[LEAPSECS] the epoch of TAI, with no more doubt

[Steve Allen]

On Sun 2019-01-20T22:49:15+0000 Michael Deckers hath writ:
>  Yes, Danjon wants a zero epoch defined, but Markowitz opines that
>  this is of interest only ("uniquement") for the IAU who should
>  decide upon it.

Danjon also expresses concern that all the labs have separate
time scales and some have more than one version based on the
same cesium standard.  Of course he does, because Danjon is
titular head of BIH which should be the one place to make a
definitive atomic time scale.

Markowitz is president of IAU Comm 31 (Time), so of course
he says that IAU should decide the epoch.

> But this seems to state something about the inputs for the data
> reduction by the BIH. It does not say that the integrated atomic
> time scale of the BIH, the BIH output, has had a step at the time,
> or a step in rate, or does it?

Of course there was a time step.  The BIH had to deal with totally
hetergeneous data from an ever changing set of contributors.  Almost
every year for the BIH there was a systematic offset from the times of
other years.  But until the cesium standard there really is little
worth in the absolute values; the importance of the numbers in
Bulletin Horaire liese in seeing and understanding the differences
between contemporary time services.

(If one reads carefully through all of the issues of Bulletin Horaire
one may be lucky enough to find the writeups when they tabulated the
systematic offsets between the final times that had been published for
given ranges of years.  It is no surprise that the Stoykos never
bothered to do this given that all of their computations had been done
by hand.  Given that there are two shelf-feet worth of volumes of
Bulletin Horaire with pages packed with tables of numbers since the
1920s it is also no surprise that Guinot never undertook to use
electronic computers to re-reduce and re-publish a self-consistent set
of UT1 values.)

Getting back to the step, in 1961 there were atomic standards for
which there already were time scales.  Some of those were constructed
by the labs with the cesium standards, and some were constructed by
the BIH.  None of these agreed with each other, and none of them had
the value that Anna Stoyko chose as a reset value for
1961-01-01T20:00:00.

> Guinot must have known, but in 2004 he said (together with Arias)
> that the origin was J1958.0. Couldn't that mean that the change on
> 1961-01-01 was designed to have no effect on A3 as published by
> the BIH?

A3 begins 1961-01-01.  It does not exist before then.  Not even when
Guinot re-interpolated all the atomic time scales in Bulletin
Horaire ser J no 7 did he extend A3 before then.  He introduced his
final reconstruction of the old atomic data with
    It is therefore possible to construct, starting from an arbitrary
    common origin, scales of Atomic Time ...
By that 1966 publication Guinot had ceased to mention 1961-01-01, but
linear interpolation of his new A3 tabulation has the value -1.4123 s
on 1961-01-01T20, the same as had been used by Anna Stoyko when she
re-set all of the BIH atomic time scales.

Guinot also indicates that he retained the jump of 1.6 ms on
1962-01-01 in his new tabulation of A3.  These various tabulations
deserve to be plotted and examined closely for a step, especially
because 1962-01-01 was also the date of the final change in the
expression for the seasonal variation of UT2 - UT1.
https://www.ucolick.org/~sla/leapsecs/seasonal.html
That change should introduce a step of about 6 ms, and this subject is
not mentioned in any of the BIH writeups.

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