It's round because of all the forces that act together on it, mainly gravity. It's why everything is spherical in space. Spin tends to make it a bit wider at the equator and shorter from pole to pole.
If it were flat, it would be a flat disk because of a high spin rate. But let's assume it was a flat disk without a high spin rate. You'd still be attracted to the center of gravity--the center of the disk--so as you went farther out, you'd lean outward. On a ship, you could sail off the edge of the world but onto the other side. On either side though, you'd tend to slide back toward the center because of gravity.
But that's why the world isn't flat. Everything tends to pull toward the center of gravity (or the center of mass). The only stable end to this is when everything is equally far from the center of it all, which is a sphere. Flattened a bit because there is a spin that distorts it.
· 5 years ago