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US Screwel teechurs thought up the Flat Earth theory in the mid 1930s.
They didn’t believe it, but they declared out of whole cloth the idea that others did.
In Inventing the Flat Earth (1991) Jeffrey Burton Russell argues that 19th century anti-Christians invented and spread the falsehood that educated people in the Middle Ages believed that the earth was flat. As one writer summarizes, “Russell also examined a large selection of textbooks and found those written before 1870 usually included the correct account, but most textbooks written after 1880 uncritically repeated the erroneous claims in Washington Irving, John William Draper and Andrew Dickson White. Russell concludes that Irving, Draper and White were the main writers responsible for introducing the erroneous flat-earth myth that is still with us today.”
Russell suggests that the flat-earth error was able to take such deep hold on the modern imagination because of prejudice and presentism. He specifically mentions “the Protestant prejudice against the Middle Ages for Being Catholic ... the Rationalist prejudice against Judeo-Christianity as a whole”, and “the assumption of the superiority of ‘our’ views to those of older cultures”
Russell is currently Professor Emeritus of History at the University of California, Santa Barbara. He has also taught History and Religious Studies at Berkeley, Riverside, Harvard, New Mexico, and Notre Dame.