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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



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