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    Re: sextant precision.
    From: Frank Reed CT
    Date: 2005 Jun 22, 06:29 EDT

    Bill you wrote:
    "My contention is that a  optically perfect shade with faces parallel will
    still offset the ray if not  perfectly perpendicular to the ray path.  You
    are probably correct that  the offset is negligible in practice, but
    nonetheless does exist."
    So  experiment! Get a piece of flat clear glass, and hold it in the light
    path.  Rotate it. Tilt it 45 degrees to the light path. Then try perpendicular to
    the  rays. Does it change the alignment of direct and reflected images in
    your  sextant? There's nothing like experimental evidence to constrain one's
    theoretical musings.
    Here's another: No sextant involved. Hold a piece of  flat glass close to
    some text on a page at normal reading distance. Rotate it.  Does the text appear
    to shift from refraction (it should)? Now hold that same  piece of flat glass
    up in front of some distant scenery. When you rotate the  glass as before, do
    objects shift back and forth (they shouldn't)? TRY this  experiment and see if
    you agree with my should/shouldn't comments. Then work out  the theory...
    Evidence constrains theory.
    42.0N 87.7W, or 41.4N  72.1W.

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