[fitsbits] Re: leap second alert

Wed Dec 15 07:29:36 EST 1999

In article <837ih3$n62 at newton.cc.rl.ac.uk>,
Pat Wallace <ptw at rlsaxps.bnsc.rl.ac.uk> wrote:

> In article <3856FDF4.A4796C01 at oblivion.nothing.com> Skywise <into at oblivion.nothing.com> writes:
> >Many types of time have been thrown about by their abbreviations (TT, TAI,
> >TCG, TDB, etc...) and I was wondering if there's an online resource where I
> >could learn all about them.
> 
> Try http://tycho.usno.navy.mil/systime.html

A summary is also given here:

=====================================================================

TAI = International Atomic Time. Defined by about a dozen atomic clocks
      distributed worldwide. TAI-UT1 was approximately 0 on 1958 Jan 1.

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. We also have
      UT1 (not corrected for seasonal variations) and UT0 (not corrected
      at all).

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        "      +61.20    -0.02
1996-01-01       +30     +61.63    +0.55
1996-07-01        "      +62.0     +0.2
1997-01-01        "      +62.4     -0.2
1997-07-01       +31     +62.8     +0.4
1998-01-01        "      +63.3     -0.1
1998-07-01        "      +63.7     -0.5
1999-01-01       +32     +64.1     +0.1

Earlier, one could get the latest status concerning leap seconds in
UTC by sending e-mail to adsmail at tycho.usno.navy.mil with a Subject:
line of 'leap' and no text. You would then receive in reply a list of
past and provisional future leap seconds.  However, this site seems
to not have been updated since 1994.  More up-to-date information
can be obtained among these sites (thanks to Matthew.Healy at yale.edu
for providing these URL's):

    http://tycho.usno.navy.mil/gps_datafiles.html

    ftp://maia.usno.navy.mil/ser7/tai-utc.dat

    ftp://tycho.usno.navy.mil/pub/series/ser14.txt

    http://maia.usno.navy.mil/

    ftp://hpiers.obspm.fr/iers/bul/bulc/BULLETINC.GUIDE

    ftp://maia.usno.navy.mil/ser7/deltat.preds

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