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Science: The Wobbly Earth

3 minute read

To the run of men, the earth seems to rotate with trustworthy steadiness. Astronomers know better. Observed with their sharp-eyed instruments, the earth’s rotation is a wobbly business. In Nature, Astronomer T. Gold of Britain’s Royal Greenwich Observatory tells how he took the wobble apart and used it to show, among other things, how Antarctica may have got its deposits of coal.

Most of the earth’s wobbling is a grand motion that makes its axis describe a 23° cone once every 26,000 years. This precession is caused by the pull of the sun and the moon acting on the slightly nonspherical shape of the earth. Superimposed is a subtler wobble. The axis keeps steady in space, but over an uneven period of about one year, the bulk of the earth moves slightly in relation to the axis.

Gold uses this motion to show that any departure of the earth from its normal axis makes the earth’s material flow plastically. The flowing “damps” the motion and keeps it from continuing for more than a small fraction of 1°. Having proved this to his satisfaction. Gold constructed a high, wide and handsome theory.

Beetle v. Geoid. If the earth were a perfect sphere, he says, it would not be stable on its axis. The “smallest beetle crawling over it would change the axis of rotation in relation to markings on the sphere” because there would be no force to resist the kickback of the beetle’s crawling. But the earth is not a perfect sphere; it is a geoid slightly flattened at the poles by the centrifugal force of its rotation. So it spins like a fat flywheel on the short axis between the poles.

But what if the shape changes because of the rise of mountains or the accumulation of glacial ice? In this case, says Gold, the axis will shift to take account of the new distribution of mass. Slowly, the plastic earth will swell in the proper places to make itself a geoid again. When this process is complete, it will settle down with its North and South Poles in new places. Gold figures that modest crustal changes could make the earth turn 90° in less than 1,000,000 years, relocating its poles on its former equator.

Trapped Axis. According to one school of geology, something of the sort may have happened many times already. The shift of the poles would explain remains of tropical vegetation found near the present poles and signs of glaciation found in the present tropics. Another proof: the magnetic particles in many ancient rocks do not point toward the present magnetic poles (TIME, Sept. 27).

Gold finds it somewhat harder to explain why the earth’s axis is not migrating appreciably at present. He suspects that it may have been caught in a “trap.” The axis tends to shift, he thinks, toward the region where glacial ice is melting fastest, moving one of the poles toward that same region. The climate there gets colder. The glaciers grow thicker. Then the axis and the pole move slowly back again.

Gold does not guess how long the trap will hold the axis. Eventually, he suspects, it will break away and carry the cold of the North Pole to Mexico or Japan.

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