I retired from the Sea, many moons past and, I continue to take observations of the celestial bodies using a home-made artificial horizon.
Using real instruments which have served me very well and re-playing, once more, the necessary techniques, it is possible to ascertain a position line within 0.1 of a nautical mile.
My peepers are not as precise as my younger days but I can manage a 4-body set of stars and have them resolve into a positional accuracy of, at best, 0.6 of a nautical mile.
Observations specifically for Longitude, by equal altitudes either side of Transit, are, at best, 0.25 nautical miles off True ( taken from Google Earth) .
These days, rather than resorting to the long-winded Haversine method, I calculate via an ordinary school grade calculator, using the sine/ cosine formulae, though I still purchase the annual Nautical Almanac.....an Almanac of Sorts, also exists on-line, it is usually extremely co-incident with the Nautical Almanac, and the only reason I don't use it, is because of its cumbersome layout. There are also some very good, easy to use and freely available on-line spreadsheets.
I also keep a watchful eye on the kind of devices being sold on the well-known auction site and, most of what takes up the bulk there , are not working instruments at all, but mere decoration items.
Oh, these suchlike replica's are interesting insomuchas they do reflect a level of contemporary likeness to the real thing and, indeed the brass effect is certainly eye catching. Visual appearance is very representative of type and are convincing, for what they are.
However, the necessary Devil for the required details needed for accurate measurement, just isn't there at all....nor was it ever meant to be.
As a member of NavList, I can state that there are current posts concerning the search for a circular devise known as a" Pocket Sextant".
These were made, large hand size with a scale accuracy resolved to One minute of arc, and were mostly just about capable of that sort of fine-ness, though the optics did not always provide a small 'scope and were thus a naked-eye instrument.
Real one's are as rare as Hen's teeth, and always exchange hands for a great deal of cash.
Unfortunately, the many replica's of this type of instrument, are as convincing as ever, just as their full-sized brethren.
The replica market is fraught and there has been a significant rise in selling prices, that have been jacked-up just enough to convince the less knowledgable into believing that such a thing is possibly a pukka deal.
If one is tempted by what seems a bargain, then just browse diligently through the whole of the listing and there MAY be a note to the effect of being a replica, some of the sellers do just that.
Thankfully, a sure way to tell replica's , is that they always come with a smart and glossy wooden box...or an unmarked valise of leather.
...why this should be so, when it is obvious that a distressed-looking case will inevitably add greatly to the artifice....I don't know for sure, though I do like to think that the 'not doing so', is a glim ***** of conscience
Regards All
Ceeg
