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    Re: New resource re ships' logs
    From: Douglas Denny
    Date: 2010 Apr 5, 23:35 -0700

    George wrote:
    "......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? ....."

    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.

    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.

    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 physiological.

    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 alone.

    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.

    Douglas Denny.
    Chichester. England.
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