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Doug shows an interest in magnetic variation; perhaps it interests others too.

Variation is the difference between the way a compass-needle points and the
North-South direction. If there's any iron around, then the needle is also
affected by the local deviation, which varies with the course of the
vessel. But in the absence of any such local deflection (or if it's been
well-compensated out), the variation is what remains, due to the fact that
the Earth acts as an immense magnet with poles that are misaligned with
respect to the Earth's axis. What's more, that misalignment changes with
time, and there are also local fluctuations: so that the variation doesn't
follow a simple pattern over the Earth's surface that a bar magnet would
produce. Worse still, infrequently the direction of the compass will even
reverse, but as this occurs at intervals of hundreds of thousands of years,
it's not going to bother us.

Even now, I doubt whether the Earth's magnetism is completely understood,
but it's attributed to the swirling motion of electrically-conducting
liquid rock deep within the Earth. (I'm not a geophysicist, so stand to be
corrected about that.)

It's an important matter for the Earth's magnetism to be well mapped, and
its variation with time predicted as far as possible, because all our
compass courses depend on the variations marked on our charts.
Unfortunately, the British team who contributed to this magnetic survey
work was disbanded a few years ago. I wonder who does it now: is there a US
survey team at work? On our charts, there's an entry by the compass rose,
that usually states something like- "4deg 50'W 1985 (10'E)", where the term
in brackets is the predicted annual change from the 1985 value. Over time,
however, the variation will start to diverge from that prediction. Where
will the updating information come from, I wonder?

When mariners were exploring unknown oceans, they needed to measure their
local variation:  for one reason, to make sense of their own compass
bearings as they travelled; for another, to bring back as information to go
with the charts they would produce.

There was another reason, too. In the early 1700s, knowledge of local
magnetic variation around the Earth was proposed as a way of "discovering"
the longitude, by Halley (of Halley's comet), the Astronomer Royal of the
time. This was before the days of chronometers and lunar-distances. The
proposal was rather doomed to fail, because it was hard to measure
variation to sufficient accuracy, because there was so much local
fluctuation, because of the variation with time, and because there were
large areas of sea over which the variation didn't change much with
longitude. Halley's proposal stimulated mariners into measuring and
reporting variation, so that Halley was enabled (with a lot of
interpolation, exrapolation, and intuition) to compile a map of variation
over the then-known world. It was useful, but not for the purpose Halley
intended.

How do you measure variation? In theory, just take a compass-bearing on the
Pole Star. Make a small adjustment depending on GHA Polaris, for the
displacement of Polaris from the Pole itself. The trouble is, it's hard to
take an accurate compass-bearing on an object that's high up in the sky.
It's easier when the object is on or near the horizon, as in the case of
the rising (or setting) Sun. Amplitude tables exist, which show the
difference of the Sun's azimuth from true East or West, when rising and
setting, usually for the moment when the horizon bisects the Sun..

In the tropics, when the Sun is rising and setting from the horizon almost
vertically, this is accurate enough. In higher latitudes, the Sun is
arriving or leaving at a shallow angle. Because refraction near the horizon
is highly uncertain, this can then affect the azimuth somewhat. But it's
hard to take a compass-bearing to better than a degree or so at the best of
times, so measuring variation at sea is at best an inexact science.

In fact, you can measure variation from the bearing of any object at all in
the sky, if you have a good idea of your own geographical position. Choose
a convenient low-altitude object, get its dec and GHA from the almanac, and
calculate its azimuth just as if you were obtaining a celestial
position-line. Any difference with the compass-bearing of that object is
the variation. Indeed, if you take that compass-bearing at the same moment
as you measure a sextant-altitude, you have killed two birds with one
stone.

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
Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
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