NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Making an artificial horizon, and leveling thereof
From: George Huxtable
Date: 2011 Jan 22, 11:55 -0000
From: George Huxtable
Date: 2011 Jan 22, 11:55 -0000
Alan wrote- "On displacement of calculated fix from GPS coordinates, many under 3 NM, a few around 1 NM, with a couple under 1 NM. Of course, there were some "wild hairs" which I attribute to "operator error", yours truly being the operator." I doubt whether his results are affected at all by the two possible reasons he suggested; twist of the shield assembly and distance of the observer from it. But if he is seeing discrepancies of around 3 miles, something seems wrong. That would involve an error in sextant reading of around 6', because that reading is halved when using a reflection. Indeed, use of an artificial horizon should be a procedure of great accuracy. Not only is there that factor of 2 in the sextant reading, but also the dip of the horizon, which is usually the biggest unknown in altitude navigation, is no longer involved. Some study of those worse results, rather than his ones that show the least discrepancy, may be productive. I suggest that Alan posts to the list an example or two of such reflected Sun altitudes, those that show discrepancies of a few arc-minutes (= miles), providing the following information- Date and time (to the second), sextant reading, index error, which limb used, and observer's position, according to GPS or map. If temperatures and pressures were recorded, so much the better, but unless those values were extreme, or the Sun was low, they will have little effect. List members may then be able to help in tracking down the origin of any discrepancies. George. contact George Huxtable, at george{at}hux.me.uk or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. You also asked , "And why do you need to stand back to see the two views?" I really don't know, just that it seemed that there was an optimum distance from which I could see both the reflected and sextant suns, this seeming to be back from the AH, rather than closer to it. As to alignment, I'm just going on the basis of the instruction sheet that came with the thing, which stated that the ah should be oritned so that there was no shadow at either side. Best. Alan ---------------------------------------------------------------- NavList message boards and member settings: www.fer3.com/NavList Members may optionally receive posts by email. To cancel email delivery, send a message to NoMail[at]fer3.com ----------------------------------------------------------------