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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Instumental error?
From: Alexandre Eremenko
Date: 2005 Apr 20, 14:45 -0500
From: Alexandre Eremenko
Date: 2005 Apr 20, 14:45 -0500
Fred, I apologize to you and to the list: I mistyped the web address. The corrected address is: http://adsabs.harvard.edu/cgi-bin/ nph-bib_query?bibcode=1888AN....118Q.383S&db_key=AST (Again, remove carriage return!) In general, this archiv contains about 30 XIX century papers with keywords "sextant", "errors", "eccentricity" etc. I am glad it is free. The author speaks of a meridian circle rather than a theodolite. Somehow on the first reading I had a wrong impression that the trick can be done with an ordinary theodolite. But when I started to explain this to the list, I encountered difficulties, so I looked at the paper again, and noticed that a hudge objective lens is required. The ordinary methods involving theodolite are probably the "outdoor methods". I mean you put some vertical sticks on a large distance from your position, and measure the horizontal angles with your sextant and then with theodolite. The large distance is required because you cannot assure that both measurements are exactly from the same position. For example, if you need 0.1' precision, and are able to keep the position to 1 inch (I doubt is is easy to do better than that, even with sophisticated tripods), the sticks should be at the distance of 3400 feet which is hard, of course. Any indoor method I can imagine would use one or several reflecting surfaces, but the problem of crating parallel rays is hard. Alex.