# NavList:

## 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: Herbert Prinz
Date: 2004 Oct 13, 15:09 -0400

```Alexandre Eremenko wrote:

> Herbert Prinz:
> "The test checks the perpendicularity of the mirror
> to the LINE passing through the upper edges of both visors."
>
> This is not so. Simple proof: move the index arm few degrees
> in any direction. The plane of the index mirror changes.
> Or move the visors on the arc. The line connecting the visors changes.
> But the test is still valid.

Yes of course. And when you are done with the test, the mirror will again
be perpendicular to the line connecting the visors.

I think we are across purposes here. It goes without saying, that if you
move the the visors, you move the line connecting them. (And you also
have to move the index arm with the mirror in order to be able to see the
visors simultaneously.) But you don't move the visors or the index arm
around during the test procedure. My point was to describe the geometry
of the situation during the test procedure. For this it is essential to
know the orientation of the plane with respect to the visors.

You said that the test checks the perpendicularity of the index mirror
w.r.t. the plane through visors and eye. It was clumsy of me to say "I
don't think so". Of course, it does do that. But I needed more. Your
condition still leaves one degree of freedom for the orientation of the
mirror. I needed the stronger condition that nails the plane down in
space (or at least with respect to the visors) at that very moment where
we do our experiment moving the eye up and down while watching the visors
getting in and out of coincidence.

Herbert Prinz

```
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