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Patrick Stanistreet wrote:

> Just curious but when Harrison created his chronometers
> there had to be some standard against which they were
> set.  Of course land based clocks were around but even
> those clocks had to be set against some other standard.
> I would guess that the ultimate standard at that time
> would have been astronomical.

Yes...


> But still what exactly
> was used to arrive at accuracy of a few seconds.
> Was the land based authority setting GMT associated
> with the Almanac office?

At Harrison's time (17xx) there was no GMT (at least not
in the sense of a zone or even universal time) and no
associated authority.


> Assuming timing of a star one ends up with a sidereal
> clock but when did land based clocks achieve sufficient
> accuracy to time a star over 24 or more hours so as
> to differentiate between sidereal and solar time?

Each observatory regularly observed the transits of the
sun, corrected for the equation of time and compared the
result with the observatory's clock. The clock was usually
never reset in order not to disturb its mechanism which
would otherwise move irregularly for a while afterwards.
The corrections were mathematically added to subsequent
clock readings which were thus calibrated to local mean
time. It was also known how many seconds the clock gained
or lost each day, so the correction could be interpolated
accordingly. A good clock did not necessarily run at the
correct rate, some error was allowed, but the error of a
good clock had to be steady and predictable.

I don't know too much about the history of clock accuracy,
but my impression is that by the 18th or 19th century a
competent astronomer with a good clock and a fresh set of
clock corrections may have been able to time an event at
the level of a fraction of a second, routinely at least at
the level of seconds.

I seem to remember dimly that Harrison observed the sun in
order to calibrate his clocks, but I don't have a source
handy.


> Any book recommendations that cover this topic in detail?

All of this has nothing to do with GMT in the sense of a standard
time which was introduced in the late 1800s. If 'this topic' is
the introduction of time zones and standard times, then this would
probably be covered in the recent

Clark Blaise
Time Lord : Sir Sandford Fleming and the Creation of Standard Time
Vintage, 2002


Bye,
  Thomas

--
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  Thomas Schmidt                  e-mail:     schmidt@hoki.ibp.fhg.de

   

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