NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Impressed with the Raft Book
From: George Huxtable
Date: 2010 Feb 1, 15:51 -0000
From: George Huxtable
Date: 2010 Feb 1, 15:51 -0000
Mike Boersma wrote- "There is a quote in Duttons that Captain Weems made 10 horizon sights on six occasions with an average error of 2 miles and a maximum error of 4 miles. Duttons does not explain how he did this. He must have had extremely good conditions." Mike doesn't provide the paragraph number in Dutton's, which could be useful in order to understand the context of that report. Weems' errors are just what are expected for altitude observations, up from a sharp horizon, by an experiened navigator with a decent instruments from a biggish ship avoiding adverse conditions. But only so long as the observed objects are well above the horizon (by 5º or more) so that refraction is predictable and can be properly compensated. It shows the sort of precision that traditional celestial obsevation can provide, under real-world conditions. On the other hand, attempts to make deductions from the timing of rising or setting of any body will always be very imprecise, as a result of the extreme and unpredictable atmospheric refraction that occurs around zero altitude. George. contact George Huxtable, at george@hux.me.uk or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
