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    Re: Longitude by Sunset
    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
    >
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    > Attached File: http://fer3.com/arc/img/119437.img_1189.jpg
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    >
    > View and reply to this message: http://fer3.com/arc/m2.aspx?i=119437
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    >
    >
    
    
    
    

       
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