Article 9058 of comp.protocols.time.ntp: Path: matra.meer.net!news1.best.com!nntprelay.mathworks.com!europa.clark.net!infeed1.internetmci.com!newsfeed.internetmci.com!in3.uu.net!198.3.192.201!diamond.waii.com!not-for-mail From: "Marc Brett" Newsgroups: comp.protocols.time.ntp Subject: Re: UTC introduced when? Date: 21 Jul 1997 13:32:29 GMT Organization: Western Geophysical, Div. of Western Atlas Int'l, Houston, TX Lines: 307 Sender: Marc Brett Distribution: inet Message-ID: <5qvodd$1ar8$1@mail1.wg.waii.com> References: <5qp29v$on2$1@shade.twinsun.com> NNTP-Posting-Host: rgs0.london.waii.com X-Newsreader: TIN [UNIX 1.3 unoff BETA 970424; IP12 IRIX 5.3] Xref: matra.meer.net comp.protocols.time.ntp:9058 L. F. Sheldon, Jr. wrote: > On 18 Jul 1997, Paul Eggert wrote: > > James Youngman writes: > > > > > When was UTC introduced? > > > > The current UTC system was introduced at 1972-01-01 00:00:00 UTC. > > > > The previous system was also called `UTC' but it operated differently; > > each day had 24*60*60 seconds, but periodically the clock > > frequency was adjusted, so that civil seconds in one period would have a > > slightly different length from civil seconds in the next period. > > > > > Before that, did GMT have leap-seconds with respect to TAI? > > > > No. The first leap second was 1972-06-30 23:59:60 UTC. > OK, I'll do it. I was hoping somebody else would, but . . . . > I am a ROF who (still) thinks "GMT" is the base of All That Is True. > What is "TAI" (besides a strange dance I see young folk doing on > occasion)? GMT is as obsolete as the British Empire. A good idea at the time, but completely eclipsed by modern realities. Depending on who you talk to, GMT might mean either UTC, UT, or UT2. Best not to use it at all. The following 2 articles (written by others, I hasten to add) should explain things better than I ever could. [MB] Here's one repost... ------------------------------ RISKS-LIST: Risks-Forum Digest Wednesday 14 May 1997 Volume 19 : Issue 14 FORUM ON RISKS TO THE PUBLIC IN COMPUTERS AND RELATED SYSTEMS (comp.risks) ACM Committee on Computers and Public Policy, Peter G. Neumann, moderator ------------------------------ Date: Mon, 28 Apr 1997 09:36:11 +0100 From: "Peter B. Ladkin" Subject: A definitive clarification of time measurement Peter Ladkin, Universitaet Bielefeld, Technische Fakultaet, Postfach 10 01 31, D-33501 Bielefeld, Germany http://www.rvs.uni-bielefeld.de +49 (0)521 106-5326 (From John Laverty via Peter Ladkin) In Britain, the National Physical Laboratory is the canonical site for questions concerning standard time, and the Royal Greenwich Observatory (which is now in Cambridge) refers all questions there. I talked to Dr. John Laverty of Time and Frequency Services, CETM (jrl@taf.npl.co.uk), who kindly supplied me with the following account of time standards. The position of the sun in the sky has been used as a basis for measuring time for many centuries. One simple example is that 12 noon in local solar time occurs when the sun is directly `overhead'. However, local solar time does not provide as uniform a time scale as that based more implicitly on the rotation of the Earth about its axis. The Earth's orbit is elliptical and its axis tilted, so that the actual position of the sun against the background of stars appears a little ahead or behind the expected position. The accumulated error varies from 14 minutes slow in February to 16 minutes fast in November. These effects can be predicted, and a more uniform timescale can be established on the basis of a hypothetical 'mean' sun that moves with uniform speed across the sky. Greenwich Mean Time (GMT) is probably the most well known example of such a time scale: GMT is the local time on the Greenwich meridian based on the position of a hypothetical mean sun. The need to coordinate time measurement and agree on a standard time was driven by improved communications, particularly by the railways, when the differences in the local time at different locations became very noticeable. Greenwich Mean Time was established as a world time standard at the International Meridian Conference in 1884. The time scales in active use today are Universal Time (UT), Coordinated Universal Time (UTC) and International Atomic Time (TAI). They are described below along with some of the reasons for their use. Greenwich Mean Time (GMT) and Universal Time (UT) are very closely related. Before 1925 January 1, the twenty four hour GMT day was taken to commence at noon, while since that date the convention has been for the GMT day to begin at midnight. The