NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Nav exercise - ex-meridian
From: George Huxtable
Date: 2008 May 20, 20:59 +0100
From: George Huxtable
Date: 2008 May 20, 20:59 +0100
Jeremy's ex-meridian exercise turns out to be of great interest. It has taken me into unfamiliar waters, as I have never even attempted an ex-meridian correction before. And it's showing up some of the problems that are involved. Am I right in thinking that this exercise goes right up to, and perhaps even beyond, the viable limits for which the ex-meridian procedure can be expected to work? Perhaps that's the lesson that we are intended to draw. With such a high Sun, it doesn't allow much difference, between observer's long and Sun's GHA, before the Sun's direction gets way off the meridian. The difficulties of using ex-meridian tables really show up here. In the tables that I have, Bowditch and a modern(ish) Norie's, the quantity A, or alpha, is tabulated only to the nearest degree of lat or dec., and changes greatly for each such step, so getting the right number calls for a tricky double-interpolation. Just by in-the-head estimation, I would put A at about 21 or so. And that leads me to an ex-meridian correction of about 14', but I can see that the correction could easily be half an arc-minute off, either way. Am I in the right ball-park, I wonder? Normally, I would have gone straight for a St Hilaire position-line. Being pushed by Jeremy into trying an ex-meridian method has only reinforced that view. George. contact George Huxtable at george@huxtable.u-net.com or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---