[LEAPSECS] Toasting Unix timestamp 1234567890

[Steve Allen]

On Sat 2009-02-14T19:27:26 -0700, M. Warner Losh hath writ:

> In message: <20090215001754.GA3000 at ucolick.org>

>             Steve Allen <sla at ucolick.org> writes:

> : And as of 1970-01-01 the TA seconds were of a length systematically

> : shorter than the TAI seconds have been since 1977-01-01.

>

> Can you explain this a little bit more, or provide a pointer to the

> explanation?


Essen's cesium chronometer was running in Teddington in the first half
of 1955.  The news about this had spread around the time service
bureaus really fast.  At that date Markowitz at USNO was chair of IAU
Comm 31.  Before the IAU meeting in that summer Markowitz proposed
that the measurements of ephemeris time using the dual-rate moon
camera be compared with the cesium using calibrations with the time
broadcasts in the US and UK.  Before the next IAU meeting in 1958 the
US and UK teams had published their calibration of cesium with
ephemeris time, and thus was born the atomic second of
9192631770 +/- 20 cycles of the hyperfine transition.
The biographies based on interviews with Markowitz indicate that he
really was rushing to get this calibration done as soon as possible.

In 1957 Rudy Mossbauer identified the lattice effects of gamma ray
absorption (and got the Nobel in 1961 only barely after Caltech
realized they had better grant him tenure first).  By 1960 Pound
and Rebka had used the Mossbauer effect to measure the gravitational
redshift.

As I understand it, the intial versions of atomic time compiled by the
BIH simply used all of the contributed measurements of frequency from
all of the participating cesium chronometers.  Sometime around the
early 1970s the comparisons had got good enough to make it evident
that a lot of the cesium atoms were running faster than ones at sea
level because a lot of the chronomters were a mile up at NBS in
Boulder Colorado or at PTB in Bavaria.

So in order to be specific about the rate of TAI, it was decided
that beginning on 1977-01-01 the frequency of TAI would be reduced
by correcting for the height of the chronometers above the geoid.
The rate difference was 1e-12.

At the 1976 IAU meeting it had finally been decided that the basis for
all astronomy had to abandon Newton and adopt Einstein.  I think that
it was at this meeting where IAU Comm 31 mentioned the impending rate
change in TAI, but I'll have to check.

I would love to find records of discussions among the astronomers
about the impact of changing the rate of TAI as compared with the rate
of ephemeris time.  In any case it is evident that the uncertainty of
the initial measurement of the cesium resonance was more like 2e-9,
so this geoid-shift of the rate of TAI was well within that range.

Furthermore, extrapolating the rate difference of old TAI and new TAI
back to the earliest records of eclipses gives a difference of around
0.1 second, which is clearly below the measurement uncertainty of any
sort of calibration that could be made at the dawn of human writing,
so it's not like any result could be affected by the change of TAI.

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