NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: sextant index error measurement
From: George Huxtable
Date: 2006 Nov 3, 23:36 -0000
From: George Huxtable
Date: 2006 Nov 3, 23:36 -0000
Commenting on the dialogue between Bill and me, developing Paul Hirose's suggestion about a striped index-alignment board, Jim Martin has written- According to "Nicholls concise guide"vol.1,a close by object | or straight line should not be used to determin index error,because it | does not allow for sextant parallax and atmospheric refraction.I can | give the textbook explanation if it is of interest. But I think he has missed the point. What we were discussing, though perhaps the context did not make it completely clear, was a way of establishing what was the vertical offset, between the two viewlines of a sextant, and then setting up some target specially marked to compensate for that offset. That vertical offset between the viewlines is the basis of the sextant parallax that Jim refers to. If it's measured, for a particular sextant, and then allowed for in the marking of the target, then it no longer becomes a problem, and the warning in Nicholls, though reasonable as a general rule, no longer applies. As for refraction, the difference in refraction of two light rays, separated vertically by only a couple of inches, and only a few tens of metres long, would presumably be entirely negligible. But if Jim thinks that I have got things wrong, I hope he will explain further. 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 -~----------~----~----~----~------~----~------~--~---