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Re: Assumed positions, WAS: IN HONOR OF JEREMY...
From: George Huxtable
Date: 2009 Sep 25, 17:41 +0100
From: George Huxtable
Date: 2009 Sep 25, 17:41 +0100
I think there may be a bit of misunderstanding about Douwes of Amsterdam, and the problem to which he provided the solution. Antoine, and presumably others, have devised an analytic solution to the question of two intersecting circles of equal altitude, drawn around two known geographical positions, and the resulting ambiguous-pair of intersections. These geographical positions could be of a single body at two times, allowing for vessel's motion in between, or of two bodies at the same time. However, Cornelius Douwes was working around 1740, long before chronometers had arrived on the scene, and without knowing the (Greenwich) time, those geographical positions could not be known. Douwes was just trying to find the latitude. There was no way longitude could be established. Anyone can find latitude from a Sun altitude if it's taken at local noon, but what if the Sun happened to be obscured then? Douwes problem was to find latitude from a pair of Sun altitudes, neither of which was taken at noon, in the days when GMT was unavailable. But navigators, although lacking chronometers, might well have a hack-watch, good enough to assess the time INTERVAL between those Sun observations. So that was the problem that Douwes tackled; given two Sun altitudes at unknown times, and the time interval between them, what was the latitude? By the way, Cotter states that "Douwes's method requires the use of an estimated latitude". Douwes wasn't the first to tackle this question, though. In 1728, Facio Duillier, of Swiss origin, who had worked with Isaac Newton (and, by the way, invented jewelling for timepieces), had published a method which was considered particularly unwieldy. Several other methods followed that of Douwes; Dunn, Ivory, Ainsley, Riddle, all feature in Cotter's "History of Nautical Astronomy". Don't expect me to expound on any of this, though. I've just quoted, without trying to take it all in. As for modern papers, deriving a pair of intersections of the position circles in latitude and longitude, at known times, there are several papers. For example, G G Bennett, "General Conventions and solutions- their use in Celestial Navigation" Navigation, 26 (4) 1979-80 Torben Kjer, "Unambiguous two-body fix methods derived from crystallographic principles", Navigation, 28 (1) Spring 1981. I seem to recall that Andres has noted his procedure for such a calculation on Navlist. Peter Hakel added- "My response: Such tedium can be bypassed to some degree if the problem can be reduced to some standardized form. Then its solutions can be calculated once and tabulated for practical use on a sufficiently fine grid. So I suppose nobody has printed any "Douwes sight-reduction tables."". On the contrary, Douwes devised solar tables for use with his method, which were referred to in 1760, and improved by Maskelyne in the second edition of his "Tables requisite", 1781. 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. --~--~---------~--~----~------------~-------~--~----~ NavList message boards: www.fer3.com/arc Or post by email to: NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---