NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Plastic sextants and prejudice. was: GPS shortcomings.
From: George Huxtable
Date: 2005 Jun 9, 16:30 +0100
From: George Huxtable
Date: 2005 Jun 9, 16:30 +0100
Peter Fogg wrote, about an experience with a plastic sextant- >When I've had the chance to examine them I've found them disconcerting to >use - for example, the index error was different each time I checked on one >instrument, before and after taking a sight. It didn't inspire confidance. I >know and accept that they have been used to cross oceans with, and possibly >they are ideal for the task of teaching students. The effect Peter describes is most likely to be a loose mounting of one of the mirrors, which can happen with any sextant if the adjustment screws have been overslackened, perhaps during adjustment by a "sextant-worrier". If a corner of a mirror is no longer being firmly grasped between an adjusting screw and a backing spring, what he describes is the inevitable consequence. It's easy to check for, by pushing, ever-so-gently, against the mirror corners. Was that tried? As the user of a plastic sextant over many years, I always check index errors before AND AFTER a measurement (or series), just in case, and don't detect a difference. Nor have I observed any problem, in either of my Ebbco sextants, with the warping Peter has referred to, and wonder which is the sextant model in which he has observed this fault, and whether in reality it presented any problem. I don't pretend that such sextants are perfect, and we have discussed some defects more than once on this list. But Peter Fogg's comments read more as a rehearsal of prejudice than a reasoned evaluation. George. ================================================================ contact George Huxtable by email at george@huxtable.u-net.com, by phone at 01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. ================================================================