NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: The Noon Fix
From: Frank Reed
Date: 2009 Apr 8, 12:00 -0700
From: Frank Reed
Date: 2009 Apr 8, 12:00 -0700
George H wrote: " But his posting raises a few questions in my mind." Again... :-) George, by focusing on very small details, you are throwing out the proverbial baby with the proverbial bathwater. One can, in fact, apply this seemingly "heretical" method of getting a fix with excellent results. You've really missed out by not investigating this further. Of Jim's book, you wrote: "One problem is that, presumably, he reserves the details of his arguments to his book, perhaps in order to encourage us to buy it, but that inhibits informed on-list discussion of his points." NavList is surely not the target market for such a book. Jim, you do whatever you want, but be careful not to fall into the trap of posting the material contents of your small book on the net just for the amusement of us on NavList. While NavList members are likely to be able to help spread the word regarding your book, this is not the primary market for it. So what IS the market for this type of navigation? I would suggest that nearly anyone new to sextants in this GPS age should learn this method of navigation (or one of its variants) FIRST. It's easy. It's reasonably accurate. And it takes the student from first sights to a fix in latitude and longitude in the shortest possible time. Additionally, and importantly, it's very easy to re-learn this technique in just a few minutes. A navigator who has learned "latitude and longitude by noon Sun" has the fundamentals in hand and, if desired, the full methodology of line of position navigation and its many, many variants can always be learned later. And George, you wrote: "What is hidden, under that catchy title of "The noon fix", or "longitude at noon", is that it's actually longitude AROUND noon" But everyone knows this, George. It does not detract from the method in any way. And: "comparing observations over a time span that extends well before and well after noon. The shorter that time span is, the less accurate is the result. Unless that time-span is a well-extended one (Bowditch recommended 30 minutes each side of noon), then "longitude around noon" becomes impractical, a fact that its proponents often gloss over." The focus on time interval, here and in Bowditch, may be misleading. What matters is a change in the Sun's azimuth (just convert the method to its equivalent lines of position to see why). For reasonable accuracy, one should take sights over a sufficiently long period such that the Sun's azimuth changes by roughly twenty degrees. When the Sun is fairly low in the sky, that's fairly close to the suggested "30 minutes" on each side of noon. But when the Sun is higher, the time interval can be quite a bit shorter. Also, just from the standpoint of terminology, the idea that you have to take sights over a somewhat extended period to get longitude near noon is really not that different from the common procedure for getting latitude at noon. Most students, when learning the LAN sight, are taught to take occasional sights leading up to noon and then catch that moment when the Sun "hangs" at its maximum altitude. So even for latitude, it's really latitude "around noon". The difference for the longitude case is that we also want some sights after noon to make a complete noon curve. -FER PS: And while we're at it, for those who haven't heard of it, among Hewitt Schlereth's numerous books on navigation, there is "Latitude and Longitude by the Noon Sight" which is occasionally available from used book dealers on abebooks.com, ebay, etc. --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---