Absorbing account of how a 16th-century cartographer put America on the map

BOOK OF THE DAY: The Fourth Part of the World: The Epic Story of History's Greatest MapBy Toby Lester Profile, 464pp, £25

IN AN era of satellite imagery and internet technology it can be difficult to appreciate the challenge that visualising our world once posed. Scholars in medieval Europe viewed the Earth as centre of the cosmos, and constructed a three-part world view based on the landmasses of Europe, Africa and Asia being encircled by ocean.

For the compiler of an early medieval encyclopaedia, Isidore of Seville (d 636), there was also a fourth part, still unknown and beyond the ocean.

The broad theme for this enjoyable and absorbing book is the overturning of that world view as Europeans probed east, south and west during the later middle ages.

The cast for what Toby Lester calls his "macro-history" brings together a stimulatingly diverse range of characters. Ptolemy, St Brendan the navigator, Genghis Khan and Prester John are among those with walk-on parts. All lead towards the Portuguese and Spanish voyages of the late 15th century.

When Columbus sailed westward in 1492, he famously sought to explore new trade routes to east Asia and the Indies. Other voyages quickly followed, but it took a little longer to appreciate the immensity of what was being discovered: a major new land mass, the fourth part of the world.

That conceptual breakthrough was crystallised at St Dié in Lorraine, eastern France, in 1507.

There, Martin Waldseemüller produced a small globe and a huge map that represented a new understanding of the world. A short accompanying book, probably co-authored with the humanist Matthias Ringmann, notices "the fourth part of the world, discovered by the kings of Castille and Portugal".

What was revolutionary was how that area appeared on the map. In historic firsts, the new landmass is shown surrounded by water, and its southern part is labelled "America".

Lester focuses his "micro-history" on Waldseemüller, his innovative work and its subsequent diffusion and development. Much about the map and the linked commentary was controversial, not least the downplaying of Columbus and the prominence given to Amerigo Vespucci, the Florentine merchant who claimed to have made four voyages to the New World between 1497 and 1504.

Controversial too was the depiction, six years before the first-known European sighting of the Pacific, of an America surrounded by water.

On this issue, as well as on his designation "America", Waldseemüller had second thoughts, and he offered a revised, more conservative world map in 1513. But 1,000 copies of the 1507 map had already been circulated across central Europe. The name America was gaining wider currency and the existence of the Pacific was about to be confirmed among Europeans. The detail of the New World had yet to be refined, but its outline could not be retracted.

Lester is good on the subsequent history and impact of the Waldseemüller map, for example on how it may have contributed to the thinking of Copernicus. He also outlines how the map "disappeared" to such an extent that during the 19th century it was believed "lost".

In 1901 a single copy of the original 1507 map was discovered in a castle in southern Germany.

In contrast to the now widely discredited Vinland map that was once considered an early representation of northeast America, the credentials of the Waldseemüller map appear close to impeccable. Certainly the US Library of Congress thought so. In 2003, it paid $10 million (€6.7 million), at that time "the highest price ever paid publicly for a historical document", for the only known complete copy.

Arnold Horner lectures in geography at University College Dublin. His book, Mapping Sligo in the Early 19th Century, will be published by Wordwell in 2010