NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Appeal to forum members to collect data
From: George Huxtable
Date: 2007 Oct 25, 22:50 +0100
From: George Huxtable
Date: 2007 Oct 25, 22:50 +0100
Jim wrote- | I would like to collect some data from a variety of observers using a | variety of marine sextants taken in a uniform manner. I have no wish to pour cold water on such an admirable aim, but I think Jim's attention should be drawn to a serious difficulty. The biggest error in measuring altitudes above the horizon is in establishing the direction of that horizon. The dip of the horizon can often differ by a significant fraction of a minute, and occasionally by much more than that, from its "book" value. This is the effect known, when it becomes serious, as "anomalous dip". It can occur at its worst in coastal waters, particularly semi-enclosed waters, as Alex can testify from recent experiences in Kiel. It can't be predicted, and its effects can be of the same order as any errors that are likely to be found in a sextant, and will mask or confuse those errors. How will Jim become aware of any dip errors, and disentangle them? By the way, Jim didn't ask for an estimate of the observer's height of eye, and that can be a significant matter. ============== But I would like to ask a slightly-cynical question. Unless you are going to use your sextant to measure lunar distances, does the possibility that it might be out by a minute or two matter a scrap to your navigation, in a small craft? Just thought I'd ask... 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. --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To , send email to NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---