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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Bowditch tables and sexant parallax
From: Frank Reed CT
Date: 2005 Apr 28, 17:55 EDT
From: Frank Reed CT
Date: 2005 Apr 28, 17:55 EDT
Bill wrote: "How the heck do I correct for parallax error for two points at varying distance? " I don't think anyone bothers with this. It might be an interesting mathematical puzzle, but I don't think it would ever matter in practice. And: "and a three-arm protractor is precise to 0.1'." But is it that accurate? Let's suppose the arms are 50 cm long. Placing the end points at 0.1 arcminute accuracy would be equivalent to placing the tips of the arms with a linear accuracy of 0.015 millimeters while the width of a line on a chart is probably around 0.5 mm in most cases --that's over 30 times larger. I wouldn't count on angles laid out with a three-arm protractor to be any better than 0.1 degree accuracy. Then again, maybe I've missed something. As an aside, here's a little mantra: Angles *ARE* Ratios. An angle of one arcsecond is a ratio of 1:206,265. An angle of one arcminute is a ratio of 1:3438. And an angle of one degree is a ratio of 1:57.3. Memorize any one of these and you never need trig for small angle calculations. -FER 42.0N 87.7W, or 41.4N 72.1W. www.HistoricalAtlas.com/lunars