Why is the earth round?

Answer

The Earth is round because it has so much mass.

All masses attract each other by gravity. A stone, or ourselves, are not round, because the gravitational pull we exert on ourselves is not very great at all. What holds us together are rather chemical bonds.

But if you keep putting mass together, there’s a point where the gravitational force the body exerts on itself really becomes so great that it starts to define the shape of that body. And that shape is a sphere. The reason is that gravity depends on distance, and a sphere is a body where every point on its surface is equidistant from the center.

Think the earth was all sea, and try to make a mountain in the sea. You immediately feel the water flowing down, until everything is flat. That ‘plane’ is then a sphere, the surface from which nothing can flow downwards. You can make mountains with stones, because the friction is greater there, but eventually they also crumble.

The Sun is also a sphere, because it has much more mass than the Earth. And all planets are spheres, even some of their satellites (like our Moon) are. The border is somewhere around a 500-mile radius. Planetary moons smaller than that can take on quite erratic shapes.