The earth isn't flat or circle. It is a sphere. Well, it was discovered centuries ago by a traveler in which the name I cannot remember. He traveled the world and put a mark in different places, he found out that the world is sphere because of the shadows of the markers. Also, as of the modernity today, it is proved by the satellites/astronauts that have been outside the earth.

It's so cool to have this kind of topics you know.. Anyway, you'll find more facts in the internet.