NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Lat/Lon by "Noon Sun" & The Noon Fix PROVE IT
From: James N Wilson
Date: 2009 Apr 25, 21:11 -0700
From: James N Wilson
Date: 2009 Apr 25, 21:11 -0700
George: Your simulated data has thrown me for a loop. I expected to learn, but not to find that what I proposed is not what I expected. I started with your first example, not expecting to do any more. First, I had to convert your decimal data, which is what the computer needs, to that which is what is commonly used with the sextant and the almanac.Degrees, minutes and tenths. A bit of work, but no big deal. All of my programs input and output data in that format. The conversion to decimal and back is done out of sight. My first computer lessons stressed that, "The computer should work for you and never vice versa." Alas, grossly violated these days. The results of that first try were not good. The altitude-time slopes looked fine, and the ascending slope was flatter than the descending one, which is what would be expected from the northward motion. But the time from highest altitude to LAN was only three minutes, and I knew it should be about five. So, I reluctantly tried the second set. Worse! I was surprised to find that the slopes for that example were significantly flatter than for the first. Giant difference! Astonishingly, the ratio looked okay--the ascending slope was flatter than the descending one. So, I had to sit back and think. My method is dependent on determining the slope of the altitude-time lines, and therefore it is vulnerable to factors that distort those. But I never expected to see such a huge variation. As I wondered what I would do if this were a real situation, I realized that I would have to start taking sights earlier. The standard quoted is twenty to thirty minutes before maximum altitude. Your example started twenty-five minutes before. But with such a low altitude, I should start earlier. Hindsight, but that's what I would do faced with the situation. That intensifies errors due to changes in course and speed, but it's the only way out. I make my students plot their sigths, and they reluctantly do so at first. But they have increased confidence in them when they see that they fall on a straight line. The idea there is to help them select the best sight for reduction. Now, I once was invited to introduce a boatload of scouts to celestial navigation on one of the LA Maritime Institute's brigantines. I instructed them as to how to hold and use my sextant, and how to take readings. Then I turned them loose. I gave each one a single opportunity, and recorded the time and altitude. I couldn't reduce the sights, but I did plot them as they took them. I was pleased that they all fell on a straight line. That meant that they were doing it right. Their first sights ever, and they were all good. Except for my granddaugher's. Anyway, I'm a believer in plotting runs of sights. Too see such an extreme variation in your simulated data shakes my confidence. So, I need a run of sights starting earlier. Maybe forty minutes before maximum altitude. Sorry to make more work for you, but if we're trying to simulate real life, it's necessary. The idea of the double altitudes method is to take sights far enough away from LAN to get a reasonable attempt at determing it. At higher latitudes, starting earlier is the only way to do that. Hoping for better results next time. Jim Wilson ____________________________________________________________ Get Info on Money Market Account from 14 search engines in 1. --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---