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    Re: Magnetic Variation.
    From: Brooke Clarke
    Date: 2004 Feb 12, 09:39 -0800

    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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