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## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Perpendicularity and other qstns.
From: Frank Reed CT
Date: 2004 Oct 13, 01:34 EDT
Bruce Stark earlier wrote:
"Without the vanes, just viewing the two parts of the arc, the alignment changes from the bottom of the mirror edge, where it's nearly perfect, to the top".

And Alex E replied:
"This is EXACTLY what I was saying."

Well, that's great, but you seem to be jumping to conclusions. I've tried your experiment with three very different sextants, and I do not see the effect that you describe.

And Alex wrote:
"3. The effect can be demonstrated mathematically. And I can post this math if there is interest to it on this list."

I would love to see the math. Unless I have completely misunderstood what you are trying to describe, I'm skeptical that there is a real effect here.

Herbert P earlier wrote:
"You are describing a parallactic effect which can only be caused by a different distance of
the individual cylinders from the front edge of the index mirror."

Alex replied:
"No, this is not what I am describing."

You're saying that the standard arc test for perpendicularity of the index mirror depends on where the observer's eye is located. That is clearly not the case with most sextants. Herbert provided, what I found to be, an insightful analysis that could explain why you see something that other people do not. I think you may want to re-read what he wrote. Of course, I don't have any way of knowing if that really DOES explain what you're seeing since I can't see what you see, but it seems like a good start to me.

And wrote:
"No, this has nothing to do with front silvered mirrors. I suspect this was a mistake in Tamaya manual."

Are your mirrors front-silvered? It may be a little early to assume that the Tamaya manual is mistaken.

And
"It has to do with LARGE mirrors."

Come on now. If the effect is visible in large mirrors, then a smaller effect should be visible in smaller mirrors. It would still be present unless it depended in some significantly non-linear way on the viewing angle, which I find implausible.

And:
"The cylinders indeed help (thanks to Jim Thompson), because of their flat
horizontal tops."

By the way, for anyone who does not feel like spending money on something as silly as special index mirror testing "cylinders", a stack of coins works very nicely. I use five nickels (US five cent pieces).

And Alex wrote:
"This is a very basic test. Test Number 1 in all books I know. It is hard to believe that the book authors just copy what previous books say, without really testing this."

Right. But the simple fact is that people DID test this. Remember, sextants were important instruments of astronomical observation especially in the 18th century. Some of the world's very best and brightest astronomers studied the behavior and errors involved in using these instruments. I do believe that you have discovered something unusual related to the design of your instrument, but I do not believe (yet!) that you have discovered something that was missed by almost three centuries of  analysis.

And finally wrote:
"2. The effect is LARGE. Few milimeters if measured on the right edge of the index mirror (as seen from 1 foot distance), so I am extremally surprised that several people who say they actually performed the test, do not see it."

Clearly if we do not see it (and again, I tried three very different sextants), and you do see it, then this is an issue which is not universal to all sextants.

Frank R
[ ] Mystic, Connecticut
[X] Chicago, Illinois

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