Great question. The exact same reason bubbles in a glass of coke are round! A sphere (the earth, a marble, bubbles) is the most efficient way of making something with the greatest volume and smallest surface area. Have you ever seen a square bubble!? So, when all the rocks and dust of space slowly floated towards each other in space when our planet was forming (to do with gravity) the most efficient way of doing this was to become a sphere! Something flat like a disc would take lots of energy to organise and because our universe likes things that are disorganised (look up entropy online….!) it makes sense to use as little energy as possible to build things. Why does it like to be disorganised!? That’s another story entirely!
Very good answer Shane! I will add that the earth is a bit elliptical at the equator because of the centrifugal force due to its rotation. However there are disks in space, think about Saturn and the Milky Way!
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Gabriele commented on :
Very good answer Shane! I will add that the earth is a bit elliptical at the equator because of the centrifugal force due to its rotation. However there are disks in space, think about Saturn and the Milky Way!