NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Star-star distances for arc error
From: George Huxtable
Date: 2009 Jun 24, 16:44 +0100
From: George Huxtable
Date: 2009 Jun 24, 16:44 +0100
Douglas Denny ended his last posting with a farewell, writing- "I object for instance to being dismissed out of hand with an offahnd and glib: "clear indication I have not tried it" - when I have. So I think it time to leave." Well, it's a pity to lose a new member who is clearly interested in the topic, although reluctant to consider views other than his own. I sympathise with his objection to the phrasing of the words he complains about. But just in case he hasn't quite closed his eyes just yet, and to put the record straight, I will have a go at addressing some aspects of that departing posting that seem clearly wrong to me. It's helpful that he has identified his Hughes sextant as a micrometer instrument, in which case the "platinum divided scale", indicating just whole degrees, serves not much more than cosmetic purposes. The provision of a calibration certificate becomes useful in correcting for any scale errors, and provided those corrections are applied and not ignored (and they vary smoothly over the arc) imply that any such scale errors do not degrade the overall accuracy of the instrument. The extent of the maximum correction, of 1' 30", does not of itself prevent the instrument being read and used to 10". Although my old Vernier sextant has a Kew certificate of zero measurable error (which I think implies no more than 10",) at all calibration points, I would be just as happy to use Douglas's Hughes sextant, after making the appropriate corrections. A corrected error isn't an error at all. We were informed that the attempted tests, which failed, were tried using a sextant that was clamped in place, not hand-held, and I suggest that was one reason for the lack of success. It prevents the sextant being "swung" about , to achieve proper coincidence. Indeed, it's hard to see how a user would achieve that alignment in the first place, before tightening a clamp. And than, that plane will swing around the Pole, at 15� per hour. No the sextant should have been used held in the hand, just as it's designed for. Mariners measuring lunars at sea, for whom such clamping was impossible, had to put up with the bodies in view "dancing around" far more than did Douglas on land, who complains "Some form of rest at the very least is needed, or a clamp with some degree of freedom. The stars dance about too much for accuracy otherwise." That's exactly the motion that a sextant is designed to cope with. I have tried to make my view clear, that measuring star-star lunars is tricky, calling for skill and practice, and a clamped instrument provides no easy answer. Douglas is clearly wrong in suggesting that - " The resolving power of the telscope is not the issue here, but the ability to superimpose star images - accurately. In other words: eye acuity is the issue.". And elsewhere, he goes into different aspects of human-eye acuity in some detail. But all that is relevant only if he is using a peep-tube rather than a telescope. That's the purpose of using the telescope, to enhance the acuity of observation, by a factor of approximatly the magnification of the telescope. So those acuity values we were given should have been improved by the factor of magnification, 3 or 5, or whatever. I am surprised that an optics professional has failed to take such matters into account. ================== Let's turn to Douglas' views on lunars, about which he seems less than expert. He wrote "Lunar distances were very poor indeed." (whatever that means) and quotes Dyson, in 1922 stating that mariners using lunars couldn't do better than about 20 miles. Which is in my view, is a fair assessment, but of course to measure a lunar to 20 miles or 20' at the equator requires measuring an angle to within 40 seconds, under at-sea conditions. Which already undermines Douglas' claim that one can't do better than 1', even from on land. He continues- "And this means with use of the reflecting circle too - more accurate than a sextant - eliminating some of the inherent errors of the sextant.". Nonsense! Dyson was writing about sextants. The circle was a proposed solution hardly ever used by navigators at sea, though occasionally, until the early 19th century, by surveyors and hydrographers. It was doomed when machine division of sextants arrived around 1780. He adds "It took at least an hour for the astronomers on board ship when the method was tested to 'clear the distance' to find longitud", Also nonsense! The clearing process wasn't what took the hour's calculation. That was the time it took to calculate the position of the Moon in its orbit, and the need for doing that disappeared when precalculated lunars were published in the first Nautical Almanac of 1767.c George. ontact George Huxtable, at george@hux.me.uk or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---