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    Re: GPS time for celestial
    From: Richard B. Langley
    Date: 1997 May 20, 10:48 EDT

    On Mon, 19 May 1997 captainmike@XXX.XXX wrote:
    > At 23:45 5/18/97 -0700, -=Tony=- wrote in part:
    > >Scott:
    > >
    > >	1.  GPS time is different from GMT/UTC.
    > >
    > >	-=Tony=-	San Francisco
    > Tony:
    > GMT and UTC are different from one another also.  It is not my intent to
    > start a discussion of the difference, since the time signals we all use for
    > practical navigation are given nowadays in UTC.
    GMT and UTC, as these terms are now typically used (GMT is the name for the
    standard time of zone Z in the U.K.), are identical. See my article below.
    -- Richard Langley
       Professor of Geodesy
                  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@XXX.XXX
       (original version: 3 February 1990; this version: 23 December 1995)
    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 under the title
    "A Few Facts Concerning RGO, GMT, and UT".  For the current version, I
    have modified the title slightly.
    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!
    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. 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 the nominal
      day. 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.  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 Sevres, France, determine when
      a leap second is needed.
    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
    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. 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
    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 published
    in 1980 by the Oxford University Press.  Although the book is out of
    print, you may be able to find it in your public library.  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 and published by University Science Books, Mill
    Valley, CA.  ISBN 0-935702-68-7.  There is also a wealth of information
    on time at the U.S. Naval Observatory's Directorate of Time Web site
    (URL: http://tycho.usno.nay.mil/time.html).  For information on Queen's
    University's International Study Centre at Herstmonceux Castle, visit
    their Web site (URL: http://castle.isc.queens.ca/isc/welcome.html).
     Richard B. Langley                         Internet: LANG@XXX.XXX
     Geodetic Research Laboratory               BITnet:   LANG@UNB or SE@UNB
     Dept. of Geodesy and Geomatics Engineering Phone:    (506) 453-5142
     University of New Brunswick                FAX:      (506) 453-4943
     Fredericton, N.B., Canada  E3B 5A3         Telex:    014-46202
          Fredericton?  Where's that? See: http://www.city.fredericton.nb.ca/
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