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This was some excellent research. I particularly
liked the part about the black ball used to
signal 1300 hours to ships at anchor.




Thomas Schmidt wrote:
> 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.
>
>
> Yes, it would. And indeed that seems to be what Harrison did.
> Thanks for supplying the keyword "chimney"; a Google search for
>               harrison clock star chimney
> finds these links, among others:
>
> http://www.clockswatches.com/papers/stars.htm
> http://www.harrisonclocks.co.uk/lincs.htm
> http://www.peg-gear-clock.com/wooden-gear-pendulum-clocks.html
>
> So Harrison used the disappearance of a star behind the neighbor's
> chimney to regulate his reference clock ("His best wooden-gear
> regulator was adjusted to an accuracy of 1 second per month"),
> and he then could compare his marine chronometers at leisure
> with this reference clock.
>
> Of course, you can't use the same star during the whole year,
> but with some extra care you can use a suitable succession of
> stars for year-round regulation of a chronometer
>
> Observatories also used stars to regulate their clocks; an
> advantage over oberving the Sun's transit is that the Sun
> tends to heat the instrument unevenly, distorting it slightly.
> Extra precautions must be taken to keep the relevant parts of
> the instuments in the shadow. This is not necessary when you
> are observing stars.
>
> On the other hand, mean time is defined with respect to the Sun,
> and an observation of the Sun directly supplies the correct time
> (after application of the equation of time). To use a star as
> a time indicator, you need to know its position, and its right
> ascension can only be measured by timing its transit with respect
> to mean time which again depends on the Sun. Once you have this
> position, you are independent of the Sun, but any error in the
> position will slightly affect your time measurement, and different
> stars will have different errors and thus give slightly inconsistent
> time readings.
>
>
>
>>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.
>
>
> The clocks were either set by a clockmaker or by a nearby
> observatory. They could also remain abord ship and be set
> with respect to a time signal supplied by a time ball or a
> noon cannon. The clocks would thus presumably be set to the
> port's local mean time (before the introduction of standard
> time zones).
>
> 
http://www.nmm.ac.uk/site/request/setTemplate:singlecontent/contentTypeA/conMuseumAsset/contentId/2051 
:
> | With increasing numbers of ships carrying marine chronometers and needing
> | to set their instruments to Greenwich Mean Time, the Observatory installed the
> | first Time Ball in 1833, signalling 1pm every day to enable the master to set
> | his chronometer before leaving the docks on the Isle of Dogs opposite
> | Greenwich. At 12.55pm, the Ball rises half way up the mast, at 12.58 it climbs
> | to the top, and at 1pm it drops; the instant it begins to move down signals the
> | precise time. The Ball is dropped at 1pm and not 12 because the
> | astronomers are too busy observing the sun transiting the meridian at noon
> | and setting their clocks. The present Ball dates to 1919.
>
>
> Bye,
>   Thomas
>
> --
>   -------------------------------------------------------------------
>   Thomas Schmidt                  e-mail:     schmidt@hoki.ibp.fhg.de
>
>

   

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