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    Re: Coordinates on Cook's maps
    From: Alexandre Eremenko
    Date: 2007 Apr 20, 08:04 -0400

    
    Dear George,
    
    I am impressed with your list
    of error in Moscowitz papers.
    You would be an ideal referee for a scientific
    journal; did any of the two Navigation journals
    ever use your service?
    (I would be delighted if all my papers were refereed
    so carefully:-)
    
    Yesterday I re-read Moskowitz's paper on the
    first sextants, and found that it actually contains
    some relevant info to our recent discussion of
    the circle division.
    
    According to Moscowitz, Bird claimed 0.001
    (one over thousand) of an inch accuracy in his
    manual circle division. Thus he divided large sextants
    to 1' and smaller sextants to few minutes.
    Moscowitz carefully studied the arcs of surviving sextants
    but apparently made no attempts to verify
    Bird's accuracy claims.
    
    Now when the first dividing engine was invented,
    a sextant was divided on this engine,
    "in the presence of Longitude Comissioners"
    and then the sextant was brought to Bird for
    inspection. Bird conclusion were that
    
    a) Division error on this sextant does not exceed 15", and
    b) A larger error would not escape his scrutiny.
    
    Most interesting is the second statement.
    How exactly did Bird examine the arc, remains an
    enigma, at least for me.
    
    Thus Bird himself recognized that machine division
    is more accurate than his own by a factor of at least 4.
    
    I should add that this 15" arc division accuracy exceeds
    (is better than) that of many modern sextants,
    judging by their certificates.
    Cassens-Plath claims 10" and SNO-T claims 12".
    But this is in their advertisements, not verified
    by any independent testing.
    While the very first machine-divided sextant was
    found to be accurate to 15" by an independent
    testing, in fact by a competitor:--)
    
    And on testing in Kew observatory:
    I have a paper describing the testing arrangement;
    apparently this equipment was installed in 1867.
    And indeed, I've never seen certificates older
    than that. I don't know when this testing was discontinued,
    but the middle XX century British
    sextants already have certificates
    issued by their manufacturers rather than by
    the Kew Observatory.
    
    Alex.
    
    
    
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