NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Brad Morris
Date: 2014 Jan 20, 17:49 -0500
Thanks for that Frank. It slipped by me in the Star-Star discussion, as it was just on passing, and not the focus of the conversation.
Today, concentricity (eccentricity or centering error) is denoted in linear units, not angular. I'm interested how that translates.
I've approached Richard Dunn, who has promised to try and dig out the information. We may hear from him in a few weeks on this topic.
One reference I found seemed to indicate that 'Class A' sextants needed to resolve to at least 10 seconds. That may be true as lesser resolutions were produced.
That said, I think that there is more to the Class A rating than resolution and centering error. What about accuracy of scale, etc? You've got perfect centering and 10 second resolution, but the arc could be divided by a drunken monkey.
This story is not complete.
Brad
Hi Brad,
My understanding of the Kew "Class A" rating was that it was an overall rating. It was the certification required for sextants given to Royal Navy cadets. It combined several factors, and the instrument had to meet various standards on several tests.
You may remember a NavList discussion a few years back about tables of "star distances" published in about 1905 for use with Lord Ellenborough's method of testing sextant arc error at sea (*). In the introduction, the authors say that a "Class A" certification implies among "other things" that the centering error (or "arc error" as we would call it today) amounted to less than 1' of arc maximum. Classes B and C would presumably permit progressively greater arc error, and this same source says that the sextant would be "rejected" (in other words, worse than class C) if the arc error was greater than 3'.
*that discussion was in March 2010, and here's my first message on thee subject, specifically addressed to you personally, in fact. :)
-FER
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