The Wandering Poles

Instead of spinning smoothly, the earth wobbles on its axis. By sighting telescopes on distant fixed stars and carefully measuring their apparent movement, scientists have determined that the North and South poles--the points at which the imaginary axis of rotation pierces the earth's surface--are continually on the move. Over the course of a year, they wander about the polar regions in roughly circular paths about 50 ft. in diameter.

The cause of the erratic wobbling motion has long been a geophysical mystery. Elaborate calculations have shown that without great forces to sustain the wobble, it would be effectively damped out in as little as 30 years. Now, after dis carding several other theories, scientists have found convincing evidence that it is caused and sustained by major earthquakes.

Analyzing data at the University of Western Ontario, Geophysicists Lula Mansinha and Douglas Smylie found that the circular path traced by the North Pole between 1957 and 1968 was actually composed of interrupted arcs that spiraled almost imperceptibly inward. The inward motion, they decided, was an indication that the earth's wobble had begun to decrease for short periods of time. But between each of the arcs comprising the circle there was a break, marking a time when the wobble suddenly increased. Significantly, Mansinha and Smylie reported in Science, nearly all of the breaks occurred at the time of major quakes.

The two geophysicists also discovered that there were noticeable changes in the path of the wandering pole from five to 20 days before many of the major earth tremors. By tracking the pole path more regularly, and by placing sensitive instruments along the earth's major fault zones, they suggested, scientists may some day provide advance warning not only on where and when an earthquake will occur, but how severe it will be.

Impressed by the apparent correlation between wobble and earthquakes, Columbia University Physicist James Heirtzler offers a different theory in the current issue of Scientific American. The prequake variations in the path of the pole suggest to him that the wobble is responsible for--rather than a result of--the earthquakes. Furthermore, he speculates, the wobble may also cause climate changes, mountain-building, and even the occasional reversal of the earth's magnetic field. But Heirtzler's theory still leaves wide open the question that Mansinha and Smylie believe they have correctly answered: What causes the wobble?

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