NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Calibrating a sextant scale
From: George Huxtable
Date: 2007 Nov 24, 23:43 -0000
From: George Huxtable
Date: 2007 Nov 24, 23:43 -0000
Alex wrote- 4 years ago, when I joined the list, one of | the list members (I think it was George) wrote that he | challenges anyone to determine his (modern, metal) | sextant arc correction from | stars. I tried to do this. I made several hundred observations | of stars, and failed. Not sure that it was me, though it was a good question. I would like to ask some supplementary questions. Has anyone on this list, by measuring star-star distances or by any other method, ever discovered reproducible errors, outside the terms of a calibration certificate or maker's warranty, in a sextant? Has anyone made calibration measurements of his own, in which he has more confidence than in the manufacturer's scale readings, corrected as necessary by the box certificate? And if the answer is yes, what's the magnitude of those errors? ===================== With the discussion about inter-star differences, I remembered that John Karl's new book, "Celestial Navigation in the GPS age", devoted several pages to helping users to calibrate or check their own sextants that way. He selected 12 pairs of bright stars (with rather a Northern-hemisphere bias), to provide a suitable spread of angles to calibrate, ranging from Bellatrix to Betelgeuse, at about 7 deg 30', as far as Betelgeuse to Spica, at over 113 deg. For each such pair, he provides a table, showing how the refraction alters the odd minutes and fractions of that separation, based on the observer's latitude, and on the altitude of the first-named star. In the explanation he claims- "Since the observer's latitude and the star's altitude determine the altitudes of any other star ... the altitude of the second star is not needed". On the face of it, it seems a good simple scheme, dead easy for a user to implement. But on reflection, I'm not convinced. I have been worrying about that statement. I don't think it is true. It's all a bit more complicated than that, I fear. Given a latitude, and a star with known declination, and an observed altitude, it's true that one can deduce a local hour angle. That local hour angle will be the same in amount, corresponding to that altitude, whether the star is rising or falling in the sky, before or after culmination, but will be opposite in sign. And there will therefore be two completely different Greenwich hour angles. And therefore two completely different possible values for the local hour angle, and thus the altitude, of the second star. Therefore, as I see it, there should be two different tables for the refraction correction, depending on whether the first star is to the East or the West of the observer. The table as given, for, say, Bellatrix to Betelgeuse, tells only half the story. I haven't investigated the matter deeply enough to discover which half. Am I missing something, somewhere? Have I misunderstood? Can anyone help? John Karl himself, perhaps, if he still tunes in to Navlist, though we haven't heard from him recently. 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 -~----------~----~----~----~------~----~------~--~---