A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Alexandre Eremenko
Date: 2020 May 24, 08:47 -0700
Notice that some certificates are graded in minutes and tenths of a minute. Perhaps it depends on the manufacturer, and on the device which is used to measure this correction.
Speaking of your suggestion, it seems reasonable, but manufacturers do not really care, and just follow the tradition.
In fact, as Frank said once, the arc correction of modern sextants is usually so small, that you cannot measure it yourself, if you do not have this special device. I checked this statement myself with my own sextants: there is no correlation between my actual errors and errors shown in the certificate issued by Freiberger.
This suggests that the whole business of arc correction is of little relevance, and this is consistent with Cassens and Plath attitude: their certificates only say "Good for practical purposes" and give no correction numbers.
Just out of curiosity I asked Cassens-Plath to certify the same SNO-T of mine. Unlike Freiberger, they did not permit me to be present at the process:-) and to see their device. And they issued a certificate which says "Good for practical purposes". The largest correction that Freiberger's certificate shows on 0-110 degrees part of the arc is 10" and for 120 degrees 14".
I did a lot of observations in better conditions then the usual sea practice (from my balcony, and mainly star-to-star distances, Lunar distances, and Sun with artificial horizon). But I was never able to see any correspondence between my actual errors and these arc corrections.