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    Re: Sextant Accuracy and anomalous dip
    From: Trevor Kenchington
    Date: 2003 Mar 23, 15:56 -0400

    Fred Hebard wrote:
    
     > This leads us back to Jared's question, where they did seem to be
     > establishing their observatory in a random spot.  However, perhaps it
     > wasn't too difficult to measure the distance to a known spot in the
     > vicinity, or perhaps the spots were known and had been located by
     > observing the moons of Jupiter or some such.
    
    
    Is it possible that, contrary to the written account, they were making
    their observations not at some convenient random point but at either
    established survey triangulation stations that had already been tied in
    by geodetic surveyors or benchmarks surveyed relative to such stations?
    That might explain why they ended up at forts: Trig stations need to be
    on hilltops with good visibility all round, which tend to also be
    favoured spots for forts, while it helps to have the trig station on
    government property. Streets in the middle of towns, which the
    cable-ship navigators also found themselves in, are likely places for
    benchmarks.
    
    The best place to check the going of your chronometers would obviously
    be a proper time observatory. If that isn't possible (and it would
    rarely be -- time observatories being few and far between), your own
    observations for time from a point of known location would be the next
    best thing, with stations established by geodetic survey being the only
    ones known with adequate precision for cable-laying purposes. But to try
    checking the going by observation without knowing where you are, and
    without setting up an observatory to determine the time (which the cable
    ship's personnel do not seem to have done), appears hopeless.
    
    I note George's suggestion that they found time from cable signals and
    hence the precise location of the cable hut. But if, as George suggests,
      the written account relates to checking the chronometers following a
    cable break, using the known location of the cable hut and observations
    for time, why were they taking the observations at forts and on town
    streets, rather than alongside cable huts?
    
    Unless all they were checking was the rate of going of the chronometers,
    not the accumulated error, the written version seems to be deficient in
    at least one detail: It seems that they must either have known the time
    (from cable signals) or else their position (from prior geodetic survey
    or from observations using time from cable signals). And if they knew
    the time precisely, they would not have been taking observations to
    check it. Hence, I suggest that (by one means or another) they knew
    exactly where they were when taking their observations.
    
    
    Trevor Kenchington
    
    
    --
    Trevor J. Kenchington PhD                         Gadus@iStar.ca
    Gadus Associates,                                 Office(902) 889-9250
    R.R.#1, Musquodoboit Harbour,                     Fax   (902) 889-9251
    Nova Scotia  B0J 2L0, CANADA                      Home  (902) 889-3555
    
                          Science Serving the Fisheries
                           http://home.istar.ca/~gadus
    
    
    

       
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