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    Re: What is a "Class A" sextant?
    From: Dave Walden
    Date: 2014 Jan 21, 12:59 -0800

    Here's some numbers:


    Journal of Navigation
    Journal of Navigation / Volume 1 / Issue 03 / July 1948, pp 266-268Copyright © The Royal Institute of Navigation 1948 DOI: http://dx.doi.org/10.1017/S0373463300027685 (About DOI), Published online: 18 January 2010
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    Table of Contents - 1948 - Volume 1, Issue 03 Author Index
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    Research Article
    A Critical Table for Sextant Corrections
    J. P. G. Worlledge
    It is the present usage by the National Physical Laboratory and the Trade to give, on its Certificate, the ‘constant’ (graduation and centring) errors of a sextant only at stated points on the scale. If these errors are of a magnitude which is worth while applying as a correction, this usage is not quite what the user requires. The N.P.L. Class A Test, issued for instruments reading directly to 10″ (or o′·2 in the case of decimal subdivision) allows combined graduation and centring error up to 40″ or 4 scale divisions of the micrometer or vernier; the Class B Test for instruments graduated to 20″ permits such error up to 2′ or 6 scale divisions. Thus, even with instruments that pass these tests, it may be worth the marine navigator's while to apply the error as a correction. What he wants to know, however, are the limits of such errors as are worth his while to apply. In other words, he requires a Critical Table of Corrections, of the type used in the Air Almanac and in certain mathematical tables by L. J. Comrie, by whom, it is understood, the method was introduced.
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