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Hi George:

There are a number of magnetic observatories located all over the
world.  They have permanetly mounted magnetometers recording the X, Y
and Z components of the Earth's magnetic field.  From this data a
mathematical model of the Earth's field is made once every 5 years.  You
can use this model on line given a Lon, Lat and date.  This is the model
that's built into some GPS receivers, allowing the receiver to tell you
the true bearing to a way point.  Since GPS receivers have o compass
functionality, the receiver says something like waypoint # 57 is 37
Degrees (true) from here, and it's up to you make use of that
information.  You can see more about the Earth's magnetic filed on my
web page at:
http://www.pacificsites.com/~brooke/Sensors.shtml#Earth%27s%20Magnetic

Have Fun,

Brooke Clarke, N6GCE
http://www.PRC68.com

George Huxtable wrote:

>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.
>
>================================================================
>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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