The Treaty of Tordesillas, between Spain and Portugal, divided the world outside of Europe in an exclusive duopoly along a north-south meridian 370 leagues, or 970 miles (1,560 km), west of the Cape Verde islands. However, as it was not possible at the time to correctly measure longitude, the exact boundary was disputed by the two countries until 1777. The completion of these negotiations with Spain is one of several reasons proposed by historians for why it took nine years for the Portuguese to follow up on Dias's voyage to the Cape of Good Hope, though it has also been speculated that other voyages were in fact secretly taking place during that time.