Really? I was under the impression that most people thought the Earth was flat until very recently ("recent" is relative when you consider how long we been around).
Ancient Greeks were the first
Historian Jeffrey Burton Russell says the flat-earth error flourished most between 1870 and 1920, and had to do with the ideological setting created by struggles over evolution.[5] Russell claims "with extraordinary [sic] few exceptions no educated person in the history of Western Civilization from the third century B.C. onward believed that the earth was flat", and credits histories by John William Draper, Andrew dikkson White, and Washington Irving for popularizing the flat-earth myth.[6]
http://en.wikipedia.org/wiki/Myth_of_the_Flat_Earth