Ailly, Pierre d' (1350–1420)

French geographer and theologian. As a cardinal he became involved in the Great Schism, arguing for the supremacy of Church councils over popes. He is best remembered for his book Imago mundi/Image of the World (about 1410), one of the leading geographical texts of the period, which strongly influenced Christopher Columbus.

Born at Compiègne and educated at the university of Paris, d'Ailly pursued a career in the Catholic Church, rising in 1411 to the rank of cardinal. Caught up in the Great Schism, in 1408 he broke with Pope Benedict XIII, and later attended the Concil of Constance, during which he published his Tractatus super reformatione ecclesiae (1416), an influential defence of Church councils.

He found most of his material for his Imago mundi in the works of classical writers, and paid little attention to the growing travel literature of his own day. A related work, Compendium cosmographiae/Cosmographical Compendium (1413), did little more than repeat the geography of Ptolemy. However, while Ptolemy had assumed that both land and sea covered about 180° of longitude, d'Ailly extended the land mass to 225°. The implications of such a framework were not lost on Columbus, a careful reader of d'Ailly.