[time-nuts] Absolute time accuracy pre-Cesium?

[paul swed]

If I am reading the paper correctly they used the moon as the reference. I
would have thought it was the sun. But the moon gives a very clean edge
definition. And now I know how the 770 came about. One more bit in the
knowledge bunker.
Thanks
Paul
WB8TSL

On Mon, Mar 25, 2019 at 8:03 PM Tom Van Baak <tvb at leapsecond.com> wrote:

> > Does anyone have a pointer to information about the absolute time
> > accuracy (not stability) that was available via PZT or other techniques
> > prior to the Cesium definition?  I'm doing a presentation and want to
> > show the evolution of accuracy.  My Google-fu has failed me in finding
> > anything pre-Atomic.
> >
> > Thanks!
> > John
>
> A nice example of how good astronomical timing was is how they calibrated
> cesium atomic time against astronomical time. The original 1958 paper is
> here:
>
>
> http://leapsecond.com/history/1958-PhysRev-v1-n3-Markowitz-Hall-Essen-Parry.pdf
>
> What you see there is that they spent 4(!) years and took 4(!) data points
> to precisely compare the best astronomical clock with the first cesium
> clock. It appears they got millisecond accuracy in their timings. Compared
> against the existing astronomical clock standard, the four measurements of
> cesium frequency were:
>
> 9 192 631 761
> 9 192 631 767
> 9 192 631 772
> 9 192 631 780
>
> Do the math: the mean is 9 192 631 770 +/- 8 Hz. That, literally, is where
> the magic 9192.631770 MHz cesium number and definition of the SI second
> comes from. That suggests the precision was 8 Hz / 9192631770 Hz, which is
> 8.7e-10, the equivalent of 75 us/day, or 2 ms/month, or 27 ms/year.
>
> As a practical matter a more accurate value of 9192631770 would have been
> useless because the earth is less stable than 8e-10 anyway. Here, for
> example, is how different UTC and UT1 would be depending on how the cesium
> SI second had been defined:
>
> http://www.leapsecond.com/pages/ut/cs9192-ut1-ani.gif
> http://www.leapsecond.com/pages/ut/
>
> In retrospect we would have had fewer leap seconds if they had chosen
> 9192631950 Hz instead of 9192631770 Hz. But at the time it wasn't a choice;
> it was just a measurement.
>
> /tvb
>
>
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