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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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