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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Dip, dip short, distance off with buildings, etc.
From: Frank Reed CT
Date: 2006 Jan 5, 20:44 EST
From: Frank Reed CT
Date: 2006 Jan 5, 20:44 EST
In Bowditch and elsewhere, there are formulas for dip, dip short, Table XV for distance based on measured height, maximum visibility distance, etc., and they all have various mysterious corrections for "mean refraction". I've got this stuff all figured out pretty well now, and it turns out that there is a really easy, though somewhat bizarre (!), way of thinking about the effect of refraction in terrestrial, or coastal navigation, situations. You can calculate dip or the altitude of a tall building peeking up from beyond the horizon using straight Euclidean geometry and trigonometry ignoring refraction completely. Then to include refraction, you simply change the radius of the Earth from R to R/(1-x) where x depends on the temperature gradient of the atmosphere. On average it's about 0.15 but it can easily be anywhere in the range 0.13 to 0.17 and sometimes it's as low as 0 or as high as 1.0 (temperature inversions yield higher values of x). This works perfectly to derive the equations in Bowditch for dip, dip short, Table XV, and apparently everything else where terrestrial refraction is involved. Details upon request... -FER 42.0N 87.7W, or 41.4N 72.1W. www.HistoricalAtlas.com/lunars