We can't give you a direct answer but we are allowed to help in a sense.
We have to go way back, and think Aristotle, he believed that the earth was indeed round, and the sun, stars, and moon all orbited around us. That philosophy lived on for centuries, Copernicus's theory was wrong but had a better idea. The first true philosopher who had the idea was Aristarchus of Samos.
Take the information given here put it to use, and do some research! :)