term Universal Time (UT) was introduced in 1928 by astronomers to denote GMT measured from Greenwich Mean Midnight, to be clear about the convention for the start of the day. Now there are actually three different definitions: UT_0, UT_1, UT_2 (using underscores to denote subscripting). UT_0 is based on `direct' observation of the earth's rotation on the prime meridian, UT_1 is adjusted to account for the small movements of the Earth relative to the axis of rotation (polar variations), and UT_2 adjusts for seasonal variations. The maximal difference between all three is of the order of a few tens of milliseconds. The term `UT' thus crudely refers to all three for large granularities, and for finer granularity, the term is ambiguous and one needs to specify which of the UT's one is referring to. Starting in the 1930's with the development of quartz crystal oscillators, but particularly in the 1950's with the introduction of atomic clocks, better measurements have been available. As a consequence of studies comparing atomic clocks and astronomical observations, it was realised that atomic clocks offered a more much more stable time standard than one based on the rotation of the Earth. In 1967, the SI second was redefined as "the second is the duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom". The international time scale based on the SI second is International Atomic Time (TAI). TAI was synchronised with UT at the beginning of 1958. It is a more stable time scale than UT, but UT and TAI naturally drift apart because they are based on different principles. Universal Coordinated Time (UTC) is a compromise between TAI and UT and was established in its current form on 1 Jan 1972. It uses the SI definition of the second, but introduces leap seconds by convention in order that the difference between UTC and UT shall never be more than one second. There have been 20 leap seconds introduced since January 1972; the first at 1 July 1972. The 21st leap second is scheduled for 1 July 1997. So UTC and TAI run in lockstep, but with conventional separation, which is now 30 seconds and will become 31 seconds on 1 July 1997 (By the beginning of 1972 TAI and UT had drifted apart by 10 seconds from the `synchronisation' point at the begining of 1958, which accounts for the extra 10 seconds in addition to the leap seconds). UTC is the current world time standard, as indicated by the recommendations of the International Telecommunications Union (ITU) for example. There are some 50 or so centers around the world which measure TAI/UTC using commercial atomic clocks, with just a few laboratory based 'primary' caesium standards which are are able to measure the time with greatest accuracy. The PTB in Germany has the distinction of having the longest running and most reliable primary caesium standards. The NPL, having developed the first caesium atomic clock in the 1950's, is currently working on the `next generation' standard based on the caesium fountain method demonstrated at the LPTF in Paris. There are other primary caesium standards at NIST in the US, NRC in Canada, CRL in Japan and in Moscow. The institute responsible for maintaining TAI and UTC is the BIPM in Paris, and the decision as to when to introduce leap seconds is made by the IERS, also in Paris, who measure UT also. The Royal Greenwich Observatory (RGO) no longer maintain their own time standard. It is recognised that GMT and UT are equivalent, so that now the IERS provide the information necessary to determine GMT. However, the appropriate definition of UT should be used instead of GMT if the distinction between UT_0, UT_1 and UT_2 is important for a given application. The time standards that are so carefully measured by astronomers and metrologists need to be made available if they are to be of use, and radio time signals are one of the most common ways of making UTC available. In Western Europe, NPL broadcasts the UK time on 60 kHz from the BT Radio Station at Rugby (call sign MSF), and similarly, the PTB broadcasts Central European Time from Frankfurt (call sign DCF77) on 77.5 kHz. There are similar transmitters operated by other countries around the globe. The other common means of accessing standard time is through the Global Positioning System (GPS) navigation system, where accurate position and time information allow a receiver to calculate its position from the times of flight (at the speed of light) of signals from a number of GPS satellites. The GPS system was developed, as its name implies, for positioning, but a welcome spin-off is