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    Re: GPS & Magnetic Bearings
    From: Robert Eno
    Date: 2003 Feb 10, 07:04 -0500

    Interesting you should mention this topic. The last theory that I read was
    that the magnetic goes in cycles; that is, it reaches a peak strength in one
    polarity and then gradually decreases to almost zero, after which it starts
    to increase in strength although not necessarily in the same polarity.
    According to Dutton (14th edition, Maloney):
    "...it is known that there have been 12 reversals of magnetic polarity over
    the past nine million years and that we are overdue for another. The earth's
    field intensity has decreased over 50 percent in the last 4,000 years, and
    one study predicts that in 1,200 years or so magnetic compasses will become
    useless -- there will be no field to direct them and they will point in any
    direction. Although all decreases do not result in reversal, when the field
    returns a few centuries later, its polarity is more likely than not to have
    I have further read that mass extinctions may have occured during these
    period of magnetic "calm". This is because the magnetic field provides some
    protection from incoming cosmic rays and other extra-terrestrial nasties.
    Without the field, we get a good saturation from all directions.
    Another theory that I read -- and this is based on a book that was written
    on the topic --  that the reversals happen suddenly. According to the author
    (whose name escapes me) the collapsing magnetic field induces a huge
    electric "shock" that rips through the oceans, killing everything in its
    path. I should point out that the author of this book had zero scientific
    credentials and admitted as much. In fact he felt that because he was
    unencumbered by scientific dogma, he was more qualified to offer a fresh
    Whatever theory one chooses to believe, I find it a depressing thought that
    my old reliable compass will some day be as useless as a GPS without
    batteries. Although I won't be around to ponder the problem.
    ----- Original Message -----
    From: Jared Sherman 
    Sent: Monday, February 10, 2003 12:14 AM
    Subject: Re: GPS & Magnetic Bearings
    > Robert-
    >  Sounds spot on. I understand there is even still some debate on how fast
    the poles reverse, and whether that might take us all unawares one day. (Not
    literally in one day.)

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