section_path: Atlantic Ocean (1419–1507)/Portuguese exploration
section_name: Portuguese exploration
paragraph_text: Portuguese exploration
  
  In 1297, King Denis of Portugal took personal interest in exports. In 1317, he made an agreement with Genoese merchant sailor Manuel Pessanha, appointing him first admiral of the Portuguese Navy, with the goal of defending the country against Muslim pirate raids. Outbreaks of bubonic plague led to severe depopulation in the second half of the 14th century: only the sea offered alternatives, with most population settling in fishing and trading coastal areas. Between 1325 and 1357, Afonso IV of Portugal encouraged maritime commerce and ordered the first explorations. The Canary Islands, already known to the Genoese, were claimed as officially discovered under patronage of the Portuguese, but in 1344 Castile disputed them, expanding their rivalry into the sea.
  
  To ensure their monopoly on trade, Europeans (beginning with the Portuguese) attempted to install a Mediterranean system of trade which used military might and intimidation, to divert trade through ports they controlled; there it could be taxed. In 1415, Ceuta was conquered by the Portuguese aiming to control navigation of the African coast. The young prince Henry the Navigator was there and became aware of profit possibilities in the trans-Saharan trade routes. For centuries slave and gold trade routes linking West Africa with the Mediterranean passed over the Western Sahara Desert, controlled by the Moors of North Africa.
  
  Henry wished to know how far Muslim territories in Africa extended, hoping to bypass them and trade directly with West Africa by sea, find allies in legendary Christian lands to the south like the supposed long-lost Christian kingdom of Prester John and probe whether it was possible to reach the Indies by sea, the source of the lucrative spice trade. He invested in sponsoring voyages down the coast of Mauritania, gathering a group of merchants, shipowners and stakeholders interested in new sea lanes. Soon the Atlantic islands of Madeira (1419) and the Azores (1427) were reached. The expedition leader who established settlements on Madeira, was Portuguese explorer João Gonçalves Zarco.
  
  Europeans did not know what lay beyond Cape Non (Cape Chaunar) on the African coast, and whether it was possible to return once it was crossed. Nautical myths warned of oceanic monsters or an edge of the world, but Henry's navigation challenged such beliefs: starting in 1421, systematic sailing overcame it, reaching the difficult Cape Bojador that in 1434 one of Henry's captains, Gil Eanes, finally passed.
  
  From 1440 onwards, caravels were extensively used for the exploration of the coast of Africa. This was an existing Iberian ship type, used for fishing, commerce and military purposes. Unlike other vessels of the time, the caravel had a sternpost mounted rudder (as opposed to a side-mounted steering oar). It had a shallow draft, which was helpful in exploring unknown coastlines. It had good sailing performance, with a windward ability that was notable by the standards of the time. The lateen rig was less useful when sailing downwind – which explains Christopher Columbus (Italian: Cristoforo Colombo) re-rigging the Niña with square rig.
  
  For celestial navigation the Portuguese used the ephemerides, which experienced a remarkable diffusion in the 15th century. These were astronomical charts plotting the location of the stars over a distinct period of time. Published in 1496 by the Jewish astronomer, astrologer, and mathematician Abraham Zacuto, the Almanac Perpetuum included some of these tables for the movements of stars. These tables revolutionized navigation, allowing the calculation of latitude. Exact longitude remained elusive from mariners for centuries. Using the caravel, systematic exploration continued ever more southerly, advancing on average one degree a year. Senegal and Cape Verde Peninsula were reached in 1445 and in 1446, Álvaro Fernandes pushed on almost as far as present-day Sierra Leone.
  
  In 1453, the Fall of Constantinople to the Ottomans was a perceived blow to Christendom and established business links with the east. In 1455, Pope Nicholas V issued the bull Romanus Pontifex reinforcing the previous Dum Diversas (1452), granting all lands and seas discovered beyond Cape Bojador to King Afonso V of Portugal and his successors, as well as mostly cutting off trade to and permitting conquest and increased war against Muslims and pagans, initiating a mare clausum policy in the Atlantic. The king, who had been inquiring of Genoese experts about a seaway to India, commissioned the Fra Mauro world map, which arrived in Lisbon in 1459. In 1456, Diogo Gomes reached the Cape Verde archipelago. In the next decade captains at the service of Prince Henry, discovered the remaining islands which were occupied during the 15th century. The Gulf of Guinea would be reached in the 1460s.