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 <t...@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 > > > _______________________________________________ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.