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    Re: Historical Magnetic Variation/Declination
    From: Frank Reed CT
    Date: 2004 Jun 19, 16:11 EDT
    Bill B wrote:
    "If I understand it correctly the earth reverses magnetic poles (approx. 100,000 year cycle?) and we are *way* overdue, with signs that this is starting to happen.  How quickly it happens is open to speculation, with an inconclusive study of volcanic lava flow indicating it has historically flipped in a matter of months in one case."

    There does not appear to be any periodicity to the polarity flips, so we're not "overdue". There's a nice graphic in the FAQ section of the USGS geomagnetism site that shows the polarity changes during the past 50 million years or so. There are some short cycles, some long cycles, no real pattern. There was a recent episode of Nova (science program shown on PBS, maybe also seen in the UK and elsewhere under a different name) which played up th possibility of an impending polarity flip but this is really more in the realm of a "what-if" speculation. There's no significant evidence that it's happening. On average, these flips are estimated to take about 7000 years to complete.

    And:
    "Supposedly, while this reversal is happening, there are large areas of magnetic anomalies, with four, six, eight of more poles occurring worldwide. That could pretty well make a compass useless on an extended blue-water voyage."

    In the late 18th and on into the 19th century, it was common practice to observe the compass bearing of the Sun at sunrise/sunset. A simple table look-up or calculation then yields the variation of the compass. With a complicated multipolar field, you could still do this. On the other hand, compasses don't work well near poles and in regions of high field inclination so there would likely still be many "dead zones".

    And:
    "But what about GPS or other satellite radio-based aids to navigation? I have totally lost signal for fifteen or more minutes at a stretch over the past year because of solar flares/magnetic storms that interfered with the radio transmissions from the satellites."

    Has anyone else had similar experience with GPS during magnetic storms? Just curious what the anecdotal evidence is.

    Frank R
    [ ] Mystic, Connecticut
    [X] Chicago, Illinois
       
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