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    Re: Johnny Walker Commercial on TV
    From: George Huxtable
    Date: 2009 Oct 28, 15:28 -0000

    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.
    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.
    NavList message boards: www.fer3.com/arc
    Or post by email to: NavList@fer3.com
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