System.Time.Clock Design Issues

[Gregory Wright]

On Feb 4, 2005, at 7:00 AM, Ashley Yakeley wrote:

> In article <87vf9835x7.fsf at sefirot.ii.uib.no>,
>  Ketil Malde <ketil+haskell at ii.uib.no> wrote:
>
>> Isn't UTC prior to the epoch defined as UT1?  I seem to remember
>> reading this, but couldn't find the reference atm.
>
> I believe civil time prior to 1972 (which is more a date of 
> establishing
> UTC than an epoch) was defined as UT1. It was called "Greenwich Mean
> Time" in those days.
>

Actually, before 1972 civil time was complicated.  "Greenwich Mean 
Time",
referring to the timescale used in celestial navigation, was basically 
UT1.

Civil time in the United States from the 1950s until the introduction 
of UTC
was determined by the time signal distributed by the Master Clock at the
US Naval Observatory, with frequent 20 millisecond adjustments (and a 
few
60 millisecond adjustments) to keep the civil timescale close to UT1. 
The
time signal was broadcast by the radio station WWV.  A table
of these adjustments can be found in the revised edition of the 
Explanatory
Supplement to the Astronomical Almanac.

In essence, civil time in the US before the introduction of UTC had 
seconds
which were usually 1000 SI milliseconds long, some that were 1020 SI 
milliseconds
and a few that were 1060 SI milliseconds.

The Naval Observatory Master Clock kept a timescale that closely 
approximated
what was to become TAI, i.e., it kept SI seconds and did not have a 
phase discontinuity
when TAI was introduced in 1972.


Best Wishes,
Greg



> -- 
> Ashley Yakeley, Seattle WA
>
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