# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

**Re: A-10 Sextant Manual**

**From:**Gary LaPook

**Date:**2009 Jun 11, 09:12 -0700

From looking at his chart and the Nautical Almanac it is clear that the highest altitude he measured on sun was about 51 degrees which would produce a three-tenths of a mile offset at 45 miles (Table 19) and maybe one NM two degrees along the LOP, nothing to worry about. For those curious about computing winds based on drift sights I have attached an excerpt from Air Force Manual 51-40. gl Gary LaPook wrote: > Chichester did use Mercator charts, actually plotting sheets, for > this flight. I have attached his charts. The first two are > illustrations from his books. The third is figure 41 from Chichester's > own explanation of the navigation of this flight published in the > Observer's Book, which I have also attached. This chart is a > simplified copy of the following charts, the actual chart that he used > on his flight from Norfolk to Lord Howe. The last chart is his actual > chart used for flying to Norfolk from New Zealand. I have also > attached Hughes' explanation of the navigation. > > The actual charts may look confusing because of the arcs and the > radial lines which was Chichester's method of determining the wind > encountered in flight based on his observations of his angle of drift > on two different headings. More modern practice is to plot the drift > lines on an E-6B rather than on the chart itself to determine the > winds. If you are just interested in the celnav aspects, you can > ignore all of these lines as Chichester did in his simplified figure > 41 in the Observer's Book. > > Your concern over plotting a curved LOP as a straight line on a > Mercator chart is unwarranted. I have attached Table 19 from Bowditch > which shows how much the curved LOP departs from a straight line. You > can see that, except for very high altitudes, this factor can be > ignored especially given the lower level of precision available with > celnav in flight. > > > You can download the complete American Practical Navigator for free > from this website, choose in the "select" box. > > http://www.nga.mil/portal/site/maritime/?epi_menuItemID=35ad5b8aabcefa1a0fc133443927a759&epi_menuID=e106a3b5e50edce1fec24fd73927a759&epi_baseMenuID=e106a3b5e50edce1fec24fd73927a759Christian > Scheele wrote: > > gl >> Hello Gary, >> >> I hear you and I largely agree with you but please bear with me for a >> moment while I try to work something out. If you're assuming that a >> navigator is plotting on a Mercator chart ( I think Chichester was), >> I wouldn't say that deliberate offset navigation is a modern >> implementation of latitude sailing. Latitude sailing, using the noon >> method, gives you a latitude line and this means a position line that >> can be plotted on a Mercator chart as a straight line without the >> need for any corrections. When you find a position line on the basis >> of a sun shot at any other time other than noon, your plotted >> position line, because it is plotted as an uncorrected straight line >> on a Mercator chart, will always be "a bit" assumed unless..... you >> are actually plotting a real great circle arc, i.e. a position line >> corrected for the map projection. Could the astrograph do this? I >> assume not, because the curves on its foils are rigid, unless it >> actually came with maps to fit the curves, rather than curves to fit >> the maps, but that would be impractical way to do something I would >> imagine the astrograph was supposed to make easy in the first place. >> I am assuming, therefore, that the astrograph was, from a precision >> point of view, not a "perfect" plotter. I know that in world war 2 >> some navigators had an astrograph over their tables, but how many I >> don't know, I don't know much about the military stuff. In any event, >> Chichester didn't use an astrograph. I'm still assuming that >> Chichester used a Mercator projection. Chichester was using sun, not >> star shots. When using the stars, if you have a reasonably good idea >> of where you are, say you are sure that you must be within about 2 >> degrees, depending on which region you are in, of a point on the the >> position line which, again, is a small arc of a great circle track, >> and assuming that the global position of the sun is far away enough, >> then the aforementioned problem should not exist as you can then plot >> your position more or less as a straight line - the great circle >> radius is so big - just like Weems did in his books plotting star >> altitude curves. I say 2 degrees because I am taking an extract of >> Weems pages showing a latitude and great circle radius for which this >> arc size to gradient of position line relationship holds. You can't >> plot the "sun curves" in the same way as you would star curves, >> because of the rapid change of declination. When I said Chichester >> had an assumed position line rather than a position line, I was >> assuming that such a relationship which allows one to plot a great >> circle arc as a straight line may not have been available to >> Chichester, had his D.R. navigation not been so good, all the while >> still assuming Chichester was plotting position lines as >> "uncorrected" straight lines onto a Mercator map.... But I am not >> entirely sure, maybe I should have phrased it as a question...here's >> what I was thinking.......Let's say his D.R. had not been so good and >> he had been on the unfavourable side of the position line, that is >> away from the island, further than he had planned to be at the >> precomputed time. Let us assume that this error had brought him >> beyond 2 degrees (120 miles) or so of the predetermined point on the >> position line. Assuming this scenario, how serious would it have >> been had he plotted his position line as a straight line on Mercator >> chart? Imagine: The straight now really becomes a curve if, only a >> very slight one. Could this produce a substantial error? Norfolk >> island is 35sqkm. Lord Howe is 56sqkm. For this last stage of the >> flight, D.R. is of course used again. Did he have the drift, wind >> speed indicators that should have been around by the time of world >> war 2? What about his compass? Did he have more than one? An >> induction compass like Lindbergh - who did not use astro on his epic >> flight - to check the magnetic one? Let's say some of these issues >> come in and another error, albeit a much smaller one, is added on to >> the existing one. Could a pilot miss the island due to the combined >> sum of such errors? Very important in this regard is: How high was >> Chichester flying, i.e. what what was the range of his visible >> horizon? Perhaps I'm wrong and it wasn't that serious and any error >> would have been "innocuous" due to the far visible horizon. In >> referring to Chichester's navigational method, I was implicitly >> including this speculation.... >> Again, I largely agree with you ....but there's just that shred of >> doubt. >> I'll take a thorough look at your website as soon as possible. >> Thanks, I appreciate the tip. >> >> >> Christian Scheele >> >> ----- Original Message ----- From: "glapook@pacbell.net" >>>> To: "NavList" >> Sent: Thursday, June 11, 2009 4:01 AM >> Subject: [NavList 8612] Re: A-10 Sextant Manual >> >> >> >> Well, I don't know that you can call it "reckless" since Chichester's >> deliberate offset navigation procedure became the standard navigation >> method for finding islands and was taught to thousands of navy and air >> corps navigators during the second world war. It was actually a just a >> modern implementation of the centuries old method of latitude sailing, >> approaching to one side of a destination, east or west, and then >> following the latitude LOP to the destination. >> >> See the texts I have posted on my website at : >> >> http://www.geocities.com/fredienoonan/ >> >> Go to "List of topics" then to "Single LOP landfall procedure." >> >> gl >> >> >> >> >> >> >> > --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To unsubscribe, email NavList-unsubscribe@fer3.com -~----------~----~----~----~------~----~------~--~---