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: Frank Reed
Date: 2009 Jul 3, 17:22 -0700
From: Frank Reed
Date: 2009 Jul 3, 17:22 -0700
Bill Morris, you wrote: "What you say about his lack of data is fair enough, but isn't estimating index error by using a star the simplest case of a star-star sight? If one accepts this, may we then accept that sights using two stars are likely to have similar errors?" For short distances, less than five or ten degrees, sure, of course, they're similar, and identical at zero degrees, but there's a substantial difference with larger angles. You 'swing the arc' (as George also has noted in a couple of recent messages) when you bring the two star images together, and it's much easier to see the alignment in those cases. You wrote: "In my case, using a SNO-T sextant with x6 telescope clamped atop a theodolite tripod the standard deviation of index error observations using a star was 0.17 minutes (n = 30). " Sounds quite reasonable! Just about what I would expect with a SNO-T. Have you tried this unclamped? Also, what do you find when you average sets of four? Does your s.d. decrease by a factor of two? And may I ask, how are you "clearing" these observations (just out of curiosity)? You wrote: "I am not a fan of Galilean 'scopes and Gordon seems to have been particularly unlucky with his if he found "...a star image will superimpose over a range of 5'." " Yeah, that's an amazingly bad result (Gordon's 5' range, that is). It could either be an unusually bad telescope or possibly that issue of dark adaptation that Bill B. and I have experimented with. It turns out that many people see bright stars as distorted, flared "blobs" when their eyes have dark-adapted but the images are much sharper when we first step into the dark. Traditionally, most navigators have considered full dark-adaptation a pre-requisite for star sights in order to see the horizon clearly, but that may work against star-star sights. Also, Gordon's daylight observations of altitudes are much better than this 5' error range he finds with stars. Something is clearly amiss. I don't think we should worry too much about Gordon's results way back in the early 1960s. He was at that time a young, newly-graduated engineering student with a passion for boating and an enthusiasm for celestial navigation. His results are what they are: the observations and ruminations of one individual using (mostly) one sextant. There are some nice points in his paper, but also some very weak points. -FER --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---