
NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2025 Jan 21, 06:30 -0800
Alex E, you wrote:
"Sorry if I misunderstood how the index error was checked."
I don't think you did now. From Peter's follow-up with photos, it now appears that you had it right. He's making a basic "crude" error.
You added:
"The misunderstanding comes from 2 different interpretation of the expression "on top"."
Yes, and this is one of the problems with recommending the "Sun on Sun" method. It's hard to explain in words, and I find that many newbie navigation enthusiasts are confused by it. Of course there are plenty of explanations with diagrams, and today there are also plenty of videos. Of course, this is also mostly wasted effort. The standard horizon test, which can be done with any object with a distinct edge at a distance greater than a mile or so, is all that's needed. I suspect newbie navigators spend time on these un-necessary tests because there's a "natural" tendency to think that "skillful" adjustment of the sextant is somehow important to the art of the navigator.
You concluded:
"On the other hand, the disagreement with the hirizon method was very large (as I understood from the original message), and I do not believe that a shade in a sextant from a reputable manufacturer can have large prismatic error."
This is surely true for a shade from a "reputable manufacturer", as you say, but you don't know that this is the origin of the shades in advance. Many modern sextants received as "gifts" are decorative pseudo-sextants, of course, and there are also near-functional fakes on the market now (you've the seen the "Tamaya" fakes, I assume?). Until you see the instrument or see high-quality photos of it, you don't know whether it's a good sextant or not.
Also, brand new sextants are relatively rare while used sextants on the market have often been repaired or modified by prior users. A shade on a sextant originally from a "reputable manufacturer" can turn out to have been sourced from bad stock or a even pulled from one of those fake sextants. I vividly remember a sextant I owned a couple of decades ago that had a shade that looked like a decent, high-density shade, but the glass was solid black, 100% opaque. This was mounted on a Tamaya sextant, probably an after-market modification. How it acquired such a useless, non-shade, I could not guess... I eventually sold that sextant to a buyer in-person and explained to him that this shade was useless. Of course in the years since that sextant has probably been re-sold more than once, and each new owner has probably puzzled over that opaque shade...
None of that changes the simple fact that you apparently guessed right -- he was simply doing it wrong.
Frank Reed