Handout #18 -The Magnetic Earth


As we have seen, the Earth acts like a big magnet, as freely rotating magnets will align themselves to the large, external field provided by the Earth itself.  The north and south magnetic poles of the Earth, however, are not in the same places as the geographic north and south poles, which lie exactly on the axis of the rotation of the Earth.  Rather, the magnetic poles are located at considerable distance from the geographic poles.  The north magnetic pole is situated in the vicinity of Ellef Ringnes Island in northern Canada, about 1300 km (about 800 miles) from the North Poles.  The south magnetic pole is located just off the coast of Wilkes Land, Antarctica.  It is 2550 km (1600 miles) from the geographic South Pole.

The Earth’s magnetic field is not perfectly symmetrical, but is wavy and somewhat irregular.  Because of this irregularity, a compass some distance away from a magnetic pole may not point directly toward the magnetic pole; rather, the compass follows the direction of the Earth’s magnetic field at that location.  The path created by following the compass in the direction of the poles may be wavy, but it will end at a magnetic pole.  Most maps indicate the directions of both true north, meaning the direction of the geographic north pole, and magnetic north, the direction in which a compass points.  The difference in angle between the direction of true north and magnetic north at any given point on a map is known as the declination.  Knowledge of magnetic declination is critically important when navigating with a compass and a map.

Below is a chart of declinations across the Earth.  If you are on the “0” line, your compass ACTUALLY points to geographic (true) north.  If you are on a “10” or “-10” degree line, your compass points 10 degrees too far East or West of geographic north.  The estimated declination for our present position: latitude 37.97 N, longitude 122.29 W, elevation 0.0 km is 14 degrees 46 minutes East.  The current rate of change it 6 minutes per year Westward.

Below is a picture of the magnetic field of the Earth based on actual measured data.  This shows that although the overall shape of the Earth’s magnetic field is that of a torus (like a bar magnet), there are significant complexities close to the surface of the Earth and especially inside it, where the field lines are much denser and have a tendency to twist around each other.