NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: lunars with and without altitudes
From: George Huxtable
Date: 2006 Nov 12, 19:56 -0000
From: George Huxtable
Date: 2006 Nov 12, 19:56 -0000
Frank Reed wrote- | If the lunar distance is short, you have to be more careful in the altitude. | At a distance of ten degrees, an error of one minute in either altitude will | yield a tenth of a minute error in the clearing process. That didn't fit my expectations, so I cooked up some trial values. I presumed an observed star altitude, above the true horizontal, of 70 degrees, an observed Moon altitude (of its centre) of 80 degrees, and an observed lunar distance (between centres) of 10 degrees. With such high altitudes, I ignored refraction altogether, so the only correction to make was for Moon parallax, for which I assumed a value of horizontal parallax to be 1 degree. Those figures correspond to a deduced azimuth difference, between Moon and star, of zero degrees, which seemed entirely reasonable. Correcting the observed Moon altitude for parallax increased it to 80.173648 deg., and then the corrected lunar distance became 10.173648 degrees. But say that there was an eror in the Moon altitude determination, which resulted in a smaller altitude, by just 1 arc-minute. Everything is as before, except that the observed Moon altitude is now 79.983333 degrees, and its corrected value is 80.157268 degrees. What would be the resulting change in corrected lunar distance? To fit those observations, the azimuth difference between Moon and star must now become 2.36 degrees. With that value of azimuth difference, and our new corrected Moon altitude of 80.157268, and star altitude 70, and observed lunar distance 10 degrees, just as before, I now get a value for corrected lunar distance of 10.173371 degrees, just .0165 arc minutes less than before. That is, an error of one minute in Moon altitude results in a 60th of a minute error in the clearing process, not a tenth of a minute as Frank quoted. One of us must be wrong. Considering my recent lamentable record in trig errors, it could be me. 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 -~----------~----~----~----~------~----~------~--~---