[time-nuts] The TAI zero epoch of 1958

[Steve Allen]

On Thu 2019-10-24T16:51:53-0700 jimlux hath writ:
> I suspect that 1 Jan 1958 was chosen because it was in the middle of the
> International Geophysical Year (IGY) - perhaps the promulgators of TAI did
> it as part of IGY?

No, not really.  The original UK atomic time scale "Greenwich Atomic"
(GA) started its counting from a date mid 1955.  The original USNO
time scale A.1 put its match with UT2 at 1958-01-01T00:00 UT2(USNO)
which was 0.035 s different from the international value of UT2.
Other labs started their time scales from various different dates, and
some labs counted their scales using seconds which were of a different
duration than what became the SI second.

All of the scales were constructed by hand, hand integrations
(quadratic?  trapezoidal?  who knows, but Runge-Kutta is unlikely) of
the frequency offsets which had been measured by eye off of
oscillographs, and hand typing the tabulations.  BIH obtained all the
tabulations they could, and by hand they compensated for the time
offsets and the frequency offsets, and by hand they re-interpolated
them to points at 20h UT2 in order to combine them, and by hand they
tabulated the results, and by hand the printers transcribed them to be
published in Bulletin Horaire, and by hand they plotted them.  BIH
chose to set their original alignment with UT2 at 1958-01-01T20:00
UT2(BIH).  And in 1964 by hand BIH decided to re-align and re-set all
their atomic time scales as of 1961 and create the A3 that eventually
became TAI, and preserving only approximately a value equal to UT2 on
1958-01-01.  And in 1966 BIH re-interpolated all their atomic time
scale points from 20h UT to 0h UT.

I have transcribed many of these tabulations into digital form,
plotted them, and I can all the "by hand" because it is painfully
evident where there were typos and where some integration went awry
for several points before returning to match again.  Not possible to
see, of course, are cases where the numbers went awry and stayed
that way.

--
Steve Allen                    <sla at ucolick.org>              WGS-84 (GPS)
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