A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2015 Apr 18, 16:16 -0700
Bruce, you wrote:
"The mirror floating in pool is really elegant. I’m jealous. I now regret all of my effort to use a steel plate with leveling screws, plus all of the other refinements I built into my “system” so I could use it on steep uneven ground etc."
Well, don't throw it out in the name of "elegance" quite yet... :)
Are you sure that the system he has there actually works?? I'm not. A mirror in a wooden frame floating in water is essentially a flat-bottom boat floating in a pond. Is the deck of a flat-bottom boat necessarily level? The buoyancy created by the wooden frame is surely not equal in all directions. You would have to go through the same procedures of placing levels on the mirror that you would have to use with a mirror resting on solid ground. Also, that wooden frame sounds good, but it will absorb water in a non-symmetrical way, changing the buoyancy and from there the tilt of the mirror. This probably won't matter much for a short run of practice sights, but it means that any adjustments for leveling would have to be repeated on each use.
As for his "1 mile accuracy" claim, people who set up demos like this are optimists, of course --and nothing wrong with that. When we show our tools to the public, we emphasize best case scenarios. On a good day with the mirror properly leveled and with the index error of that plastic sextant stable for some period of time, I am sure that he can get +/- 2 minutes in observations most of the time (that is, 1 s.d. error about to 2 min. of arc), and after halving, that implies +/- 1 mile in the resulting lines of position. Those "good days" are rare things... Based on my own experience with Davis plastic sextants and simple artificial horizons like this one, even when we're doing everything right I would expect something more like +/- 10 minutes of arc (in the 1 s.d. sense) in the angles and therefore +/- 5 miles in the LOPs. With a good-quality metal sextant and a properly-leveled high-quality artificial horizon (and much more attention to small details), the accuracy can be much higher. There's no reason not to expect +/- 0.25' in the observations and then +/- 0.125' in the LOPs.