Question: Why is the world round?

James M Monk answered on 22 Jun 2011:

The force of gravity does not have a preferred direction – it pulls equally in all directions into the centre of the Earth. Since it is gravity that is holding the Earth together in a lump, the surface of that lump follows the points where the gravitational pull is equal, which is a ball. This shows the importance of symmetry in physics – just from the fact that the gravitational field is symmetrical (the same in all directions) we can show that the Earth must be a sphere.

In fact the Earth isn’t quite perfectly spherical because it rotates, which causes a bulge at the equator because of the inwards acceleration needed for the rotation.