By Sid Perkins
Concerned about your weight? Don’t go to the North Pole, where you’re about 20 km closer to the center of Earth–and therefore a pound or so heavier–than at the equator. Head, instead, for India. There, you’d be standing over a less-dense landscape with a gentler gravitational pull. Yes, what you weigh depends on where you are. Your body doesn’t change from place to place, but the gravitational field does. Topography, crust composition, and the planet’s rotation-induced equatorial bulging are among the factors that influence Earth’s gravitational pull at different locations. Furthermore, this uneven gravitational field changes slightly with the seasons, as precipitation carries moisture’s mass from the oceans onto the continents.
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For more than 30 years, scientists have been monitoring the planet’s tug with several dozen satellites and sensitive instruments carried into the field. But the global gravitational model that they’ve compiled from that data has just been rendered obsolete by a pair of satellites that were launched last March.