NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Noon Fix
From: John Karl
Date: 2009 Apr 11, 19:28 -0700
From: John Karl
Date: 2009 Apr 11, 19:28 -0700
I've read with interest this discussion of the noon fix. So I though I would run off some exact results that simulate Jim's example -- as far as I understand it. I thought such a simulation would give us some insight to the problem. So attached is a plot of altitude versus time observations around LAN with our boat sailing with four different components of northerly speed, the maximum being 10 knots. The top red curve is for no speed, the bottom curve for the 10 knts. The Dec = N11 degrees, the initial latitude at t = 0 min is N69 degrees. The zero-speed LAN is at 30 min with Ho of 32 degrees, virtually Jim's Ho, but I don't know his Dec or Lat. As expected, you can see the northerly velocity component shifts the maximum Ho progressively earlier as the speed increases to the 10 knts, and depresses Ho since the boat is moving progressively away from the sun. The apparent LAN for the 10-knt boat is shifted 6 min early, resulting in a apparent Lon shift of 1.5 degrees -- a huge amount. The apparent Ho is depressed 4.3', small in comparison to the Lon shift. As we all know the geometry of the spherical nav triangle, at times, can be counter intuitive. So I would hesitate to adopt Jim's simple linear correction as gospel without further study. From the little bit of experimentation I've done, it seems like the example discussed here is somewhat on the extreme side. Other Lats & Decs have produced smaller effects for the 10 knt case. Depending on Lats & Lons, some only had 2-min time shift, but the 4' depression was typical. I haven't had the time, maybe not even the inclination, to develop an approximate sight-reduction method along Jim's idea to confirm his method. In fact, like George, not sure I understand exactly what his method is. So far, I view this whole discussion as rather academic considering how easy the intercept method is, and how satisfactory the standard noon shot is, even with its horrible Lon accuracy, for standby or emergency situations. One last academic thought: The "noon fix" is exactly just that. We're trying to measure both the sun's altitude and its azimuth ( zero or 180) which does indeed give a true fix. And as commonly appreciated, we can't measure the azimuth very accurately -- even when we're dead in the water. --JK --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---