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Re: An "alternative" sextant?
From: George Huxtable
Date: 2009 Jan 11, 21:03 -0000
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. George. 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 To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---