How Two Kings Drew a Line Through an Unmapped World

On June 7, 1494, in the Castilian town of Tordesillas, envoys for Isabella and Ferdinand of Spain and John II of Portugal signed a treaty dividing the unexplored, non-Christian world between them. The boundary was a line of longitude running pole to pole, 370 leagues west of the Cape Verde Islands. Everything east of the line — Africa, the Indian Ocean, eventually most of Brazil's bulge — was Portuguese. Everything west, including most of the Americas, was Spanish.

The treaty was a renegotiation. The previous year Pope Alexander VI, an Aragonese by birth, had drawn a line only 100 leagues west of Cape Verde — a position that gave Spain almost everything. John II of Portugal, whose navigators already knew more about Atlantic winds than anyone alive, refused. He pushed the line another 270 leagues west.

What John knew, or strongly suspected, has been argued ever since. When Pedro Álvares Cabral made formal landfall in Brazil in 1500, the eastern bulge fell on Portugal's side of the new line. Some historians think Portuguese ships had already sighted that coast and the negotiation was informed; others credit Cabral's landing to the South Atlantic gyre, which sweeps any westbound vessel toward the South American coast.

The practical problem was that no one could actually measure longitude at sea. The treaty specified a meridian, but for two centuries the precise position was a matter of diplomatic argument and strategic ambiguity. Both crowns kept exploring across each other's lines, and rival powers — France, England, the Dutch — simply ignored the whole arrangement. "The Pope may divide the world as he pleases," Francis I of France reportedly said, "but I should like to see the clause in Adam's will that excluded me."