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    Re: Compass - southern hemisphere/northern hemisphere
    From: Glendon
    Date: 2004 Oct 21, 10:46 +1000

    Lisa,
    
    Compass needles line up with the earth's magnetic field. This means that
    near the north magnetic pole the north tip of the needle points down from
    the horizontal to be in alignement with the field, and up, near the south
    magnetic pole. Near the "magnetic equator", so to speak, the needle sits
    more or less horizontal. Manufacturers of compasses can weight the needle to
    counterbalance the pointing down or up effect, so that the needle sits
    horizontally...when it is used in that part of the earth's magnetic field it
    is designed for. Compass manufacturers have settled on dividing the earth up
    into 5 magnetic zones, and manufacture compasses to work in one or more of
    those zones. Alternatively, the manufacturers may sit the needle on a
    special bearing, such that it sits horizontally when used all over the
    earth....called a global needle.
    
    See the Silva or Suunto sites for further discussion, and charts of the 5
    zones. Or do a Google on compass zones. Note that there is quite a
    convergence of zones immediately north of Australia, while the US and most
    of Europe is covered by one zone.
    
    It sounds like you will find the needle or card on your handbearing compass
    "sticking" once you move north of Australia. This will be because the
    bearing the needle or card sits on is binding due to uncompensated tilt.
    
    Some compasses provide for compensation by adjustment.Check yours. I am very
    unclear on binnacle mounted compasses. The adjustment devices on them are
    clearly designed to make compensatory adjustments in the horizontal plane, I
    am not so sure about the vertical plane. I suspect it varies by compass
    make. Maybe others might like to comment on this point...I have always
    wondered
    
    Lee Martin
    
    
    ----- Original Message -----
    From: "Lisa Fiene" 
    To: 
    Sent: Thursday, October 21, 2004 8:30 AM
    Subject: Compass - southern hemisphere/northern hemisphere
    
    
    > Our hand bearing compass is one designed for the southern hemisphere, as
    > we're in Australia. I remember being taught that it's important to have
    > a southern hemisphere compass in the southern latitudes and a northern
    > hemisphere compass in the northern latitudes.
    >
    > What's the principle involved here, and how did the early navigators (eg
    > going from Europe to rounding Cape Horn etc.) compensate?
    >
    > Thanks, Lisa
    >
    
    
    

       
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