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    Re: An "alternative" sextant?
    From: George Huxtable
    Date: 2009 Jan 11, 21:03 -0000

    A month ago, in [6740], I put forward a suggestion for a sextant-type 
    instrument with an entirely different geometry to the conventional sextant. 
    In response to questions from Scott Owen, in [6772], I extended the 
    explanation, with a crude sketch, in [6777]. This drew further comment from 
    Scott, [6924], and also from Richard Pisko [6894], who had amended the 
    threadname to "Sextant and quintant limits, and an alternative sextant". I 
    thank them both for taking an interest.
    It now seems to me that Richard has found the flaw in the argument, and 
    explained why nobody has ever made such a device. He has indeed shot the 
    whole notion down in flames, and for that I'm rather grateful.
    I will use his words, when describing the behaviour of such a (hypothetical) 
    instrument, when used for measuring horizontal angles-
    "Unfortunately, determining the angular distance between two vertical 
    objects such as power poles or church steeples seems to be more difficult; 
    as the direct view of the one pole would still be vertical, but the 
    reflected view of the other pole would be horizontal as seen from a 90 
    degree included angle at the observer's position."
    That looked as though it would present a serious problem, and it seemed best 
    check it out in practice, rather than relying on pure reason. Not having a 
    wide horixon, sprinkled with power poles, I've tried in out it a more 
    domestic environment, from my armchair. As most of my walls are covered with 
    bookshelves, I could compare a direct view of the vertical spines of books 
    facing me, with a twice-reflected view of vertical book-spines in a shelf on 
    the wall to my right, about 90� away horizontally. The two mirrors were 
    simply hand-held one above the other, each tilted at 45�, so that the first 
    one sent light, from the shelf on the right, vertically downward, and the 
    second sent it back horizontally to my eye. This was similar to the geometry 
    I had been proposing for an "alternative" sextant, when used for observing a 
    horizontal angle of 90�.
    And this confirmed just what Richard Pisko had predicted. In reflected view, 
    the book-spines from the shelf on my right appeared to be lying flat instead 
    of standing upright, so there was no way that the their images could be made 
    to align with my direct unreflected view of the vertical spines of the books 
    facing me. The reflected image had been twisted through 90�. And if that 
    happened with books, and power poles, so would it happen with stars in a 
    constellation, making any such instrument quite unusable.
    I had suspected that there must be some flaw; otherwise someone would have 
    produced such an instrument long ago. So my thanls to Richard for pointing 
    it out, which leaves me a wiser man, though not necessarily a sadder one. 
    And thanks, too, to Scott Owen, for taking an intelligent interest, and 
    asking perceptive questions.
    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. 
    Navigation List archive: www.fer3.com/arc
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