NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: A Lunar
From: Frank Reed
Date: 2008 May 28, 08:35 -0400
From: Frank Reed
Date: 2008 May 28, 08:35 -0400
Jeremy, you wrote: "I am wondering what people are typically getting for errors? I am thinking that this is a bit much and I think with a bit more practice I can do better." Yes. But in may not be so much about practice... These are the critical factors: 1) Sun not stars. Most people seem to get much better results with the Sun as the other body. I also find that I get great results with Jupiter and Mars. When the other body is a star my results are typically about 50% worse. In addition, when you look through historical logbooks, it's clear that the Sun was generally the object used in the majority of lunars (on the order of 80% or so). 2) telescope magnification. The unaided human eye (corrected with eyeglasses or contacts) has a resolution of about 1 minute of arc. If you want to see differences as small as, say, one-tenth of a minute of arc, then you need a telescope with a magnification of 10x. 3) index error. Obvious, yes, but it's much more important with lunars. Us the Sun limb method (or the same with a gibbous Moon in twilight, which is my favorite) and do a bunch of trials and average. You need the best possible index correction you can get. 4) general sextant adjustment. Go through and double-check all the little alignment errors. The mirrors need to be perpendicular to the frame. The telescope axis needs to be parallel to the frame. Etc. One nice thing about doing lunars is that it forces you to test all these things carefully. 5) overall instrument quality. If the instrument has any "slip" or looseness in the components, that will show up here. Not much you can do if this is the problem. 6) arc error. After you take care of everything else, if you still get errors larger than a fraction of a minute of arc, then you probably have arc error. This is supposed to be listed on the table in the case, but frequently those tables are old or filled with zeros that may indicate cheating on the testing. The nice thing about arc error is that it is correctable. This is yet another good reason to shoot lunars. If you get reliable errors for certain angles, you are essentially compiling a fresh error table. When you have accumulated enough data, your sextant is good as new --when the new errors are applied-- and every sight you take (lunar or otherwise) will benefit. 7) sight geometry and astronomical conditions. It helps when both objects are higher than five or ten degrees. The sky should be clear (one can do normal altitude sights through high cirrus but not lunars). Some uncomfortable orientations can make the process much harder. Myself, on individual lunars, I get errors with a standard deviation of about 0.25 minutes of arc when all the above are working in my favor. When I take four lunars in a row and average, which was a common technique historically, I get a mean error about half as large: around 0.1 minutes of arc standard deviation. Please note: your mileage may vary! Also note that a standard deviation is not a maximum error. When we meet in Mystic in July, let's plan a time when the Moon is "in distance" (as they used to say). -FER --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---