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    Re: A-10 Sextant Manual
    From: Christian Scheele
    Date: 2009 Jun 11, 14:10 +0200

    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 :
    Go to "List of topics" then to "Single LOP landfall procedure."
    Navigation List archive: www.fer3.com/arc
    To post, email NavList@fer3.com
    To unsubscribe, email NavList-unsubscribe@fer3.com

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