NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: longitude around noon (a twist)
From: Greg R_
Date: 2008 Jun 3, 08:53 -0700
From: Greg R_
Date: 2008 Jun 3, 08:53 -0700
--- frankreed@HistoricalAtlas.net wrote: > That's a good point about sights very close to the zenith. I had > mentioned previously on the list that there may be a special case > when the Sun is close to the zenith. I still haven't thought through > whether it really screws up the graphical technique or merely > requires more stringent rules for its application. Something sticks in my mind from way back when I was first learning celnav that for objects near the observer's zenith (i.e. Hc ~89 degrees or greater) the resulting LOP should be plotted as a circle of position (do we call that a COP?) around the object's GP vs. a straight line as we normally do. But I don't have any personal experience doing that, so I can't vouch for the technique or its accuracy. -- GregR > I made one last try at convincing George Kaplan (a name some of you > know) to > join us in Mystic this week, but alas, it's not in the cards. > > While I was at it, I chatted in my email a bit about longitude around > noon > and asked him this: > "Which leads to a question: is there an established name in the > literature, > or even in your own jargon, for a fix resulting from a series of ten > or > twelve sights taken over a relatively short period of time? I've been > > calling it a "rapid-fire fix". Do you know another name?" > > His reply: > "I don't know of a special name. You're correct, of course, if you > can get > a bunch of sights on either side of noon, you can get good enough > geometry > to get a 2-D position. It works with the LOPs, too, in that they > provide a > good spread of azimuth around then. There is a slight catch, > however, and > that is, the higher the Sun is in the sky (and therefore the more > rapid the > altitude and azimuth change near noon) the more you have to worry > about the > curvature of the LOPs. In some near-degenerate cases (sun within > several > degrees of the zenith), the usual straight-line plotting -- or math > that > assumes straight-line LOPs -- may not provide the right fix." > > That's a good point about sights very close to the zenith. I had > mentioned > previously on the list that there may be a special case when the Sun > is > close to the zenith. I still haven't thought through whether it > really > screws up the graphical technique or merely requires more stringent > rules > for its application. > > -FER > > > > > > --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---