accurate time. The GPS time signals offer high-accuracy UTC (one microsecond time accuracies are readily achievable) and global coverage, but the LF radio time signals, although limited to a range of typically 1500 km, offer the advantage of broadcasting the local time including summertime changes (to millisecond time accuracies). ------------------------------ [MB] And another... ------------------------------ In article , Paul Boven wrote: TAI = International Atomic Time. Defined by about a dozen atomic clocks distributed worldwide. UTC = Coordinated Universal Time. Differs from TAI by an integral number of seconds. When needed, leap seconds are introduced in UTC to keep the difference between UTC and UT less than 0.9 s. UTC was introduced in 1972. UT = Universal time. Defined by the Earth's rotation, and determined by astronomical observations. This time scale is slightly irregular. There are several different definitions of UT, but the difference between them is always less than about 0.03 s. Usually one means UT2 when saying UT. UT2 is UT corrected for pole wandering, and seasonal variations in the Earth's rotational speed. ET = Ephemeris Time. Was used 1960-1983, and was replaced by TDT and TDB in 1984. For most purposes, ET up to 1983 Dec 31 and TDT from 1984 Jan 1 can be regarded as a continuous time-scale. TDT = Terrestial Dynamical Time. Used as a time-scale of ephemerides from the Earth's surface. TDT = TAI + 32.184. Formerly called ET, Ephemeris Time. TDB = Barycentric Dynamical Time. Used as a time-scale of ephemerides referred to the barycentre of the solar system. Differs from TDT by at most a few milliseconds. TT = Terrestial Time. Used instead of TDT or TDB when the difference between them doesn't matter. GMT = Greenwich Mean Time. It's ambiguous, and is now used (although not in astronomy) in the sense of UTC in addition to the earlier sense of UT. Prior to 1925, it was reckoned for astronomical purposes from Greenwich mean noon (12h UT) TDT = TAI+32.184s ==> UT-UTC = TAI-UTC - (TDT-UT) + 32.184s Starting at TAI-UTC TDT-UT UT-UTC 1972-01-01 +10 +42.23 -0.05 1972-07-01 +11 +42.80 +0.38 1973-01-01 +12 +43.37 +0.81 1973-07-01 " +43.93 +0.25 1974-01-01 +13 +44.49 +0.69 1974-07-01 " +44.99 +0.19 1975-01-01 +14 +45.48 +0.70 1975-07-01 " +45.97 +0.21 1976-01-01 +15 +46.46 +0.72 1976-07-01 " +46.99 +0.19 1977-01-01 +16 +47.52 +0.66 1977-07-01 " +48.03 +0.15 1978-01-01 +17 +48.53 +0.65 1978-07-01 " +49.06 +0.12 1979-01-01 +18 +49.59 +0.59 1979-07-01 " +50.07 +0.11 1980-01-01 +19 +50.54 +0.64 1980-07-01 " +50.96 +0.22 1981-01-01 " +51.38 -0.20 1981-07-01 +20 +51.78 +0.40 1982-01-01 " +52.17 +0.01 1982-07-01 +21 +52.57 +0.61 1983-01-01 " +52.96 +0.22 1983-07-01 +22 +53.38 +0.80 1984-01-01 " +53.79 +0.39 1984-07-01 " +54.07 +0.11 1985-01-01 " +54.34 -0.16 1985-07-01 +23 +54.61 +0.57 1986-01-01 " +54.87 +0.31 1986-07-01 " +55.10 +0.08 1987-01-01 " +55.32 -0.14 1987-07-01 " +55.57 -0.39 1988-01-01 +24 +55.82 +0.36 1988-07-01 " +56.06 +0.12 1989-01-01 " +56.30 -0.12 1989-07-01 " +56.58 -0.40 1990-01-01 +25 +56.86 +0.32 1990-07-01 " +57.22 -0.04 1991-01-01 +26 +57.57 +0.61 1991-07-01 " +57.94 +0.24 1992-01-01 " +58.31 -0.13 1992-07-01 +27 +58.72 +0.46 1993-01-01 " +59.12 +0.06 1993-07-01 +28 +59.55 +0.63 1994-01-01 " +59.98 +0.20 1994-07-01 +29 +60.38 +0.80 1995-01-01 " +60.78 +0.40 1995-07-01 " +61.2 0.0 1996-01-01 +30 +61.6 +0.6 1996-07-01 " +62.0 +0.2 1997-01-01 " +62.4 -0.2 1997-07-01 +31 +62.8 +0.4 For the latest status concerning leap seconds in UTC, send e-mail to adsmail@tycho.usno.navy.mil with a Subject: line of 'leap' and no text. You will receive in reply a list of past and provisional future leap seconds. -- ---------------------------------------------------------------- Paul Schlyter, Swedish Amateur Astronomer's Society (SAAF) Grev Turegatan 40, S-114 38 Stockholm, SWEDEN e-mail: pausch@saaf.se paul.schlyter@ausys.se paul@inorbit.com WWW: http://spitfire.ausys.se/psr -- updated daily! LEAP SECONDS ARE ANNOUNCED IN TIME SERVICE ANNOUNCEMENT (TSA) SERIES 14 AS SOON AS NOTIFICATION IS RECEIVED FROM THE CENTRAL BUREAU OF THE INTERNATIONAL EARTH ROTATION SERVICE (IERS). SUCH NOTIFICATION OCCURS 8 TO 10 WEEKS IN ADVANCE. IF A LEAP SECOND IS TO OCCUR, THE PREFERRED DATES ARE 1) DECEMBER 31 OR 2) JUNE 30. IF REQUIRED, A LEAP SECOND MAY BE ANNOUNCED FOR EITHER MARCH 31 OR SEPTEMBER 30. ALL LEAP SECONDS OCCUR AT 23H 59M 60S UTC. ------------------------------ -- Marc Brett +44 181 560 3160 Western Atlas Marc.Brett@waii.com 455 London Road, Isleworth FAX: +44 181 847 5711 Middlesex TW7 5AB England