Apr. 12

Isaac Newton first proposed that Earth was not perfectly round. Instead, he suggested it was an oblate spheroid—a sphere that is squashed at its poles and swollen at the equator. He was correct and, because of this bulge, the distance from Earth's center to sea level is roughly 21 kilometers (13 miles) greater at the equator than at the poles.

Earth, however, is not even a perfect oblate spheroid, because mass is distributed unevenly within the planet. The greater a concentration of mass is, the stronger its gravitational pull, creating bumps around the globe.

Earth's shape also changes over time due to a menagerie of other dynamic factors. Mass shifts around inside the planet, altering those gravitational anomalies. Mountains and valleys emerge and disappear due to plate tectonics. Occasionally meteors crater the surface. And the gravitational pull of the moon and sun not only cause ocean and atmospheric tides but earth tides as well.

In addition, the changing weight of the oceans and atmosphere can cause deformations of the crust. There's also postglacial rebound, with the crust and mantle that were depressed by the huge ice sheets that sat on the surface during the last ice age now rebounding upward on the order of a centimeter a year.

Moreover, to even out Earth's imbalanced distribution of mass and stabilize its spin, the entire surface of the Earth will rotate and try to redistribute mass along the equator, a process called true polar wander.

Scientists now position thousands of Global Positioning System receivers on the ground that can detect changes in their elevation of a few millimeters. Another method, dubbed satellite laser ranging, fires visible-wavelength lasers from a few dozen ground stations at satellites. Any changes detected in their orbits correspond to gravitational anomalies and thus mass distributions inside the planet. Still another technique, very long baseline interferometry, has radio telescopes on the ground listen to extragalactic radio waves to detect changes in the positions of the ground stations.

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