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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

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

   

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