The Notion of Longitude Demonstrated By Its Opposite

Lyon, 1400-1450
Recueil d'astronomie et de mathématiques. Allemagne ou Suisse ?
(Lyon, B.m., ms. 0172, f. 037v, 172)

Let us suppose that the Earth were flat. In that case, at sunrise the light of the Sun would reach two distant cities at the same time--which was known to be false in the Middle Ages. Indeed, medieval astronomers were familiar with longitude and knew how to calculate it (although the practical difficulties of doing so would not be surmounted until the 18th century).