NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Calibrating a sextant scale
From: Alexandre Eremenko
Date: 2007 Nov 21, 13:52 -0800
From: Alexandre Eremenko
Date: 2007 Nov 21, 13:52 -0800
Dear Fred, On Nov 21, 1:33 pm, Fred Hebardwrote: > Nice post Alex Thank you. It is encouraging to know that someone reads my postings. The "ultimate accuracy" of sextant observations seems to be too exotic subject, even on this list:-) I can share some experience. For several years, I had systematic overshots with my sextant (roughly by 0.3'). I thought a lot about this, I had my arc checked by Freiberger and Cassens-Plath, I tried to investigate all imaginable causes of this error, I complaned to the list:-) Finally I concluded that this is either some misterious "personal error", or even more misterious "irradiation". After making several 100-s of shots I learned JUST HOW should the picture in the telescope look, so that there is no error. So now I can measure very precisely Sun- and Venus/Jupiter- Moon distances. What I wanted to comment on is something about Sun-Moon case. It matters very much how CONTRAST the Moon is against the sky. The best results are obtained when the sky is deep blue, and the moon is bright and slightly yellow. Normally it is not so. The Moon is usually white and the sky is sort of white-blue. Under these (normal) nonditions, what I think is the Moon's edge is not really the edge. When I really touch the edges of the sun and the Moon, the Sun's edge near the touching point looks distorted (indented). And this seems to be the real source of the about +0.3' error. If I adjust the sextant angle so that the Sun edge is PERFECTLY ROUND, then a visible gap remains between the Sun and Moon edge. Trying to liquidate this gap, distorts the Sun edge image near the touching point. After many observations, I learned how exactly the gap should look under different sky-Moon relative brightness conditions. So I can obtain nearly perfect Sun-Moon results in two ways in two ways: by leaving this gap when I shoot, or by actual touching the limbs, disregarding the resulting distortion of the Sun's edge and then subtracting additional -0.3' from my observation. One can call this -0.3' whatever one wishes, but I am inclined to believe that this is not a "personal error". This is why I post this observation on the list:-) Alex. "toNavList" 62 lines, 2199 characters --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To , send email to NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---