When Did People Start To Believe The Earth Is Flat?

People just started believing the earth was flat around 6th century BC. Though Flat Earth Society started around 1956 by Samuel Shenton. Eratosthenes Greek mathematician, astronomer and geographer(276–195 B.C.). Was the fist to believe that the earth was, in fact, not flat but round. he believed this because he used the sun to measure the earth and found that it was round. though, People just thought he was insane.