Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Marshall on 31/08/2009 09:38:06
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Why the earth and other celestial objects are round?
Why all are rotating?
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Why the earth and other celestial objects are round?
Earth other celestial objects are not an exact spheres, they are oblate sphereoids (http://en.wikipedia.org/wiki/Oblate_spheroid): they bulge out at their equators because of their rotation.
Why all are rotating?
Conservation of angular momentum: the cloud from which the planets formed (http://en.wikipedia.org/wiki/Protoplanetary_disc) was rotating.
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Large celestial objects are round (or approximately round as RD says) because this is the shape of lowest energy state that the matter can be in as a result of the mutual gravitational attraction of the matter of which they comprise. The matter making up the bodies behaves in a fluid way, and in many cases will have started literally as a fluid resulting from the heat generated by high velocity collision. In some cases (like many stars or a gas giant, like Jupiter) they remain fluid. Smaller bodies, like some moons or asteroids, are quite irregular in shape. This is probably because the energies involved when they conglomerated their material was relatively low because of their low gravitational attraction and the forces of gravity were insufficient to pull the more solid material into its lowest energy state of a sphere (or if rotating, a spheroid).
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All celestial bodies rotate because of the law of conservation of angular momentum. this is one of the most fundamental laws of the universe and comes from the fact that the laws of the universe do not change dependant on the direction we are looking.
All celestial bodies have at some time condensed out of large clouds of gas and dust under the influence of gravity. these clouds were probably moderately turbulent so most of the angular momentum canceled out but there would always be a small residual in one direction representing the net rotation of the whole cloud. As the cloud gets smaller its rate of rotation increases by a factor of many thousands and this becomes the rotation of the body.
one of the main reasons stars have planets and are also frequently double or triple stars is that the excess of angular momentum in the cloud would make a single body unstable because it is rotating too fast. Most of the angular momentum of the solar system is concentrated in the planets and if this was dumped into the sun it would rotate so fast that it would fly apart.