'Flat Earth belief is ancient and natural common sense'

"Most people in the Middle Ages thought the Earth was flat - it is the natural, common-sense view, and only the educated have to be taught to doubt it."

The opposite is true. The spherical-Earth idea appeared with Pythagoras (6th century BC); by about 330 BC Aristotle had assembled strong empirical evidence; Eratosthenes measured the circumference around 240 BC; by the 1st century AD Pliny the Elder could say that everyone agreed the Earth was spherical. It is a historical myth that medieval Europeans generally thought the Earth was flat - that myth was created in the 17th century by Protestants arguing against Catholic teaching, and only gained currency in the 19th century. Modern flat-Earth belief is a documented 19th-century revival: Rowbotham's Zetetic Astronomy (1849 pamphlet, 1865 book) is generally credited with it, and the 21st-century resurgence is a social-media phenomenon.

The timeline: Pythagoras (6th c. BC) spherical idea, Aristotle (~330 BC) empirical evidence, Eratosthenes (~240 BC) circumference measured, Pliny the Elder (1st c. AD) 'everyone agreed', Rowbotham (1849/1865) the modern revival, 2010s social-media surge. A 2018 US study found firm flat-Earth belief under 2% in every age group. The 'medieval flat-Earth myth' has no medieval source - it was coined in the 17th century.