Physically considered, the Earth is a globe; all the authors of the high Middle Ages are agreed on this. In the earlier 'Dark' Ages, as indeed in the nineteenth century, we can find Flat-earthers. Lecky, whose purpose demanded some denigration of the past, has gleefully dug out of the sixth century Cosmas Indicopleustes who believed the Earth to be a flat parallelogram. But on Lecky's own showing Cosmas wrote partly to refute, in the supposed interests of religion, a prevalent, contrary view which believed in the Antipodes. Isidore gives Earth the shape of a wheel. And Snorre Sturlason thinks of it as the 'world-disc' or heimskringla --the first word, and hence the title, of his great saga. But Snorre writes from within the Norse enclave which was almost a separate culture, rich in native genius but half cut off from the Mediterranean legacy which the rest of Europe enjoyed.
The implications of a spherical Earth were fully grasped. What we call gravitation--for the medievals 'kindly enclyning'--was a matter of common knowledge. Vincent of Beauvais expounds it by asking what would happen if there were a hole bored through the globe of Earth so that there was a free passage from the one sky to the other, and someone dropped a stone down it. He answers that it would come to rest at the centre. [...]The most vivid presentation is by Dante, in a passage which shows that intense realising power which in the medieval imagination oddly co-exists with its feebleness in matters of scale. In Inferno, XXXIV, the two travellers find the shaggy and gigantic Lucifer at the absolute centre of the Earth, embedded up to his waist in ice. The only way they can continue their journey is by climbing down his sides--there is plenty of hair to hold on by--and squeezing through the hole in the ice and so coming to his feet. But they find that though it is down to his waist, it is up to his feet. As Virgil tells Dante, they have passed the point towards which all heavy objects move. It is the first 'science-fiction effect' in literature.
The Discarded Image, "Earth and her Inhabitants" (1964)