Pierre d'Ailly, Imago Mundi [Image of the World] (1480) was one of the books that Christopher Columbus read and annotated while planning his great voyage. As it happened, d'Ailly thought that the Atlantic Ocean was not so very wide. Columbus used d'Ailly's book to calculate that each degree of longitude along the Equator was the equivalent of 56.67 Roman miles...and this error led Columbus to underestimate the circumference of the earth by about 25%. If he knew it was such a long journey to the other side, he might not have started it in the first place. Here is the book itself, with Columbus's annotations:  In his copy of this book, and in other places, Columbus inscribed his signature:  |