Geophysics: 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...