Subject: UTC, GMT, time resolution - clarification requested

From: Chris Lilley <[email-masked]>
Date: 17 Aug 1995 11:13:41 GMT

Greetings

I searched back a couple of thousand articles in this group but did 
not see a suitable FAQ that would answer my question. If there is one, 
please point me to it. I need to know about time - UT in particular.

My question:

HTML is currently undergoing internationalisation [1] and as part of this
date and time formats are being defined so that browsers can display dates
and times using local conventions (removing sources of confusion - is
 12/9/95 12th of September or December 9th?). I have two questions about the
time format, which I believe the astronomical community is best placed to
answer:

1) What resolution is "enough" for most purposes? I know that in some 
cases you need to state time to the femtosecond; the point is to strike a
balance so that the vast majority of times can be expressed. Currently, 
the proposal has

 <!ELEMENT TIME - O #EMPTY>
             <!ATTLIST TIME
                     %attrs;
                     ZONE      CDATA  #IMPLIED
                     VALUE     CDATA  #REQUIRED

The contents of VALUE should be in hh:mm:ss.ss format. Is hundredths of a 
second enough? Would extending this to milliseconds hh:mm:ss.sss be an
improvement? I am trying to avoid a situation where the new time element 
is not used for scientific Web  documents because it is nearly, but not 
quite, good enough.

2) Could someone clarify whether GMT has any formal definition and how this
relates to UT? I seem to remember there is a UTC1 and a UTC2 ... I would
appreciate a reference to the differences between these.

The current proposal states:

  ZONE should contain a string representing the offset of the zone
  from GMT of the form "+HHMM" or "-HHMM".  If omitted, Universal 
  Time (GMT) should be assumed. 

I know this is wrong, and seek your help in formulating a more precise 
wording. I will also be suggesting that several "scientific zones" such 
as UTC1 be accepted as valid zone information. Would that be useful?

Thank you for your time and cooperation. I hope we can make the Web a good
medium for scientific communication and collaboration.

[1] ftp://ds.internic.net/internet-drafts/draft-ietf-html-i18n-00.txt

-- 
Chris Lilley, Technical Author
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---

From: Richard B. Langley <[email-masked]>
Date: 17 Aug 1995 13:06:17 GMT

> [1] ftp://ds.internic.net/internet-drafts/draft-ietf-html-i18n-00.txt

For time formats and resolutions, consult ISO 8601.  I think you'll find what
you need to know about GMT and UTC in the following article.  If not, question
away.

                A Few Facts Concerning RGO, GMT, and UT                   
                ---------------------------------------                       

                           Richard B. Langley                                  
   
                      Geodetic Research Laboratory                
               Dept. of Geodesy and Geomatics Engineering                  
                       University of New Brunswick              
                   Fredericton, N.B., Canada  E3B 5A3                      
                           E-mail: [email-masked]          

   (original version: 3 February 1990; this version: 27 January 1995) 

In answer to the question "Does anyone know the exact difference between 
GMT and UTC?" here are a few facts concerning the Royal Greenwich 
Observatory, Greenwich Mean Time, and Universal Time.

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 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 cling to the pre-1925 definition of GMT   
  although it is recommended that the term Greenwich Mean Astronomical   
  Time 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.

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.  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.  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.  So in astronavigation, GMT can imply   
  UT1, but in general communications (as it is used by shortwave   
  broadcasters for example) GMT usually means UTC.

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 synchronized to UTC by using the GPS satellite 
  signals which are picked up by a pair of GPS receivers atop 
  Broadcasting House in London.

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 itself has been sold -- to Queens 
  University in Kingston, Ontario, who operate it as a satellite campus 
  for special international studies programs.  

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.

===============================================================================
 Richard B. Langley                         Internet: [email-masked] or [email-masked]
 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
===============================================================================