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: Irv Haworth
Date: 2008 Jun 03, 11:07 -0700
From: Irv Haworth
Date: 2008 Jun 03, 11:07 -0700
Hello My 2 cents worth... High altitude sights are VERY difficult to take unless your arms are strong and steady as a rock...and indeed one plots it as a circle of position. himself -----Original Message----- From: NavList@fer3.com [mailto:NavList@fer3.com] On Behalf Of Greg R. Sent: June 3, 2008 8:53 AM To: NavList@fer3.com Subject: [NavList 5257] Re: longitude around noon (a twist) --- 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 -~----------~----~----~----~------~----~------~--~---