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The earth (and other planets) are spherical because the force of gravity organized them and they took this form as a preference. When very large masses of material accrete to form something the size of a planet, gravity increases and increases as the material comes together. The mass begins to "pack itself tighter" through the process, and as the arrangement increases in size, the "most effective shape" for the forces acting on the material will dictate that the material form a sphere.

Planets formed in this way have their constituient material heated up by mechanical energy imparted to the material as it accretes. The amount of energy added to the "mix" is almost always sufficient to cause the whole "ball" to become molten. That results in a really, really big ball of liquid rock. And liquid rock is plastic - it can be shaped easily. Gravity does this, and the optimum shape for the conditions - a sphere - will be the result.

Note that the planets usually form in such a way that they are spinning on an axis. The plastic nature of the large mass of the planet and the rotation of the mass couple to "deform" the body a little bit. This gives most planets (earth included) a little bit of a "bulge" at the equator, or, said another way, planets often exhibit a slight flattening at the poles.

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10y ago

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