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 26, 14:30 +0100
From: George Huxtable
Date: 2009 Jun 26, 14:30 +0100
Gary asked- I don't follow the discussions of lunar distance very carefully. What is the expected accuracy of the derived longitude in practice? There's an unavoidable factor of about-30 comes in, from the ratio between the length of the month and the length of the day. So the Moon travels round the sky, with respect to the stars, at only about half a degree per hour. Greenwich time is determined from measuring that motion. So if you can measure Moon position to 1 minute of arc, with respect to the star background, then you can measure Greenwich Time to about 2 minutes-worth, thus longitude to about 30 minutes. If you could measure Moon position to half a minute of arc, which was about the best anyone could hope for at sea, even in good conditions, then if there were no other errors (in prediction, in correction, in deducing local time from another observation), you could get longitude to about 15' of arc, 15 miles in the tropics. Dyson's estimate of 20 miles wasn't far out, in my view. Dyson was careful to state (from his 1920s perspective) "Unfortunately, even with perfect tables, , it is found that the most skilful mariner cannot obtain avery accurate position of his ship in this manner. With great pains and elaborate calculations he can be correct to within 20 miles." And so, Frank's qualification to that statement is irrelevant "And don't forget that an error of 20 miles included the error from the lunar distance tables in the Nautical Almanac. Since those were about 0.3' for the latter half of the period when lunars were commonly used, the implied error in the observations is even a little less.". It wasn't. Dyson was assuming perfect tables, and said so. Frank denigrates the Astronomer Royal as "not knowledgeable on lunars". I have asked Frank before now, on several occasions, to explain to us his own qualifications and experience in determining lunars fron a vessel AT SEA. He has never responded. George. contact 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 -~----------~----~----~----~------~----~------~--~---