We now know the shape thanks to modern science but before it was all up in the air. And these are the beliefs of some of the ancient civilizations.
In the Cherokee nation, they believed that earth was formed from mud that came from under water and it formed a big island which would be the earth.
Ancient Babylonians believed the earth was inside a hallow mountain, floating on a very large body of a water (sea). Everything- the sun, moon,sky,star,everything that existed was inside this mountain.
There theory is hard to believe now but had we been in the ancient days we would be probably supporting such an idea.
In ancient Egypt, they believed everything was created was by there god Keb and he himself was the earth they were living on. The star were even considered his jewels, those pretty big jewels.
Ancient Hindus believed that the earth was an upside down bowl, being carried by elephants..weird ? gets weirder..The elephants were to be thought to stand on the back of turtles, poor turtles?
No there were snakes underneath them but nothing underneath the poor snakes. That must have been one damn heavy bowl upside down up there.
Who was the first person to think that such simple thing, that the earth was just a spheric object?
Not Christopher Columbus, thought he heavily believed in a time many people laughed at the idea. It was the greeks who were first to guess the shape of the earth right.
The famous mathematician Pythagoras thought of it and many other greek diplomats supported him, one being, Aristotle.