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Frank wrote-

 As for longitude, the biggest problem with using a series of
| sights around noon is that they are not really symmetrical unless the
| observer is motionless and the date is near one of the solstices.

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.

| But these
| issues can be corrected without a whole lot of trouble and you will then
| have a longitude, too. The longitude would not be as accurate as the
| latitude but not too bad either. It depends on the details (as Bill noted
in
| another post).

Frank is minimising the difficulties here, which we have been into in some
detail on the list before. The trouble is that the altitude is changing so
slowly, near noon, that the procedure is inaccurate, unless the observation
is extended over a long period either side of noon, to determine the moment
of symmetry. And that's the biggest problem. I agree with Jeremy's
assessment of such a method of obtaining longitude as "horribly
impractical".

Indeed, the observation that Jeremy was detailing, with a Sun that's very
near the zenith, gives the best chance of making such a assessment of
longitude, because then the altitude change is much sharper, close to noon,
that it as around a noon with a lower Sun. 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.

But I fear that we are detracting somewhat from the simple message that
Jeremy was putting over with this exercise, and I regret, a bit, having
diverted the discussion down this side-alley.

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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