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    Re: Non-adjustable sextant error calibrations
    From: George Huxtable
    Date: 2004 Feb 3, 09:57 +0000

    Fred Hebard wrote, about using angles between two stars to calibrate a sextant.-
    
    >The interstellar distances need to be corrected for
    >the effect of refraction.  Several texts present tables of interstellar
    >distances, but they are simply wrong, as they have not been corrected
    >for refraction, and other effects.
    
    They are not "simply wrong", Fred, it's all that they can possibly do,
    because only the observer knows what is his position on the Earth's
    surface. And only by knowing that position, or by measurement, can the
    observer determine the altitude of a star. And only by knowing that
    altitude can the refraction be allowed for. No tables can do that job for
    you.
    
    Fred went on-
    
    >Not having tried it, I have thought an advantage of the terrestrial
    >angles method, if one has access to a good transit or theodolite, would
    >be that the sextant could be clamped in a tripod, which could make the
    >angle determinations much more accurate.  That's not possible with
    >celestial objects due to the rotation of the earth, unless the sextant
    >were mounted in what astronomers refer to as an equitorial mount for a
    >telescope.
    
    Our museum for the history of science, here in Oxford, has examples of such
    sextants, on an adjustable tripod stand so that they can be set up to
    rotate about a polar axis. The sextant frame could be clamped at such an
    angle that two celestial objects could be brought into view and would
    remain there. The word used to describe this instrument is "pillar
    sextant". Unfortunately, the same word is used for a lightweight handheld
    sextant made with two thin brass frames, kept apart by small spacers, which
    would be better described as a "double-frame" sextant.
    
    I imagine that such pillar-mounted sextants could be used for accurate and
    relaxed on-shore measurement of lunar distance when surveying the position
    of a landfall, in the days when longitudes were being established around
    the World. They were large, heavy, objects that nobody would wish to carry
    far on-land. Is there any way a theodolite could have been used, or
    adapted, to measure lunar distances, or to measure lunar-time any other
    way?
    
    Although the terrestrial method that Fred describes could well be useful
    for COMPARING two sextants (or comparing with a theodolite), I don't see
    how he is going to find terrestrial objects that have been mapped precisely
    enough to allow a sextant, on its own, to be calibrated. That job has
    already been done, to much more than sufficient accuracy, for the stars.
    
    George.
    
    
    ================================================================
    contact George Huxtable by email at george---.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    ================================================================
    
    
    

       
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