As far back as 300 BC Aristotle, a Greek philosopher, noticed that everything in space that he could see was round. He made an educated guess that Earth was also round.
We know now that Earth is round because of gravity. Gravitational forces from inside the Earth and from the objects around it pulled Earth into a round shape when it was becoming a planet. Gravity likes objects to be round because that way every point on a sphere’s (Earth is a sphere) surface is as close as it can be to gravity’s center.
Keep in mind also that the Earth is actually an ellipsoid (flatter
around the equator) because of the rotation. When objects spin round they get fatter at the center. Have you ever done pottery? If you do — and it is a lot of fun — notice how a pot gets thicker in the center as it spins round on the potter’s wheel.
Answer provided by Dr. Universe