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    Re: "Attainment of Precision" article (1964)
    From: George Huxtable
    Date: 2009 Jul 8, 12:12 +0100

    Nicolas asked, about Frank's comment
    "frankreed@HistoricalAtlas.com wrote:
    > If you want a really good value for IC, shoot the Sun or the Moon
    > limb-to-limb ... it's usually more effective than the standard sea horizon
    > test (but the standard test is usually quite sufficient and it's the one
    > that should always be taught)."
    "could you please explain why the limb-to-limb method is more effective?
    And can you explain how this is done? What I mean is: are you
    superimposing opposite limbs onto each other (so calibrating the sextant
    at the sun's/moon's diameter), or the same limbs (so calibrating for
    zero at two spots in your field of view)? I take it it is the latter
    method. Do you have any data supporting the effectiveness?"
    The main difficulty with using the sea horizon to check index error is that
    the horizon is often hazy or disturbed (especially when seen from a small
    A standard method for checking using the Sun is to put the reflected image
    of the Sun above the direct image so that they just kiss,
    tangent-to-tangent; then below it, and split the difference. That is, using
    opposite limbs, not similar limbs.
    It calls for, ideally, a very-dark-glass cap to fit to the telescope
    eyepiece, as was once a standard accessory, to make both Sun images
    viewable. (That carries a possible danger that the undiminished focussed
    heat from the Sun can crack it; an acknowledged eye-hazard with some
    astronomical telescopes, but I've never heard of it actually happening with
    the smaller telescope that's found on a sextant.) Alternatively, it calls
    for a very dark shade in the direct-view, as is always available for the
    reflected-view, and not all sextants are so fitted. And any difference in
    refraction between those shades can skew the result.
    It's quite hard to do it the other way, by superimposing precisely two
    images of the Sun, one exactly on top of the other, because when making the
    final adjustment, it's difficult to be sure which image-edge is which. Doing
    the job by averaging two displaced images, because it combines two
    independent observations, has its errors reduced by root-2 because of that
    But it doesn't provide any useful calibration of the sextant by taking the
    difference between the two readings,  about 1 degree apart, as Nicolas
    suggests. The extrapolation is too great, combined with the sensitivity to
    the observer's judgment of where the Sun-edge happens to lie.
    The Moon is another matter, and I don't see how it is possible to determine
    index error precisely by aligning opposite limbs of the Moon , because of
    its partial lighting, except at full Moon. It would call for tilting the
    scope into the plane given by the line between the horns, which in itself is
    no real problem. The difficulty comes in that those horns are just at the
    boundary between sharp-edge and shadowed-edge, and I doubt whether those
    opposite limbs would provide a pair of sufficiently-sharp targets to do the
    job well, though to be honest I have never tried the Moon for that purpose
    in real life. Perhaps Frank has done so, and will explain.
    contact George Huxtable, at  george@hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
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