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Frank Reed wrote-

>I had some time today to put together a few different ways of viewing the
>historical magnetic declination/variation maps from the USGS. There's a large
>animated GIF as well as a frame-by-frame viewer available on my web site here:
>http://HistoricalAtlas.com/mag
>
>Looking at those isolines, it's interesting to consider how nearly magnetic
>variation came to being a reasonable method for determining longitude. As
>Halley discovered in th early 18th centurt, if only the lines were as "well
>behaved" in the rest of the oceans as they were in the South Atlantic in
>that period,
>it just might have been workable. Who knows... longitude might have been
>found with a magnetic compass instead of a sextant or chronometer.

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

An interesting speculation. However, there are simple, basic, reasons why
magnetic variation simply can not be of worldwide application, and why the
lines of equal variation could not be as "well-behaved" everywhere as
Halley found they happened to be in the South Atlantic.

Consider a path around the Earth from an arbitrary starting point A. To
provide a concrete example, make it a path around the equator, going
Westwards, so that A is at lat=0, long=0, but the argument applies to any
such path.

Having gone right round the Earth, the magnetic variation must (of course)
be back to the same value that it had on setting out. That is, any
increasing variation with (Westerly) longitude along some parts of the path
must have been exactly compensated, along other parts of that path, by a
decreasing variation with longitude.

But, if the variation is going to change smoothly with longitude, then
those regions where it decreases, and those reqions where it increases,
must be separated by regions where variation is unchanging, or changing
very little, with longitude. In those regions, variation would be useless
in providing a measure of longitude.

(I'm aware that there are paths, between the corresponding geographic and
magnetic poles, for which the above argument breaks down, but in those
regions magnetic variations are unusable for other reasons.)

What finally put the nail in the coffin of Halley's proposal was the
realisation that variation varied, not just with position, but also with
time, which is why the magnetic compass roses on our charts become out of
date so quickly.

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

Willian Scoresby the younger recorded, from 1811, his annual voyages to the
"Greenland whaling", which took place in a bight in the ice which formed
West of Spitzbergen each Summer. See "The Arctic Whaling Journals of
Willian Scoresby the Younger, vol 1, ed. C. Ian Jackson, Hakluyt Society
2003".

Going North from Whitby, calling at Shetland (Zetland), there were few
navigational problems, but returning South, after two months out of sight
of land, the problem was to avoid Shetland, with its profusion of outlying
skerries (unlit in those days, of course) by sailing either East or West of
that archipelago. It didn't matter much which side Shetland was passed, but
it was important to know which.

Scoresby had a strong scientific bent, and always took an interest in
magnetic variation. Indeed, later he would submit papers on that topic (and
many others) to the Philosophical Magazine, and be elected as Fellow of the
Royal Society.

On his 1812 passage North, he had carefully noted magnetic variation when
East of Shetland, and on the return journey, again measured variation when
nearing the latitude of Shetland. He correctly deduced from its higher
value (together with clues from a sounding by the dipsy (deep-sea) lead, a
somewhat dodgy (in my opinion) lunar, and swell patterns) that he was then
West, not East, of Shetland, and shaped his course around Orkney
accordingly.

This is the only instance I know of where evidence from magnetic variation
was actually used in navigation, but no doubt there were many others.

George

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contact George Huxtable by email at george@huxtable.u-net.com, by phone at
01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
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