In 1453 Constantinople fell to the Turks. But this event had already been built into the two preceding centuries. The year 1543 has more than a numerological connection with the fall of an ancient empire; for in that year the De Revolutionibus Orbium Coelestium was published, on the very day Copernicus died. The Western mind has never been the same since; it is almost as if when the medieval curtain fell, Copernicus went to join his predecessors of the previous two centuries, whose critical explorations find their consummate achievement in his magnum opus.