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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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