On May 29, 1453, the ancient city of Constantinople, once capital of the Roman Empire, fell to the Ottoman Turks. Soon, the centuries-long order of the Middle Ages in Europe began to unravel: the armored knight on horseback ceased to inspire fear on the battlefield, stone castles no longer stopped an enemy's advance, and the pope in Rome found that half of Europe no longer accepted his teachings. European ships steered away from the familiar routes between the ports of their own continent, landing in the Americas, in India, and finally circling the world. New ideas about gaining knowledge by experiment and direct observation challenged medieval reliance on ancient authorities. Find out why in "The Day the World Changed."