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, 08:13 -0700
From: Gary LaPook
Date: 2009 Jun 11, 08:13 -0700
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---.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 NavListfirstname.lastname@example.org -~----------~----~----~----~------~----~------~--~---