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    Re: New resource re ships' logs
    From: George Huxtable
    Date: 2010 Apr 6, 17:44 +0100

    I had written-
    "......A set of lunars was taken by
    Cook and King simultaneously with different instruments, then another set,
    with the instruments swapped over, a few minutes later; a good, scientific,
    procedure. But the observations are very discordant, by over 4 arc-min,
    even though the index errors differ by only 35". And it's the two observers
    that are 4' apart, not the two sextants. And in the time interval, of over
    4 minutes, there's been no change in lunar distance. And the differences in
    deduced longitude don't reflect the differences in lunar distance. What's
    going on? It looks as if there were serious transcription errors.......Am I
    missing something?  ....."
    And Douglas Denny answered-
    "If the analysis shows it is the observers and not the sextants; why be
    reluctant to accept the conclusion it really is the observers?
    Putting it down to transcription errors with such people  as Cook, of known
    care in transcribing everything he ever wrote, is the very last thing I
    would expect."
    Cook was long dead (and eaten) by the time that volume came to be written
    "I would suggest you are missing the possibility of what is called
    'personal error' between the two observers.   'Personal error' is mentioned
    in navigation texts as something to be considered and allowed for by the
    individual seeking perfection in their sights."
    Four arc-minutes is to, say the least, a lot when "personal error" is
    concerned. It would disqualify that observer from employment at sea; or it
    ought to.
     "You have perhaps the physicists approach in thinking that because the
    sextant can read down accurately to 30" of arc that the individuals are
    actually doing that. They might not be for a variety of reasons; some
    I have already mentioned irradiation as one potential source of error with
    extended sources.  'Lunars', especially whith Sun/Moon observations I would
    expect to be a worst case when be subject to the latter error. Different
    observers might be adjusting to different tangent coincidences with this
    Eye refractive errors could be another. In Cooke's time there were very few
    people who had spectacle correction. The observers might have had different
    refractive errors such as astigmatism giving differences in the readings."
    I hope Douglas will tell us whether he has actually looked at the table
    that I pointed to, with its inconsistencies in deduced longitude, before
    attributing those results to eye defects.
    contact George Huxtable, at  george@hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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