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: Jeremy C
Date: 2008 Jun 4, 01:44 -0700
From: Jeremy C
Date: 2008 Jun 4, 01:44 -0700
> But when Hc is near 90 degrees (say, above 87 or 88), it's entirely > feasible to actually plot a circle of position. � Plot GP of the body on > the chart, open dividers (or, better, a draftsman's compass) to D, and > draw at least the part of the circle of position near one's DR. > > Lu Abel > But when Hc is near 90 degrees (say, above 87 or 88), it's entirely > feasible to actually plot a circle of position. Plot GP of the body on > the chart, open dividers (or, better, a draftsman's compass) to D, and > draw at least the part of the circle of position near one's DR. > > Lu Abel I have done this many times on paper, and most recently on my last US coast Guard exam, and a couple of times at sea with both sun and moon. It is very difficult to shoot, especially as HS exceeds 89degrees. The body literally moves in a sine wave as you rotate yourself around across the horizon. I shot the moon in one case where the body never left my 3.5x scope as I moved in a complete circle. The best thing was to shoot it both facing north and south. Plotting is actually fairly easy, and getting a fix is easy yet troubled. Take 2 sights a few minutes apart, plot the GP, then advance or retard the GP to a common time (similar to advancing a radar LOP) then draw the arc according to minutes of zenith distance. The two circles will intersect in two points, which is the issue. I just picked the nearest to my DR. On the USCG test, it was a bit trickier. I can post the problem once I get home and look it up. The very high altitude moon shoot I did (89deg 50+ min) Ho was around 89deg 57' or so so left me with a small circle of position which was in effect my fix. It is in my old nav notebook and I will post it when I get home. As my rule of thumb, I typically shoot circles when HO exceeds 88 degrees. 120 miles can be reasonably plotted on universal plotting sheets with little trouble. I tried once with an 87 degree sight and it was a bit tricky. On larger scale charts, it gets more difficult. On the exam I was able to plot on PS 923 I believe. A NM is about 2mm on that scale. It's a fun sight to shoot in the tropics. Jeremy --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---