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

[Ben Bradley]

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 at febo.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
>
>
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