NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Northing correction to Noon longitudes.
From: George Huxtable
Date: 2005 Jun 8, 17:58 +0100
From: George Huxtable
Date: 2005 Jun 8, 17:58 +0100
Perhaps there's a bit more to say about the proposed method for obtaining long. from observations of the Sun made around noon. If I refer to it as the "Frank Reed" method, I think we will all know what we are discussing. The question is: how well do we know the speed of our boat? The speed over the ground is what I am referring to. We can check the speed through the water by using the log. How well do we trust the log? Perhaps, at a guess, to half a knot. But then, we need to know the speed of the current. In my English Channel home-waters, there are tidal currents of up to 7 knots in the Alderney Race, which can make for an exciting wild ride. But there, and as a general rule for tidal waters, these are pilotage areas in which celestial navigation isn't called for. What about ocean currents, then? The Gulf Stream comes to mind, a sinuous stream of water with a sharp boundary, which wanders with time, and reaches at a guess 2.5 knots, perhaps more locally. American navigators will know more about that than I do. Of course, GPS would provide speed-over-ground very precisely, but here we are doing celestial nav by the Frank Reed method, in or (perhaps) out of the Gulf Stream. How well can we estimate velocity over the ground, in that situation? It's the Northerly component of the speed-over-ground that concerns us, so assume that our heading is North, and the current too is generally Northerly. The correction that Northing makes to the Frank Reed time-of-noon is 15.3 (tan lat - tan dec) seconds. In Florida, in Summer, that amounts to 3.5 arc-minutes of long., for each knot of boat-speed. If that speed is known precisely, then it can be corrected precisely, by Frank's proposed method or by my alternative. But any knot of error in the estimated ground-speed will give rise to an error in long of 3.5 arc minutes. It gets somewhat worse at higher latitudes. It's not an overwhelming defect of the Frank Reed method, but one that's worth bearing in mind, to add to its other problems. George. ================================================================ contact George Huxtable by email at george@huxtable.u-net.com, by phone at 01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. ================================================================
