A Line Drawn on Water
On June 7, 1494, representatives of Spain and Portugal met in this small Spanish town to sign a document with global consequences: the Treaty of Tordesillas. It aimed to settle territorial disputes following Christopher Columbus's voyages. A year earlier, Pope Alexander VI had issued a papal bull, Inter caetera, that drew a line 100 leagues west of the Cape Verde Islands, granting Spain rights to lands west of it. King John II of Portugal objected, arguing the line was too far east and would limit his nation's exploration of the African coast and sea routes.
Negotiations moved the line to 370 leagues west of the Cape Verde islands. This new line, running from pole to pole at approximately 46°30′ W longitude, granted Spain control over most of the Americas, while Portugal gained Africa and Asia. The treaty's westward shift had an large, unintended consequence: it placed the eastern bulge of South America in the Portuguese sphere, securing Portugal's future claim to Brazil after Pedro Álvares Cabral's landing in 1500.
The Science of Uncertainty
The treaty was a product of its time, built on imprecise science. The first major problem was the unit of measurement. The "league" was not standardized in the 15th century, and its exact length varied between Spain and Portugal. This alone created a zone of ambiguity about 150 kilometers wide. The treaty stated the 370-league distance should be determined by a joint voyage, but this voyage never happened.
The second, larger scientific challenge was determining longitude. While sailors could calculate their latitude with reasonable accuracy from the position of the sun or stars, longitude was a complete mystery. To know your east-west position, you must compare your local time with the time at a known reference point, an impossible task without precise clocks. Marine chronometers that could keep accurate time at sea were not invented until the 18th century. Cartographers and diplomats in 1494 had to rely on dead reckoning and flawed astronomical charts, making the exact location of the line impossible to map. This imprecision guaranteed future conflict.
The Antimeridian and an Eastern Dispute
The Treaty of Tordesillas failed to specify a dividing line on the other side of the world. As both Portuguese and Spanish explorers pushed eastward and westward, they eventually met in the Pacific. Both nations claimed the valuable Spice Islands (the Moluccas, in modern Indonesia) based on their own conflicting calculations of where the Tordesillas line's "antimeridian" should fall.
This new dispute was resolved by the Treaty of Zaragoza on April 22, 1529. To settle the matter, Spain sold its claim to the Moluccas to Portugal for 350,000 gold ducats. The new treaty established a second demarcation line 297.5 leagues east of the Moluccas. Even with this, the division of the globe was unequal; Portugal's hemisphere measured roughly 191 degrees, while Spain's was about 169 degrees.