# NavList:

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

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Re: Geometry of SNO-T
From: Bill B
Date: 2004 Oct 13, 22:19 -0500

```> Here is some simple math behind the perpendicularity test
> and Frank Reed's experiment on his kitchen floor.

Perhaps simple for someone who bemoans the simple pleasures lost when
switching from the log tables to calculators, "One important part of the
pleasure, computing with the logarithms, was gone forever."  Heck, no one is
stopping you from deleting the computer program(s) you wrote and doing it
that way.

I had posted a question a while back, and have received no response.  Since
Alex seems to have a good grasp of this now, I will ask him and others to

"If the front-silvered mirror is no longer on the axis of the rotation, will
this affect the sextant's performance?"

What I think I know is that if I scribed a line on the index mirror at it's
axis of rotation, and the plane of that mirror was on the axis, that line
would appear at the same elevation in the horizon mirror as I moved the
arm.

I also think that if the plane of the index mirror moved off the axis, that
line would move vertically as I moved the arm.

From the reference point of the angles of the index and horizon mirror, no
problem, that relationship will remain the same.

From the reference point of reflection, so what if the area of the mirror
reflecting or the body changes,  Angle in, angle out.

This is as far as my skill sets get me.  I understand the movements of a
view camera, and the differences between what the axis of rotation for
swings and tilts in relation to the center of the lens can do to the image
from an end-user standpoint, but someone with (at least) better schooling in
that area (and probably a lot brighter) did that work for me.

Now most likely I am comparing apples to oranges.  Common-sense test--
manufactures would not do something that would compromise the geometry of
their instrument, but I gnaws at me.

So I ask again.  When you move the plane of the mirror off the rotational
axis (as in a front-silvered mirror in a rear-silvered mirror mount) will
this affect the geometry of the instrument?  If so, uniformly so it can be

Thank you

Bill

```
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