A Few Facts Concerning GMT, UT, and the RGO

                           Richard B. Langley
                      Geodetic Research Laboratory
               Dept. of Geodesy and Geomatics Engineering
                       University of New Brunswick
                   Fredericton, N.B., Canada  E3B 5A3
                           E-mail: lang@unb.ca

    (original version: 3 February 1990; this version: 20 January 1999)   

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 23 December 1995 version.

The Royal Greenwich Observatory

o Prior to 1948, the observatory at Greenwich (located on a hill back
  from the Thames River with a view of the London Docks) was known as
  the Royal Observatory.

o 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!).

o 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!

o Following the closing of the RGO in the fall of 1998, the Old 
  Greenwich Observatory was renamed the Royal Observatory Greenwich (see 
  "Where's the RGO Now?" below).

Greenwich Mean Time

o 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 23-1/2 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 (i.e. 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.)

o 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.

Universal Time

o 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
  1800's held to set up the standard time system.

o 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. 

o 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.  86,400 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 86,400 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 

o 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
The Origin of UTC
o 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.

o 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 
  recently introduced "Internet Time" (Central European Time measured in 
  1/1000 of a day (a "beat")).

GMT and the BBC

o The BBC began transmitting time signals in 1924.  The chimes of Big
  Ben were first broadcast at midnight beginning 1 January and on 5
  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.

o 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.

o The time service at Herstmonceux closed down during February 1990 when
  the BBC took over the generation of the six pips.  Since 5 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 atop
  Broadcasting House in London.

Where's the RGO Now?

o In March 1990, RGO officially moved from Herstmonceux Castle to the
  grounds of Cambridge University's Institute of Astronomy. On 31 
  October 1998, the RGO was closed by the U.K. 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.

To Learn More

o If you'd 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 .

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