NavList:

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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