Article 4009 of comp.protocols.time.ntp: Path: mri-gw!psinntp!psinntp!psinntp!rutgers!sgigate.sgi.com!olivea!charnel.ecst.csuchico.edu!nic-nac.CSU.net!usc!howland.reston.ans.net!EU.net!sunic!news.kth.se!admin.kth.se!merope.saaf.se!electra.saaf.se!not-for-mail From: pausch@electra.saaf.se (Paul Schlyter) Newsgroups: comp.protocols.time.ntp Subject: Re: When the leap second happens Message-ID: <2uhevi$anf@electra.saaf.se> Date: 25 Jun 94 14:29:06 GMT References: <1994Jun23.204749.29028@integrity.uucp> <2ue2tb$hum@cruella.ee.pdx.edu> Distribution: inet Organization: Svensk Amat|rAstronomisk F|rening Lines: 115 NNTP-Posting-Host: electra.saaf.se In article <2ue2tb$hum@cruella.ee.pdx.edu>, Roland Kwee wrote: > Does anybody know of a list of the actual leap-seconds from the past > somany years? See below.... > Planned leap-seconds? Leap seconds are never planned more than a few months in advance. They're inserted as needed. They can also be removed, but this has yet not happened. > Would it not be better to count days using Julian Day (days since > 1/1/4713BC) and time of day using seconds since midnight? This will > be completely unambiguous and precise. Except that a few days will be 86401 seconds long, while a few others may be 86399 seconds.... Anyway, here's the list showing the relation between the different time scales: -------------------------------------------------------------------------- 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. 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+32.18s ==> UT-UTC = TAI-UTC - (TDT-UT) + 32.18 Starting at TAI-UTC TDT-UT UT-UTC 1972-01-01 +10.00 +42.23 -0.05 1972-07-01 +11.00 +42.80 +0.38 1973-01-01 +12.00 +43.37 +0.81 1973-07-01 -"- +43.93 +0.25 1974-01-01 +13.00 +44.49 +0.69 1974-07-01 -"- +44.99 +0.19 1975-01-01 +14.00 +45.48 +0.70 1975-07-01 -"- +45.97 +0.21 1976-01-01 +15.00 +46.46 +0.72 1976-07-01 -"- +46.99 +0.19 1977-01-01 +16.00 +47.52 +0.66 1977-07-01 -"- +48.03 +0.15 1978-01-01 +17.00 +48.53 +0.65 1978-07-01 -"- +49.06 +0.12 1979-01-01 +18.00 +49.59 +0.59 1979-07-01 -"- +50.07 +0.11 1980-01-01 +19.00 +50.54 +0.64 1980-07-01 -"- +50.96 +0.22 1981-01-01 -"- +51.38 -0.20 1981-07-01 +20.00 +51.78 +0.40 1982-01-01 -"- +52.17 +0.01 1982-07-01 +21.00 +52.57 +0.61 1983-01-01 -"- +52.96 +0.22 1983-07-01 +22.00 +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 +23.00 +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 +24.00 +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 +25.00 +56.86 +0.32 1990-07-01 -"- +57.22 -0.04 1991-01-01 +26.00 +57.57 +0.61 1991-07-01 -"- +57.94 +0.24 1992-01-01 -"- +58.31 -0.13 1992-07-01 +27.00 +59.0 +0.2 1993-01-01 -"- +59.0 +0.2 1993-07-01 -"- +59.5 -0.3 1994-01-01 +28.00 +60.0 +0.2 -- ---------------------------------------------------------------- Paul Schlyter, SAAF (Swedish Amateur Astronomer's Society) Nybrogatan 75 A, S-114 40 Stockholm, Sweden InterNet: pausch@saaf.se psr@ausys.se