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Frank Reed wrote
|
| A few days ago, in reply to one of my posts, George H wrote
| "A series of sights, taken around noon, will indeed be quite
"symmetrical",
| as long as the vessel's North-South component of speed stays constant.
It's
| simply a parabolic curve. Not symmetrical about Local Apparent Noon,
| however, but about some moment, displaced from noon by an amount which
| depends on that speed component. Allowing for that time-offset presents
one
| problem, but not the biggest one."
|
| Hi George, I'm sorry I confused you with my use of the word 'symmetrical'.
| What I was saying is that the altitudes are no longer symmetrical in the
| sense required by the usual "equal altitudes" method of determining
| longitude around noon. That's the problem that arises due to the motion of
| the vessel and the slowly changing declination of the Sun.

Frank's apology is unnecessary. He didn't confuse me; not a bit. Neither in
my mind nor in my words, which were quite free from any confusion. The rise,
and the fall of the Sun are indeed quite symmetrical, which is why his
folding-paper trick will work. It comes down as a mirror-image of the way it
went up. It's just that the centre of that symmetry is displaced from Local
Apparent Noon, exactly as I explained, when there is a North-South component
of the vessel's velocity, or a changing declination.

He continued-
| Of course, it's easy to deal with that.

Which may well be true, but nowhere has he explained how to do it.

Then Frank supposes a "typical case", which requires the navigator to be on
the bridge, taking 12 altitudes, over a period of forty minutes around noon.
Not many practical navigators would agree with Frank's claim that it isn't
impractical, when a simple pair of sights, mid-morning and noon, would do
the trick instead, and more precisely.

Frank gives us no numbers to put into his "typical case", to allow his
claims of precision to be checked. What are its limits of applicability? He
doesnt say. So let me put forward another case, quite "typical" for mariners
in the trade I describe.

Take the case of a steamer, in the transatlantic trade, aiming to pass
North-about Ireland in lat 56 deg, in midwinter. She is en route for the
Clyde or the Mersey, in World War 2, with no radio aids. Skies are
fortunately clear, and her chronometer is good. She can measure altitudes
well enough to get a noon latitude to an arc-minute. If her skipper spends
40 minutes around noon on his longitude, I ask Frank how good the result
will be, to allow her to approach the unlit Scottish coast in safety, after
dark.

We have to remember that Frank's interest, in minimising the difficulties of
measuring longitude-around-noon, relates to his pushing of it as a teaching
method, because its principle is a simple one to grasp (which indeed it is).

But much of this is old ground, which has been well-trodden on this list
before, and I doubt whether any minds are going to be changed.

================

On a different topic, I had said, in relation to high Sun altitudes, near
the zenith;

 "It isn't easy for the navigator to find the best direction to face with
his sextant, with such a high Sun, but that's another matter."

And Frank replied-

"If the navigator knows that there are TWO ways to swing the arc, then this,
too, is no problem."

I still think it is, though. Presumably, Frank refers to the swinging of the
sextant about the line to the body, rather than about the horizontal; a
technique that any decent observer always practices. But even so, accurately
measuring a high body is no picnic.

To be honest, I have never navigated in or even near the tropics, so I have
no experience of observing a really high Sun. I wonder whether Frank speaks
from such experience. I have tried, at twilight, measuring a star near the
zenith, and finding it so tricky, tried a different star instead; but then,
there are always many stars to choose from.

George.

contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.




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