- The problem of the Earth's density is a fairly complex if you're estimating. The Earth's mantle is covered by two types of crust: continental (approximately 2.7 to 3.0 grams per cubic centimeter) and oceanic (3.0 to 3.3 g per cubic cm). The oceanic mantle is denser than the continental crust due to the greater mass of water than air. The inner solid core (12.6 to 13.0 g per cubic cm) and the outer, molten core (9.9 to 12.2 g per cubic cm) also differ in density. The ocean, which covers two-thirds of the Earth's surface, is comprised of water, the substance used as the standard of 1 g per cubic cm; but the density of water increases with depth. The atmosphere, the 10,000 kilometer-deep blanket of air that envelops the Earth, has an average density at sea level of .0012 g per cubic cm. The earth, we now know, has an average density of 5.52 g per cubic cm---5.52 times the density of water at sea level.
- The significance of the density of the Earth has changed with the development of mathematics and astronomy from a matter of academic interest to a measuring device to investigate the structure of other bodies in the solar system and beyond to determine their composition and origins.
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Cavendish's experimentSir Isaac Newton took the first steps in the attempt to calculate the size of the Earth in 1687 as he developed his theory of gravitational attraction. Nearly a century later, the Royal Society of England established a committee to settle the question of the earth's density and determined the density of the Earth to be about 4.5 times the density of water. Henry Cavendish, the British chemist and physicist who discovered hydrogen and correctly determined the composition of air, used gravitational attraction between two items of known density to project the density of an unknown object (Earth) by its gravitational interaction with one of the objects. His estimate of 5.48 times the density of water. Another century passed before John Henry Poynting summarized historic experimentation and published his "conclusive" determination of 5.493 times the density of water in an essay in 1894. - Every physicist who attempted to find the density of the earth labored two disadvantages. One, of course, was the limitations of the science of his day. The main problem in trying to determine the density of the Earth, of course, is that you can't just put the earth on a scale to see how much it weighs then divide it by the volume of the Earth.
- Cavendish's experiments are today accepted as a method of finding the gravitational constant, a number which must be used to determine the relative mass of the Earth. From the 17th century onward, scientists used experiments with a pendulum, plumb line and mountain or torsion bar apparatus and came out with mean densities of between 4.39 and 6.5 times the density of water.
