As countless photos from space can attest, Earth is round—the "Blue Marble," as astronauts have affectionately dubbed it. Appearances, however, can be deceiving. Planet Earth is not, in fact, perfectly round.

This is not to say Earth is flat. Well before Columbus sailed the ocean blue, Aristotle and other ancient Greek scholars proposed that Earth was round. This was based on a number of observations, such as the fact that departing ships not only appeared smaller as they sailed away but also seemed to sink into the horizon, as one might expect if sailing across a ball says geographer Bill Carstensen of Virginia Tech in Blacksburg.


Instead of Earth being like a spinning top made of steel, explains geologist Vic Baker at the University of Arizona in Tucson it has "a bit of plasticity that allows the shape to deform very slightly. The effect would be similar to spinning a bit of Silly Putty, though Earth's plasticity is much, much less than that of the silicone plastic clay so familiar to children."

Our globe, 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," says geologist Joe Meert at the University of Florida in Gainesville.

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 "on the order of a centimeter or so," notes geophysicist Richard Gross at the Jet Propulsion Laboratory in Pasadena, Calif. "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," Meert says.

To keep track of Earth's shape, scientists now position thousands of Global Positioning System receivers on the ground that can detect changes in their elevation of a few millimeters, Gross says. 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. It may not take much technology to understand that Earth is not perfectly round, but it takes quite a bit of effort and equipment to determine its true shape.