Why Compasses Don't Point North

It is a common misconception that the needle of a magnetic compass points to a particular spot on the earth's surface. In fact, there is no particular point that can be identified as the north (or south) magnetic pole.

This is true because the earth behaves as if the source of its magnetism was concentrated at its center. The external magnetic field of the earth is the same as that which would surround an inert sphere having a very small and powerful bar magnet at its center. The earth's magnetic poles are not centers of attraction at all, but simply blurry localities at which the field lines arising from the hypothetical magnet at the center happen to emerge perpendicular to the earth's surface.

Put another way, the field lines in the area around a pole rise straight up, curve to pass over the equator horizontally, and plunge back into the earth in an area that can be interpreted as the opposite pole. Compass needles align themselves with the field lines produced by the "central magnet". For practical purposes the problem can be treated as if a north magnetic pole now lay somewhere in the vicinity of Canada's Prince of Wales Island, near 75 degrees north latitude, 101 degrees west longitude.

It is necessary to say "now" because, to complicate matters, the magnetic field lines are gradually changing their orientation. Over most of Alaska, the direction in which a compass needle points is between 20 and 30 degrees east of true north. This deflection changes slowly over the years in amounts significant enough to warrant notation on topographic maps.

It is believed that the earth's magnetic field is produced at depth by material of the earth's core slowly rotating in giant convection cells, so our central magnet is not a static thing. Taken together, these cells generate the overall magnetic field, with smaller whorls and eddies creating local anomalies.

Temporary disturbances of the magnetic field which can be of immediate concern are called magnetic storms. These occur when the earth is bombarded by particles emitted from the sun during a period of unusually high activity related to sunspots and solar flares.

In modern times, the main problems arising from magnetic storms are the communications blackouts which they cause. In Alaska, we tend to relate these eruptions on the sun with periods of intense auroral activity. To mariners, however, magnetic storms have been a curse for centuries. Luckless sailors could not understand what was causing their compasses to behave erratically, often leading them off course and onto the rocks.

At northern latitudes, the problem is worse because the field lines are steeply inclined, and the horizontal component (the one to which the compass responds) is less stable during magnetic disturbances than it is further south where the lines more closely parallel the surface. Modern measurements made at Sitka have recorded erratic deflections in the horizontal field of over 10 degrees in a period of only hours.

The Sitka magnetic observatory was established by the U.S. Coast and Geodetic Survey in 1901 after mariners had complained for over a century that their compasses often behaved strangely in northern waters. The Sitka observatory, now operated by the U.S. Geological Survey, remains today as one of the oldest continually operated magnetic (and, beginning in 1904, seismographic) observatories in North America.