NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Longitude by Sunset
From: Alexandre Eremenko
Date: 2012 May 7, 10:06 -0400
From: Alexandre Eremenko
Date: 2012 May 7, 10:06 -0400
How can this work with Sun, if the glasses are clear? One needs shades for the Sun. Alex. On Mon, 6 May 2012, Greg Rudzinski wrote: > > Alex, > > See pictured a simplified Bris that I just fabricated this evening using a 0.7mm pencil lead, two slides, and clear nail polish as adhesive. It shifts an image about 2.5*. It will work for the Sun but the daytime Moon could be a problem. > > Greg Rudzinski > > > [NavList] Re: Longitude by Sunset > From: Alexandre Eremenko > Date: 6 May 2012 21:32 > Gred, > > I am very well familiar with Bris sextant:-) > I bought the last one from Cassens Plath, and Bill B made > truly professional pictures of it: > www.math.purdue.edu/~eremenko/Navigation/Sextants/Bris > But I don't think it will help much. > > First, it is designed for Sun, has dark glass, and there is no way > to see the Moon through it. > One can make another one, of course, with clear glasses, specially > for the Moon. > > But then the advantage that you mention, "one does not need to know > the angle" disappeares. > And even theoretically, one does need to know the angles. > (When timing the settings of the Sun/Moon, the angle is known: > it is 0, or more precisely, the refraction angle). > > I actually do not know how accurate Bris can be. > I expect not better than 1'-2'. This is not enough. > > I have to confess that I have not really graduated my Bris:-( > For this one has to spend 5-6 mornings or evenings > (sunrise to noon, or noon to sunset) > on a sea shore, with excellent weather, and good horizon > unobscured in a given > direction (W or E). And with my Bris with me:-) > I never had such opportunity. (Or maybe had but missed it). > > I said 5-6 because one certainly needs averaging. > I made several observations many years ago, but not enough to graduate > the full "scale". > > > > Alex > > ---------------------------------------------------------------- > 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 > ---------------------------------------------------------------- > > > > Attached File: http://fer3.com/arc/img/119437.img_1189.jpg > > > : http://fer3.com/arc/m2.aspx?i=119437 > > >