# NavList:

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

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Re: Index mirror adjustment with offset mirror
From: Frank Reed
Date: 2014 Jan 15, 11:46 -0800

Luc Van den Borre, you wrote:
"In summary, the correct procedure (in short form) according to me is:
1) place dice or some such on the arc (to construct a plane parallel to
the arc)
2) change your view so you are sighting along the top surface of the
unreflected die
3) adjust the mirror so the top surface of the reflected die lines up
with its sibling"

Yes, and as I mentioned in a post last night, a really useful trick is to use objects (could still be dice) that are different colors. Marker caps, legos, and other small toys work great for this.

"* This procedure will work correctly for sextants with mirrors offset
any distance (including zero) from the index arm pivot
* Only not use dice if you're certain the mirror is not offset, or if
the sextant is constructed in such a way that you can sight in parallel
to the top plane of the arc."

Right. I explain it to students by saying 'this ALWAYS works, the other technique MAY work, or it may not work, and you won't know until you get it wrong!'

You also wrote:
"* It's not correct to use the top _edge_ of the dice. One must be
looking along the top plane."

I'm not sure I understand the distinction you're drawing here. You sight in line with the top edge of the two objects. Do you mean that one should not be sighting from "above" trying to get the top faces of the two dice to appear aligned? If that's what you meant, then yes, that's an important distinction. The goal is to get the two top edges to align in a straight line when viewed from exactly the height of the dice above the arc.

You listed some sources:
"(1) I consulted:
* Bowditch, ed. 2002, p 267 (which is clearly wrong for this sextant),
* Bruce Bauer's often recommended 'The Sextant Handbook' (which led my
to wrongly try and line up the arcs in the center of the mirror, though
this is followed by the correct procedure using dominoes),
* Tom Cunliffe's 'Celestial Navigation', which only works for non-offset
index mirrors
* A Tamaya manual, not helpfull
* A Davis manual, same"

Yes, this is one of those things that many sources have described incompletely for decades. And the fact that it is in Bowditch causes HUGE problems. Many experienced navigators as well as beginners in the US are taught to treat Bowditch as a "bible" of sacred scripture. It's never been that!

And:
"* A SNO-T manual, which gives the correct procedure using the placement
of 'diopters' on the arc. But these diopters appear to be L-shaped"

Yeah, I don't understand those peculiar shapes. They may have been modeled on a bit of testing gear created for the optics lab. Clearly simpler objects work just fine.

-FER
By the way, it's far better to call that Russian sextant a "SNO-T". Using "SNO-T" is liked writing CCCP for USSR, or Poccia for Rossiya (Russia), or GLONACC for GLONASS. The Cyrillic letter that looks like a western alphabet "C" represents "S" and should be written "S" when transliterated into English or any other language that uses the western alphabet. Besides "SN" stands for "Sextant Nautical" which is "close to the metal". It's the 21st century -- we can do this right.

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