NavList:

Hi Patrick:

I have a Dent Meridian Instrument - Dipleidscope with an 1843 patent date.  It was used to very accurately determine when the Sun (or a star) transited the meridian.  See my web page:  http://www.pacificsites.com/~brooke/Dent.shtml

See "A Brief History of Early Navigation" by Dava Sobel at The Legacy of Transit web page:
http://techdigest.jhuapl.edu/td1901/index.htm

Have Fun,

Brooke Clarke, N6GCE
http://www.PRC68.com


Patrick Stanistreet wrote:

I think this is along the lines of what I have been
wondering.  Will have to investigate measuring the transit
of the sun as a method for setting a clock. From a PBS Nova
video Harrison used the edge of a windowpane and a nearby
chimney to observe a star from night to night. Presumably
timing the process would give accurate sidereal time.
Once Harrisons clocks were being manufactured and distributed
to the fleet each clock would have to be set to some
standard at least initially perhaps by Harrison or his
family.  Some standard time of day/night to zero the clock
and start it running.  Also after repairs the time would
have to be reset.  Could it be that each clockmaker
independently set their own clocks and that any ship's
clock was somewhat relative in time.  I would guess not
as to take sights one would need to use a astronomical
almanac using some time standard of the era.

Thomas Schmidt wrote:

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