It Was Known in the Middle Ages That the Earth Was Round

Lyon, 1400-1450
Recueil d'astronomie et de mathématiques. Allemagne ou Suisse ?
(Lyon, B.m., ms. 0172, f. 005, 172)

Johannes de Sacrobosco (John of Hollywood) set forth the elements (doubtless already obvious to his readers) proving that the Earth is not flat. For example, an observer situated at the top of the mast of a ship can distinguish a landmark on shore, whereas a man who remains on the deck of the same ship is unable to do so.