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    Re: Magnetic Variation.
    From: Richard B. Langley
    Date: 2004 Feb 12, 14:58 -0400

    See also 
    -- Richard Langley
    
    On Thu, 12 Feb 2004, Brooke Clarke wrote:
    
    >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.
    >>================================================================
    >>
    >>
    >>
    >>
    >
    
    
    ===============================================================================
     Richard B. Langley                            E-mail: lang@unb.ca
     Geodetic Research Laboratory                  Web: http://www.unb.ca/GGE/
     Dept. of Geodesy and Geomatics Engineering    Phone:    +1 506 453-5142
     University of New Brunswick                   Fax:      +1 506 453-4943
     Fredericton, N.B., Canada  E3B 5A3
         Fredericton?  Where's that?  See: http://www.city.fredericton.nb.ca/
    ===============================================================================
    
    
    

       
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