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    Re: Position by compass variation
    From: George Huxtable
    Date: 2006 Nov 24, 23:44 -0000

    Jack Ganssle asked a good question-
    
    Wolfgang K�berer in his interesting post about the NMM said something
    in passing about determining position by variation. That sort of
    boggles my mind as variation - especially off the US East coast -
    changes slowly with distance. How does this work? And, how, especially
    long ago, did mariners determine variation with any sort of precision?
    
    ==========================
    
    Well, there was a serious proposal, in the 1600s, when variation
    had not then been well mapped, that if magnetic variation altered
    smoothly and reproducibly with longitude, and remained constant over
    time, then it could be used for determining longitude worldwide, when
    no other method existed. That was one reason why Halley made his two
    cruises around the Atlantic, right at the end of the 1600s , though by
    then it was already known that variation was inconstant. Halley's
    observations showed a complex pattern for variation, and his voyages
    ended up with a useful map of variation which might help navigators to
    correct their compasses, but was not a lot of use for determining
    longitude.
    
    Of course, the usefulness of variation in an area depended greatly on
    the direction of the isogons (contours of constant variation). Where
    they ran approximately East-West, so that variation varied from North
    to South, those variation changes added nothing to the well-known
    latitude. Halleys map shows that in certain regions (and the ocean
    West from Cape of Good Hope was one) the isogons around 1700 lay
    roughly parallel to the African Coast, generally in a North-South
    direction, and the variation was changing by roughly 6 degrees for a
    10-degree change in longitude, being about 10 degrees West at the Cape
    itself.
    
    What the speaker explained was that 200 years earlier, at the time of
    Portuguese expansion into the Indian Ocean, the variation West of the
    Cape showed a similar pattern, though at the Cape itself it passed
    through zero. Navigators, out in the Atlantic, heading for the Cape,
    could choose to sail along the right latitude line readily enough, and
    by measuring variation, to a bit better than a degree, could estimate
    how far they were off, in  longitude, to within 60 miles or so. That
    was a vast advance on dead reckoning, and would allow the dangerous
    Cape to be rounded at a safe distance, and a more Northerly course
    then steered, without bringing it into sight.
    
    It may not have been precise navigation by Jack Ganssle's standards,
    but it was a lot better than the alternative, which was little more
    than guesswork.
    
    Someone might well ask how Halley determined his longitudes, out in
    the Atlantic, in 1699, when he was surveying for variation. That's
    another story.
    
    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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