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Tuesday, April 21, 2015

Magnetic poles

Posted to Maritime Musings (by on November 20, 2009

Slow continual movement, coupled with occasional reversal

Existence of the north and south magnetic poles was postulated long after magnetic compasses came into widespread use. Prior to that, many people believed that the compass needle was attracted either to a magnetic island in the far north or to the Pole Star (Polaris) itself. Eventually it became clear that the core of the Earth was a magnet. The magnetic field protects the Earth from harmful effects of cosmic rays. Solar disturbances interacting with the magnetosphere generate the northern lights – the Aurora Borealis. While this core magnet is heavily influenced by the rotation of the Earth on its physical axis, the geomagnetic axis is offset by approximately 11°. Further, the magnetic pole is not stationary as regards the surface of the Earth. The first person to reach the North Magnetic Pole, James Clark Ross, in 1831, found it at Cape Adelaide on the Boothia Peninsula. By 2005, it had migrated to the Arctic Ocean off Ellesmere Island. Its northwestward drift is at an estimated speed of approximately 40 kilometers/year. The needle on a magnetic compass does not point to the magnetic pole. Rather it aligns itself with the local geomagnetic field, which varies due to local influences. The angular difference between magnetic north (as shown by a magnetic compass) and true north is called the magnetic declination. This declination is shown on many navigational charts and must be accounted for when courses are followed by use of a magnetic compass. Note that, while this article has been written with a northern hemisphere bias, the same principles apply in the southern hemisphere, where the South Magnetic Pole wanders around Antarctica. Over enormous time scales (tens of thousands of years), the Earth’s magnetic field reverses itself, so that North Magnetic Pole effectively becomes the South Magnetic Pole, and vice versa. The next such reversal is scheduled to occur on or about April 1, 2015.

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