emission from cesium atoms when they change between two particular energy states. The unit of Atomic Time is the atomic second. 86400 atomic seconds define the length of a nominal ‘reference’ day - the length of the day as given by the earth’s rotation around the year 1900. But because of the variations in the earth’s spin the length of the actual day can be shorter or longer than the nominal day of 86400 seconds. The time scale based on the atomic second but corrected every now and again to keep it in approximate sync with the earth’s rotation is known as UTC or Coordinated Universal Time. The corrections show up as the leap seconds put into UTC from time to time - usually on New Year’s Eve. With these leap second adjustments, UTC is kept within 0.9 seconds of UT1. Currently, the need for leap seconds is primarily due to the effects of tidal friction. The earth’s rotation in space is monitored by the International Earth Rotation Service (IERS) in Paris, France, using a global network of satellite and lunar laser ranging, very long baseline interferometry, and Navstar Global Positioning System (GPS) stations. The IERS, in consultation with the Bureau International des Poids et Mesures in Sèvres, France, determine when a leap second is needed.
In 1928, when the term Universal Time was introduced, variations in the earth’s spin were not yet known. So the term GMT was, in essence, replaced by UT1. Despite the official adoption of the term UT, the navigational publications of English-speaking countries retained the term GMT as a synonym for UT1 for some time. So, even today, in astronavigation, GMT can imply UT1. But in general usage (including that of shortwave broadcasters such as the BBC, for example), GMT now usually means the civil (atomic-second-based) time kept in the United Kingdom which is the standard time of the time zone centred on the 0 degree meridian. In this (the most common) usage, the terms GMT and UTC are identical.
But because there are two possible meanings for GMT differing by up to 0.9 seconds, the term GMT should not be used for precise purposes - particularly not in reference to GPS observations!
The concept of a coordinated universal time was introduced in 1960 when the British and American national time services initiated a program to coordinate the offsets of the frequencies and epochs (phases) of transmitted time service radio signals from Atomic Time in approximating UT2. Subsequently, other national time services joined the program. The BIH was charged with the task of monitoring and maintaining the program and introduced the term Temps Universel Coordinné or Coordinated Universal Time for the coordinated time scale in 1964. Initially, the time scale was derived by offsetting its rate from that of Atomic Time to agree with the average rate of UT2 over the past year and was held fixed at that rate for the following year. If the rate of UT2 changed significantly during the year, then an offset (from 1962, in multiples of 100 milliseconds) could be introduced on the first day of a month. This system of frequency and epoch offsets was continued until 1972 when the current practice was adopted of keeping the rate of UTC equal to that of Atomic Time and introducing leap seconds when needed to keep UTC to within 0.9 seconds (it was 0.7 seconds until January 1975) of UT1.
Sometimes the term World Time is used to denote UTC. This strange and potentially confusing term (‘UTC for dummies’?) should be avoided. Similarly, there is no clear need for the Swatch watch company’s Internet Time (Central European Time measured in 1/1000 of a day - a ‘beat’).
The BBC began transmitting time signals in 1924. The chimes of Big Ben were first broadcast at midnight beginning 1st January and on 5th February, at the recommendation of the then Astronomer Royal, Frank Dyson, the six pips time signal (officially known as the Greenwich Time Signal) was inaugurated.
Control of the BBC’s six pips was taken over by the Royal Observatory in 1949 from Abinger to where the time service had moved during the war. The time service moved to Herstmonceux in 1957.
The time service at Herstmonceux closed down during February 1990 when the BBC took over the generation of the six pips. Since 5th February 1990, the 66th anniversary of the start of the Greenwich Time Service, the six pips have been synchronised to UTC by using the GPS satellite signals which are picked up by a pair of GPS receivers on top of Broadcasting House in London.
In March 1990, RGO officially moved from Herstmonceux Castle to the grounds of Cambridge University’s Institute of Astronomy. On 31st October 1998, the RGO was closed by the UK Particle Physics and Astronomy Research Council as a cost-saving measure. Some of its research activities have been transferred to the Royal Observatory Edinburgh. Her Majesty’s Nautical Almanac Office was transferred to the Rutherford Appleton Laboratory at Chilton in Oxfordshire. With the closure of the RGO, the Old Greenwich Observatory has been renamed the Royal Observatory Greenwich. A laser ranging station and a GPS tracking station still operate at Herstmonceux but the castle and estate is now owned by Queen’s University in Kingston, Ontario, Canada, who use it as a satellite campus for their International Study Centre. Queen’s purchased the castle in early 1993 for about $8 million (CDN). This money, and an additional $4 million for renovations were gifts from Dr. Alfred and Mrs. Isabel Bader of Milwaukee, WI. Dr. Bader is a Queen’s alumnus.
If you would like to learn more about time you might look for the book Greenwich Time and the Discovery of Longitude by Derek Howse originally published in 1980 by the Oxford University Press. A second edition, titled Greenwich Time and the Longitude: Official Millennium Guide was published by the National Maritime Museum and Philip Wilson Publishers in 1997 (ISBN 0-85667-468-0). A special paperback edition is available exclusively from the museum. An excellent reference on all matters concerning time is the Explanatory Supplement to the Astronomical Almanac edited by P. Kenneth Seidelmann of the U.S. Naval Observatory (USNO) and published by University Science Books, Mill Valley, CA (ISBN 0-935702-68-7). There is also a wealth of information on time at USNO’s Directorate of Time Web site. For information on Queen’s University’s International Study Centre at Herstmonceux Castle, visit their Web site. A GPS World article discussing the future of the leap second can be found here.
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