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Science BBC World, @bbc_ciencia9 September 2014image source, ThinkstockTo know how much the Earth weighs, you have to take gravity into account and read this note to the end.And how is gravity measured? The intensity of the force that keeps the Moon orbiting around the planet and us with our feet on the ground is determined by what physics calls "G", the universal gravitational constant.image source, Wikimedia Commonscaption, Henry Cavendish was the first to calculate the density and weight of the Earth. "Newton's law of gravity described the motion of the planets and moons with amazing precision, and what we call…