In physics class we're taught that Earth's gravity is 9.8 meters per second squared. That's just a general estimate, however, good enough for schoolroom problems but not nearly precise enough for scientists studying things like how climate change is affecting sea level. But scientists have now created the most accurate gravity map ever, using data collected by a European Space Agency satellite called GOCE.
GOCE orbits the Earth at a mere 254.9 kilometers (158.4 miles) above the planet's surface. At that altitude, there is enough atmosphere that the satellite's orbit will quickly decay, so GOCE continuously fires an ion thruster with xenon gas to keep steady. GOCE carries three pairs of platinum blocks that can detect changes in gravity as small as 1 in 10 trillion. Data from November and December of last year were assembled into the colorful map above, known as the geoid.
"I think everyone knows what a level is in relation to construction work, and a geoid is nothing but a level that extends over the entire Earth," explained Professor Reiner Rummel, the chairman of the GOCE scientific consortium.
"So with the geoid, I can take two arbitrary points on the globe and decide which one is 'up' and which one is 'down'," the Technische Universitaet Muenchen researcher.
In other words, the map on this page defines the horizontal - a surface on which, at any point, the pull of gravity is perpendicular to it.
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