NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Perpendicularity check
From: Alexandre Eremenko
Date: 2004 Sep 23, 00:40 -0500
From: Alexandre Eremenko
Date: 2004 Sep 23, 00:40 -0500
Dear Frank, Thank you very much for your suggestions, especially on the star-to-star distances. (My main concern at this moment is checking my new sextant, and learning how to use it properly). On your on-line almanach, I am just curious: Why do you need to know my coordinates to tell me the Sun's GHA? I found the same strange feature in other on-line almanach as well. On Wed, 22 Sep 2004, Frank Reed wrote: > I have put together a very accurate > Nautical Almanac tool on my web site. It > can display almanac data for the standard > navigation objects accurate to about > 1 arcsecond for the period from 1750 to 2050. > The main address for the > various tools is > www.HistoricalAtlas.com/lunars > By the way, I think the "Complete On-board Celestial Navigator" > is very > clever, and its accuracy is sufficient for > the realities of standard late 20th > century position line navigation. I also think so. But my primary purpose at this moment is checking my sextant. After I reworked my sight reductions with more precise Almanach data, a strange thing happened: it looks like all the results just shifted by a fixed amount of 0.7 to 0.8'. This seems very strange to me. The "Complete on Board" has some systematic bias against the real almanach? Incredible. Concerning the sight reduction, I also used the Complete-on-Board first and the errors were up to 3'. After some thinking I decided that this is not surprising because the Complete-on-Board tables do rounding on each step to the whole minute. So I wrote a simple program (on a spreadsheet) to do the spherical triangle with high precision. This improved my results, and now the average error (over 5-10 observations with few minutes intervals between the sights) is about 0.3' to 0.4', if I exclude the observations that stick out, apparently because of crude mistakes in my measurement or in my sextant reading. The Davis artificial horizon I ordered has not arrived yet, so I am using a plate with water, and the water surface is perturbed by the slightest air motion). But I am curious whether this .3' error can be also eliminated... and what is the sextant's ultimate precision. Alex.