NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: sextant index error measurement
From: Bill B
Date: 2006 Nov 03, 19:31 -0500
From: Bill B
Date: 2006 Nov 03, 19:31 -0500
Jim wrote > 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. Thank you Jim. In the methods proposed by Frank, George, and I think Paul we are attempting to null out parallax mathematically or empirically. I am still digesting Paul's method. The gist being that with the laser we are using two dots spaced to represent the distance between the scope's line of sight and the index mirror line of sight, so parallax is not an issue. Similar with George's lower tech method. With the laser, find the distance between the scope's line of sight and the index mirror line of sight (near target). When the laser dots are the same distance apart a goodly distance away, the rays and therefore index and horizon mirror are for practical purposes parallel, which when viewing an object at close to infinity translates to no parallax error. So the setting on the sextant is the IE. Then for IC to paraphrase Frank, "When it is on, take it off; when it is off put it on." I would think refraction would be for all practical purposes identical for both laser beams or rays from a distant celestial object. Bill --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To , send email to NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---