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    Re: Variation and Amplitude
    From: Glendon
    Date: 2007 Jan 08, 21:48 -0800

    
    George Huxtable wrote:
    > Guy Schwarz quoted Tom Cunliffe as saying-
    -
    >
    > "On to Variation. How was it discovered? My guess is that navigators
    > would take a bearing on the setting/rising sun and their almanacs
    > had the amplitude tables and determined something was off? Or am I off
    > base
    >
    > Portuguese navigators, setting off deep into the Atlantic around the
    > time of Columbus, did their navigation from Polaris, and thought that
    > the compass needle was magically drawn toward Polaris. So when it
    > started to diverge as they went West, it was a source of great worry
    > and confusion.
    >
    > George
    
    My imperfect  knowledge of history is that the Portuguese were the
    first  to recognize  variation/declination  sometime in the middle of
    the 1400's, and that they began to make charts of variation according
    to their measurements and their then knowledge of the globe. In
    accordance with the times, they attempted to hold their information as
    "commercial in confidence" and not a lot is known about how they
    did things. (Can anybody correct me on this?)
    
    The modern GeoMag visualization of the earth's magnetic field goes
    back to 1590 and indicates that variation in the Mediterranean and the
    Bay of Biscay at the time was near enough zero.  This suggests to me
    that as the Portuguese began to move further afield in the late
    1400's that they  encountered recognizable variation. As George
    suggests, they may have had Polaris as their reference point.  However
    the first charts of variation made were for the west and southern
    coasts of Africa where Polaris is not visible. Clearly they had some
    method for determining north, perhaps from the sun?
    
    George mentions the Portuguese venturing deep into the Atlantic,
    whereas history mostly  records their exploration around Africa to
    India(Goa), and then Melaka.  I have read conjecture that the
    Portuguese/Basque were fishing the  cod  banks off North America before
    the recorded voyages of John Cabot and others from the mid 1490's on.
    Knowledge of variation would have been vital to dead reckoning
    navigators of the day, as the GeoMag visualization shows variation of
    20 degrees at the banks at that time.
    
    If anyone has any reliable references to these times and events, I
    would be most interested.
    
    Lee Martin
    
    
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