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    Re: Sextant calibration.
    From: George Huxtable
    Date: 2007 Apr 21, 14:32 +0100

    I wrote, in NavList 2654, about Frank's account of Mendoza's proposal, for
    calibrating sextant angles, that I-
    
    haven't yet understood, from
    | Frank's explanation, how it could be used to pin down where, in what part
    of
    | the arc, such errors occurred.
    |
    | Frank wrote-
    | "Suppose, for example, I have steeples within a  few degrees of
    || the cardinal points of the compass. Call them N, E, S, and W.  Suppose I
    | measure
    || the angle NE and find that it's 90d 10'. Then I measure angle  ES, SW,
    and
    | WN
    || and find that they are 89d 05', 91d 12', and 89d  31'. From these
    || measurements, I can deduce that there is an arc error at 90  degrees
    | of -0.5' of arc
    || --because the measured angles don't add up to 360  degrees."
    |
    | For the life of me, I can't see how that deduction follows. Why couldn't
    | that error of -0,5' of arc be elsewhere than at 90 degrees? Am I missing
    | something obvious here? How did Mendoza explain it?
    
    Ah. Second thoughts are usually best, as in this case. Now I see what Frank
    (and Mendoza) were getting at. Sorry to have been so slow on the uptake.
    
    And yet, I ask, would it really work in practice, as described? To test the
    instrument, for angles in the region of 90 degrees, called for a profusion
    of steeples (or other landmarks). Presumably, to define a common sextant
    error for all those angles, they all need to have about the same angular
    separation, say between 85 and 95 degrees. Unlikely, then, to find four such
    steeples, or other objects, in distant view, meeting such tight constraints
    in the angle between them, with nothing intervening: not even in Wren's
    London. Perhaps, from the middle of the Pool of London...
    
    One might do better if there were many identifiable trees, or buildings,
    around the shore of a large lagoon, with a vessel at short-anchor in the
    centre. Or using a boat-party, sent out to plant poles, with flags, in the
    sand, at appropriate places, about 90 degrees apart. At least, in those
    cases you could be sure that the angles were all in the same plane, so must
    add to 360 degrees.
    
    And then, that's found the precision of the sextant, measured absolutely,
    with no reference to any other standard. But just for one restricted segment
    of the arc, near to 90 degrees.
    
    Then you have to do something similar again for all the other sextant
    angles. To check the region of the sextant near to 15 degrees, by a similar
    methods, would call for 24 such landmarks, more or less equally spaced,
    around the circle! I can see ways of short-cutting that operation, using
    other parts of the sextant scale, previously determined, (such as that at 90
    degrees) but at the risk of a buildup of errors.
    
    In principle, then, it could work, but I have severe doubts about the
    practicality. What does Mayer say about that?
    
    George.
    
    contact George Huxtable at george@huxtable.u-net.com
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    
    
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