A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Bill Morris
Date: 2018 Apr 17, 17:04 -0700
Early USN MkII s simply had drums divided to half a minute, and you are correct that later ones had verniers.
As no doubt you know, the micrometer vernier illustrates what might be called empty precision. It is precise to 12 seconds or 0.2 minutes, but that is close to the absolute limit of the sextant's accuracy, which reflects errors in the cutting of the rack, periodic errors in the pitch of the micrometer worm, centring errors and so forth.
When the micrometer sextant began to gain popularity, from about 1930 onwards, several makers provided the micrometer with a vernier, Hughes and Son and Heath and Co among them, but not C Plath (the maker of the sextant shown in your left hand photograph?), while having errors on calibration well in excess of 12 seconds. Over time, accuracy has got better, but I am not aware of any maker stating that instrumental errors were guaranteed to be less than 12 seconds, with the exception of early SNO-T s.
If sextants were provided with calibration charts made using a calibration device with an acuracy and precision of, say, single seconds, with intervals sufficiently small for interpolation to be meaningful, then a micrometer vernier would begin to make some sort of sense, though not for ordinary use, where errors of observation might well be of the order of a minute or two. In any case, it is quite easy on a drum of sensible size to interpolate to a fifth of a minute by eye. It is not clear quite why the maker of the sextant in the right hand photograph provided such a small, hard-to-read drum when there is plenty of room for a larger one. Maybe the marketing department over-ruled the designers who in turn forgot to consult the mariners.