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From: phil@brl-smoke.ARPA (Phil Dykstra <phil>)
Newsgroups: sci.astro
Subject: Re: Coordinated Universal Time
Message-ID: <5335@brl-smoke.ARPA>
Date: Sun, 9-Nov-86 03:03:07 EST
Article-I.D.: brl-smok.5335
Posted: Sun Nov  9 03:03:07 1986
Date-Received: Sun, 9-Nov-86 07:09:05 EST
References: <5940@decwrl.DEC.COM> <840@tpvax.fluke.UUCP> <49@vianet.UUCP>
Reply-To: phil@brl.arpa (Phil Dykstra <phil>)
Organization: Ballistic Research Lab (BRL), APG, MD.
Lines: 32

[Another late addition...]

Universal Time: the basis of civil timekeeping; based on
 observations of diurnal motion.  Several flavors:

U.T.0: determined directly by observation.
U.T.1: U.T.0 corrected for variations in the meridian of the
       observer due to observed motion of the geographic poles.
U.T.2: U.T.1 corrected for mean seasonal variations in the
       earth's ratation rate.
U.T.C: A stepped atomic system of time which is kept close to
       U.T.1 by the introduction of discontinuities.

In theory U.T.0 -> U.T.2 are continuously variable and can
only be determined precisely after the fact (particularly U.T.2
which requires several months of observation).  This is
inconvenient to say the least so U.T.C runs at a constant
rate and inserts step discontinuities to keep in sync.
The rules are (with a couple of exceptions of course):

Prior to 1972: 0.1 second jumps when 0.1 second discrepancies occur.
Post 1972: 1.0 second jumps at 0.7 second discrepancies.  These
 jumps now only occur at 0h U.T. on the first day of a month.

The difference between U.T.C and U.T.1 is claimed to be sent
in encoded form by most of the time standard stations.

For a splendidly complex albeit very precise discussion of
time systems, see the explainatory section of "The American
Ephemeris and Nautical Almanac."

- Phil