There is a belief going that in the Middle Ages both intellectuals and common people thought how the Earth was flat. On the contrary the scholars of the time were totally aware of the sphericity of our planet, knowledge discovered many millenniums before hence it was a known fact. Despite it is not clear who the actual discoverer of such an astronomic notion was, Parmenides from the 5th century BC being a possible candidate, the very first person who scientifically demonstrate the fact that the Earth is spherical was Eratosthenes of Cyrene in the 3rd century BC: he explained not only its sphericity but he also calculated its circumference with good approximation.
Picture in the cover: Miniature from 1500 from the work of Giovanni Sacrobosco called “Tractatus de Sphaera”, the most influential astronomical book from the 13th century; next to it a Orbis Terrae Map from the 15th century.
The idea of a Flat Earth could be spread amongst common people, who anyways didn’t leave any trace as a proof of such a theory, but however rejected by the intellectuals.
The equivocal “Myth of the Flat Earth” became popular by the end of the 19th century by the positivists, in an ideological discussion between creationists and evolutionists. Probably it was an insult made by the evolutionists in order to make fun of the traditionalist attitude, who were avoiding to accept the theories about the evolution.
A myth which is as much common is the representation of the Earth as flat by the Medieval artists, and many famous painters represented out planet as a simplified spherical disk; Hyeronimus Bosch is an example of this (1450-1516), as he painted a spherical Earth in the middle of a vast space on the outer side of the well known “The Garden of Earthly Delights”
Below: Bosch’s painting
Dante himself in his Divine Comedy defines the Earth as a sphere, while Tommaso d’Aquino (1225-1274) wrote in his “Summa theologiae” such a thing:
“The sciences differ for the different method that they use. The astronomer and the physicist can both prove the same thesis, as in the Earth is spherical: the astronomer demonstrates it with the help of mathematics, the physicist through the nature of matter”.
Below: picture from 1400 by John Gower showing a spherical Earth
The myth of the flat Earth was spread through the book from 1828 by Washington Irving “A History of the Life and Voyages of Christopher Columbus”, which introduced the theory for which the well known explorer had encountered some economical struggles in order to find funds to support the expeditions, because people of his time believed that the Earth was indeed flat. In reality many aristocrats refused to fund Columbus just because they were aware of the length of the circumference of the Earth, and thought that the then technology was not advanced enough to circumnavigate the globe to reach India. By hypothesising that the American continent was not there, their observation was not entirely wrong.
Below: Columbus’ map 1490 circa
When Columbus redrawn the terrestrial circumference, he utilised the historical estimation of the ancient Greek mathematician Ptolemy, Columbus did not know that the scientist was using the Arabic Mile (approximately 2 km) and not the Italian Mile (1852 m), so his result was approximately 25% inferior to the effective terrestrial circumference. If he had used the real dimensions, maybe the aristocrats would have opposed even further resistance to his funding.
Above: the Xilografia Flammarion, made in a 16th century kind of style and used as a fake proof in the book “L’Atmosphère: Météorologie Populaire” by Camille Flammarion from 1888. The xylography was most likely commissioned by the author, an invented proof to feed the myth of the flat Earth.
Today the scholars and historians community believe a proven reality the fact that they “Flat Earth” was a fabrication from the 800’s.