Asked by tibi29 to Daniel, Anna on 27 Jun 2014.
Keywords: earth, formation
The Earth has its almost spherical shape due to the gravity. The force of gravity from the planet will attract other matter from all directions into its center of mass (the planet’s core). So, when the planet formed by collisions of rocks 4.6 billion years ago, the rocks were fused together unceasing the mass of the planet with every new rock that was added to it. The heavy part of these rocks (like iron and nickel) moved to the center of the planet and established the gravitational center of mass. The strong pull of the gravity of equal strength and in all directions towards this center will result in a close to spherical shape as we see in the Earth and other planets.
Earth is at first approximation a sphere, due to gravity. Gravity is in fact acting in all directions toward the centre of mass of the Earth, and the sphere is thus the resulting shape, if something is attracted toward the centre from all directions.
If you look more closely, Earth is not a perfect sphere, it is actually a bit squeezed at the Poles, and its radius is larger at the Equator that the the North or at the South Pole of few kilometres. This is because of the rotation of the Earth around its axis.
In fact, rotation, as you might know, induces a centrifugal force perpendicular to the rotation axis.
Earth is not a completely solid object (as we have also seen in these days, it has a fluid interior, and tectonic plates on a very thin crust), and thus is shaped by the centrifugal force, resulting in an “ellipsoid”, slight flattened at the poles, and “fatter” at the Equator.
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