NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Navigating Around Hills and Dips in the Ocean
From: George Huxtable
Date: 2003 Aug 17, 13:37 +0100
From: George Huxtable
Date: 2003 Aug 17, 13:37 +0100
George Istok wrote-> Mr. Huxtable, > >Though I am now in agreement with you, I came to that position by a slightly >different argument (that may or may not be valid). I'm delighted that we now agree, though I fear that George Istok's argument is indeed invalid, even though it gives what I believe to be correct answer! To digress slightly beforehand, when discussing gravity, one has to distinguish carefully between upper-case G, and lower-case g, which by convention mean very different things. G is the Universal Constant of Gravitation, defining the force between any two masses at a distance apart, and takes the same value anywhere in the Universe (or so our observations indicate). On the other hand, g is the acceleration due to gravity, a local matter that defines the force on a certain mass, caused by the pull of the Earth with all its irregularities, and varying with position and height or depth. It's an unfortunate choice of symbols which causes much confusion. Where George refers to changes in G, I think we should take it that he really means changes in g. George goes on to say- >I assumed a completely >isolated sphere of a light matter covered with water, a distance from the >center of the sphere to the surface of the water, a value (G) for the >gravitational attraction at the surface of the water, and that the surface >of the water at any point must always be at a distance from the center of >the sphere such that the value of G is constant. And there's the problem. The water surface (or a spirit-level) adjusts itself so that it's everywhere at a right-angle to the local direction of gravity, but that does NOT imply that over the sea surface the value of g is constant. There's no reason why it should be. Indeed, in an earlier mailing, I pointed out that g varied (at sea-level) between 978 and 983 cm per square second, between the equator and the poles. So, the basic assumption being wrong, that argument fails. He goes on to say- >I do not claim that the argument above is whole or that it is valid, but it >did convince me that there will be a mound over an anomaly where the >attraction is stronger and a dip where it is weaker. Your argument is much >simpler and even more convincing. Thank you. We are in accord. Sorry I didn't get it right first time round; it would hace saved some trouble. George Huxtable. ================================================================ 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. ================================================================