# 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 18, 15:31 -0800

Bill Morris, you wrote:
"I don't understand the references to the top surfaces of the various solid objects - dice, piles of coins, pieces of Lego - in this thread as the only requirement is for two edges of equal height above the limb"

It took me a while to imagine what he was talking about, too, but then it dawned on me. The problem is this: if you're transitioning from the concept of aligning two sections of the arc, as illustrated for example, in Bowditch, to this more general method, you might carry along the notion that you should be looking "down" on the surfaces (hence lining up the top surfaces), just as you're looking down on the the arc. This can all be explained away with an alternative diagram or photos, but it's important to recognize that this could be a point of confusion.

You also wrote:
"I am mystified as to how anyone could confuse the direct and reflected images and thus need the vanes or other objects to be of different colours."

I would say that you're "mystified" by this due to your own great familiarity with the procedure. How many times in the past five years would you guess you have done this index mirror alignment test? It's probably at least a couple of dozen times for you, wouldn't you say, Bill?

Beginners find this issue very confusing. They look at the mirror, and they say, quite reasonably, "I only see one cylinder". It's visually confusing, and using objects of different colors instantly makes it obvious that one view is reflected while the other view is direct. And considering that most sextant users will only perform this sort of alignment very rarely, maybe once in three years, only a small fraction of them will remember the process well enough to avoid being rendered back to beginner status again every time they try it out.

-FER

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