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Brad wrote, about a commercial portraying an expedition to the South Pole-

" It shows an expedition to the South Pole in 1908 and how they are getting
there via COMPASS!

The compass would not point to the geographic south pole.  It would point to
the magnetic pole.  These poles, of course, do not coincide.  Further, on a
straight line path to the geographic pole, the compass would swing
terrifically.  You could navigate to the pole this way, under the assumption
that you knew the variation of the compass for the sledging journey. Since
the 1908 journey was a pioneering expedition, how could they know the
variation?  Wouldn't they be trying to measure the variation?"

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

That isn't as silly as it might seem at first sight.

To keep a sledge travelling in a straight line, some sort of directional
pointer is needed. If the Sun can be seen, and a watch is carried, there's
no problem.
An appropriately-balanced compass, well suspended and damped, is usable
along most likely Polar routes, which do not pass near the South Magnetic
Pole. As long as it's given enough time to settle when the sledge pauses, a
compass can provide a steady direction. (In that respect, a compass is
better-off in that environment than at sea, when it can never be held
still.) It won't point toward the South Pole, but that doesn't matter; any
direction will do, as long as it stays reasonably constant. And the magnetic
variation, though enormous, won't change much from one day to another, at
the slow daily rate of sledge travel.

So the technique is to observe for magnetic variation, using a Sun azimuth
at known GMT, whenever the Sun can be seen, and then use the compass to
maintain a heading on that basis, whenever it can't. That works, and
requires no prior knowledge of variation.

George.

contact George Huxtable, at  george@hux.me.uk
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.



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