On Mon 2019-03-25T18:44:05-0400 John Ackermann N8UR hath writ:

https://www.ucolick.org/~sla/leapsecs/annastoyko.html

--
Steve Allen                    sla@ucolick.org              WGS-84 (GPS)
UCO/Lick Observatory--ISB 260  Natural Sciences II, Room 165  Lat  +36.99855
1156 High Street              Voice: +1 831 459 3046        Lng -122.06015
Santa Cruz, CA 95064          https://www.ucolick.org/~sla/  Hgt +250 m

John
http://adsabs.harvard.edu/full/1968JRASC..62..205T
indicates a timing accuracy of a few milliseconds was typical for the Calgary PZT.

Bruce

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/FFA10ED6A784AA1E39637CC0CA93B750/S0074180900036007a.pdf/div-class-title-some-advantages-and-disadvantages-of-a-photographic-zenith-tube-div.pdf
indicates a timing error of around 6 millisec

Bruce

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

On Mon 2019-03-25T16:54:28-0700 Tom Van Baak hath writ:

And it was a measurement which was performed during an interval when
everyone was surprised by the data they were seeing.  Around the
beginning of 1957 the rotation of the earth's crust shifted suddenly
as seen in the USNO plot of UT2 at the bottom of
https://www.ucolick.org/~sla/leapsecs/amsci.html

At the time that the paper giving 9192631770 was published nobody was
sure whether this was an actual change in the earth or some failure to
understand cesium frequency standards.  It was a few years before it
had become clear that earth rotation has a power spectrum of random
fluctuations.

Over time the BIH had the opportunity to watch cesium vs.  Ephemeris
Time for more years than the original papers.  In 1964 Anna Stoyko
found a value of the cesium frequency 9192631799 Hz w.r.t.  Ephemeris
Time  (Bulletin Horaire ser 6 no 7 p 186).

--
Steve Allen                    sla@ucolick.org              WGS-84 (GPS)
UCO/Lick Observatory--ISB 260  Natural Sciences II, Room 165  Lat  +36.99855
1156 High Street              Voice: +1 831 459 3046        Lng -122.06015
Santa Cruz, CA 95064          https://www.ucolick.org/~sla/  Hgt +250 m

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@leapsecond.com wrote:

For independent standards (not quite what you asked) I recall from
"The Science of Clocks and Watches" (a book with much technical info
if you're interested in these mechanical devices) that the most
accurate mechanical/pendulum clock was the Shortt Clock that used a
pendulum in a vacuum chamber for its standard. Mechanical clocks were
replaced by more stable electronic quartz crystal oscillators, and
then finally by atomic clocks.

Perhaps closer to your question: I recall in my readings about
clockmaker John Harrison (likely either in "The Quest for Longitude"
or Dava Sobel's "Longitude") that he would look from the edge of his
window at a particular star each night and note (while counting the
ticks he heard from his clock) the exact moment it would disappear
behind a nearby chimney, and knowing the Earth's rotation takes four
minutes and some (I forget) seconds off from a day, he used this to
calibrate and test the precision and accuracy of his long clocks. It
was suggested he could get within less than second with this method.
This was around age 21, so the year would be about 1714. Looking
online for PZT (photographic zenith tube), I didn't find much about
it, but it was surely first made a couple centuries after this.

The Sobel book (all about how Harrison won the Longitude prize) is
more a popular book and less technical, but "Quest" has many
mostly-technical articles, mostly about Harrison, as well as beautiful
photos of his clocks. One or two of the articles is by the man who
made (or made the parts for it, the story is complicated) the
one-second-in-100-days "Clock B" pendulum clock, built from Harrison's
writings and claims of just that accuracy in the book he wrote shortly
before his death.

On Mon, Mar 25, 2019 at 7:00 PM John Ackermann N8UR jra@febo.com wrote:

Hi:  I have a some what related question.

I'm just curious how far back in time do the current time scales extend ?  (Ie.  When was the first "second hack / synchronization"  that can be related to our current time.)

Thanks in advance for any answers.

Mark Spencer

mark@alignedsolutions.com
604 762 4099

All -- thanks much for all the great references!  I am giving the preso this afternoon (to a bunch of university space science students) so this will be a big help.  And it looks like there's a lot of great reading for when I have time to breathe.

Thanks again.
John

On Mar 25, 2019, 10:03 PM, at 10:03 PM, Ben Bradley ben.pi.bradley@gmail.com wrote: