Strange but True: Earth is Not Round - Scientific American

13/09/2015 04:11Strange but True: Earth Is Not Round - Scientific American

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Space Strange but True

Strange but True: Earth Is Not RoundIt may seem round when viewed from space, but our planet is actually a bumpy spheroid

By Charles Q. Choi | April 12, 2007

As countless photos from space can attest, Earth isroundthe "Blue Marble," as astronauts haveaffectionately dubbed it. Appearances, however, can bedeceiving. Planet Earth is not, in fact, perfectly round.

This is not to say Earth is flat. Well before Columbussailed the ocean blue, Aristotle and other ancientGreek scholars proposed that Earth was round. Thiswas based on a number of observations, such as thefact that departing ships not only appeared smaller as they sailed away but alsoseemed to sink into the horizon, as one might expect if sailing across a ball saysgeographer Bill Carstensen of Virginia Tech in Blacksburg.

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

Instead of Earth being like a spinning top made of steel, explains geologist Vic Bakerat the University of Arizona in Tucson it has "a bit of plasticity that allows the shape todeform 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 sofamiliar to children."

Our globe, however, is not even a perfect oblate spheroid, because mass is distributedunevenly within the planet. The greater a concentration of mass is, the stronger itsgravitational pull, "creating bumps around the globe," says geologist Joe Meert at theUniversity of Florida in Gainesville.

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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. Occasionallymeteors crater the surface. And the gravitational pull of the moon and sun not onlycause ocean and atmospheric tides but earth tides as well.

In addition, the changing weight of the oceans and atmosphere can causedeformations of the crust "on the order of a centimeter or so," notes geophysicistRichard Gross at the Jet Propulsion Laboratory in Pasadena, Calif. "There's alsopostglacial rebound, with the crust and mantle that were depressed by the huge icesheets that sat on the surface during the last ice age now rebounding upward on theorder 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 theequator, a process called true polar wander," Meert says.

To keep track of Earth's shape, scientists now position thousands of GlobalPositioning System receivers on the ground that can detect changes in their elevationof a few millimeters, Gross says. Another method, dubbed satellite laser ranging, firesvisible-wavelength lasers from a few dozen ground stations at satellites. Any changesdetected in their orbits correspond to gravitational anomalies and thus massdistributions inside the planet. Still another technique, very long baselineinterferometry, has radio telescopes on the ground listen to extragalactic radio wavesto detect changes in the positions of the ground stations. It may not take muchtechnology to understand that Earth is not perfectly round, but it takes quite a bit ofeffort and equipment to determine its true shape.

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August 24, 2012, 12:27 AMEpistimonas

Because gravity is an inverse square force. Squared coordinates = sphere

nothing is perfect though, its not a perfect sphere

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Strange but True: Earth is Not Round - Scientific American

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