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Discs are naturally occurring shapes in the world, because any soft object experiencing rotation on a single axis will stretch itself into a disc shape such as pizzas and galaxies due to centrifugal forces caused by rotation. Galaxies and pizzas are "soft" in the sense that the tensile forces holding the structure together are not strong enough to resists being stretched by rotation. Rocks on the other hand, do not stretch when rotating because the average tensile forces between the atoms of a rock are strong enough to resist centrifugal forces, given that rotation is not too fast. If Earth is mostly fluid, why does it not stretch as it rotates?

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  • $\begingroup$ It does. The Earth is not exactly spherical. $\endgroup$ – Javier Dec 28 '19 at 19:01
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    $\begingroup$ "The equatorial bulge is a difference between the equator and polar diameters of a planet, du to the centrifugal force exerted by the rotation about the body's axis" (WIkipedia) $\endgroup$ – Bob D Dec 28 '19 at 19:51
  • $\begingroup$ Also related: physics.stackexchange.com/questions/8074/…? $\endgroup$ – dmckee --- ex-moderator kitten Dec 29 '19 at 5:41
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Earth actually does stretch out a bit due to rotation. The radius to the equator is bigger than to the poles by about 1 part in 300.

Why doesn’t it stretch more? It’s not due to strength of material: water defined the basic shape, and it can flow. Rather, it’s due to gravity: the surface is heavy, and the centrifugal effect of earth’s somewhat slow rotation only requires a little bit of “lift” to cancel it.

Less gravity or faster rotation would make for a flatter shape.

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