Geographic and Magnetic Poles
Earth's geographic poles are fixed by the axis of Earth's rotation. On maps, the north and south geographic poles are located at the congruence of lines of longitude. Earth's geographic poles and magnetic poles are not located in the same place – in fact they are hundreds of miles apart. As are all points on Earth, the northern magnetic pole is south of the northern geographic pole (located on the polar ice cap) and is presently located near Bathurst Island in northern Canada (approximately 1,000 miles (1,600 km) from the geographic North Pole. The southern magnetic pole is displaced hundreds of miles away from the southern geographic pole on the Antarctic continent.
Although fixed by the axis of rotation, the geographic poles undergo slight wobble-like displacements in a circular pattern that shift the poles approximately six meters per year. Located on shifting polar ice, the North Pole (geographic pole) is technically defined as that point 90° N latitude, 0° longitude (although, because all longitude lines converge at the poles, any value of longitude can be substituted to indicate the same geographic point. The South Pole (geographic pole) is technically defined as that point 90° S latitude, 0° longitude. Early explorers used sextants and took celestial readings to determine the geographic poles. Modern explorers reply on GPS coordinates to accurately determine the location of the geographic poles.
Earth's magnetic field shifts over time, eventually completely reversing its polarity. There is evidence in magnetic mineral orientation that, during the past 10–15 million years, reversals have occurred as frequently as every quarter million years. Although Earth's magnetic field is subject to constant change (periods of strengthening and weakening) and the last magnetic reversal occurred approximately 750,000 years ago, geophysicists assert that the next reversal will not come within the next few thousand years. The present alignment means that at the northern magnetic pole, a dip compass (a compass with a vertical swinging needle) points straight down. At the southern magnetic pole, the dip compass needle would point straight up or away from the southern magnetic pole.
The magnetic poles are not stationary and undergo polar wandering. The north magnetic pole migrates about 10 km per year. The magnetic reversals mean that as igneous rocks cool from a hot magma, those that contain magnetic minerals will have those minerals align themselves with the magnetic polarity present at the time of cooling. These volcanic rocks preserve a history of magnetic reversals and when found in equidistant banded patterns on either side of sites of sea floor spreading, provide a powerful paleomagnetic proof of plate tectonics.
Navigators using magnetic compass readings must make corrections both for the distance between the geographic poles and the magnetic poles, and for the shifting of the magnetic poles. Moreover, the magnetic poles may undergo displacements of 40–60 km from their average or predicted position due to magnetic storms or other disturbances of the ionosphere and/or Earth's magnetic field. Angular corrections for the difference between the geographic poles and their corresponding magnetic pole are expressed as magnetic declination. The values for magnetic declination vary with the observer's position and are entered into navigation calculations to relate magnetic heading to true directional heading.
See also Bowen's reaction series; Cartography; Continental drift; Earth's interior; Global Positioning System; Latitude and longitude; Magnetism.
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