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    Re: How was GMT originally established ?
    From: Patrick Stanistreet
    Date: 2004 Jan 28, 16:38 -0800

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

       
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