by Adam Maloof, Geologist, Princeton University

Pole shifting is a fascinating and important process in geological history, but will have nothing to do with the Mayans or with 2012. Read on to understand why.

Earth is not a rigid sphere. In fact, the solid Earth deforms on a range of timescales, from the twice-a-day lunar-solar tide, to the rebound of the arctic regions over the past 10,000 years as the ice sheets melt and return their stored water weight to the oceans. Just under the influence of her own spin, Earth deforms into an oblate spheroid. In other words, Earth is fatter around the equator than it is at the pole by about 20 km (Fig. 1). It is this equatorial bulge that sticks out into the solar system, interacts with the moon, planets and sun, and leads to the orbital wobbles such as precession and obliquity. Precession is the wobble of Earth's spin axis that causes the zodiac to rotate through time (i.e., polaris has not always been the north star). Precession is the process that John Major Jenkins thinks the Mayans were able to observe and use to predict certain alignments on which to base their long count calendar. Obliquity is the changing tilt of Earth's spin axis with respect to the stars. Together, these wobbles (Fig. 2), known as Milankovitch cycles, modulate the amount of solar radiation that reaches Earth's surface and are thought to pace the comings and goings of ice ages over the past few million years.

Figure 1

Figure 2

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