From chris.lilley@mcc.ac.uk Mon Aug 21 10:52:40 1995 Path: solitaire.cv.nrao.edu!hearst.acc.Virginia.EDU!caen!zip.eecs.umich.edu!newshost.marcam.com!usc!cs.utexas.edu!howland.reston.ans.net!tank.news.pipex.net!pipex!sunsite.doc.ic.ac.uk!yama.mcc.ac.uk!usenet From: Chris Lilley Newsgroups: sci.astro Subject: UTC, GMT, time resolution - clarification requested Date: 17 Aug 1995 11:13:41 GMT Organization: Manchester and North HPC Training & Education Centre Lines: 65 Message-ID: <40v895$eav@yama.mcc.ac.uk> NNTP-Posting-Host: cguhpc.cgu.mcc.ac.uk Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit X-Mailer: Mozilla 1.1N (X11; I; HP-UX A.09.03 9000/735) X-URL: news:sci.astro 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 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 +-------------------------------------------------------------------+ | Manchester and North HPC Training & Education Centre | +-------------------------------------------------------------------+ | Computer Graphics Unit, Email: Chris.Lilley@mcc.ac.uk | | Manchester Computing Centre, Voice: +44 161 275 6045 | | Oxford Road, Manchester, UK. Fax: +44 161 275 6040 | | M13 9PL BioMOO: ChrisL | | URI: http://info.mcc.ac.uk/CGU/staff/lilley/lilley.html | +-------------------------------------------------------------------+ From lang@unb.ca Mon Aug 21 11:15:26 1995 Path: solitaire.cv.nrao.edu!hearst.acc.Virginia.EDU!caen!zip.eecs.umich.edu!newshost.marcam.com!news.mathworks.com!gatech!howland.reston.ans.net!torn!news.unb.ca!geomac.se.unb.ca!lang From: Richard B. Langley Newsgroups: sci.astro Subject: Re: UTC, GMT, time resolution - clarification requested Date: 17 Aug 1995 13:06:17 GMT Organization: University of New Brunswick Lines: 201 Distribution: world Message-ID: <40ves9$mfm@sol.sun.csd.unb.ca> References: <40v895$eav@yama.mcc.ac.uk> NNTP-Posting-Host: geomac.se.unb.ca X-UserAgent: Version 1.1.3 X-XXMessage-ID: X-XXDate: Thu, 17 Aug 95 12:57:41 GMT In article <40v895$eav@yama.mcc.ac.uk> Chris Lilley, chris.lilley@mcc.ac.uk writes: > 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 > > > %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 > +-------------------------------------------------------------------+ > | Manchester and North HPC Training & Education Centre | > +-------------------------------------------------------------------+ > | Computer Graphics Unit, Email: Chris.Lilley@mcc.ac.uk | > | Manchester Computing Centre, Voice: +44 161 275 6045 | > | Oxford Road, Manchester, UK. Fax: +44 161 275 6040 | > | M13 9PL BioMOO: ChrisL | > | URI: http://info.mcc.ac.uk/CGU/staff/lilley/lilley.html | > +-------------------------------------------------------------------+ 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: lang@unb.ca (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: LANG@UNB.CA or SE@UNB.CA 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 ===============================================================================