Christopher Columbus was likely born in October 1451 in Genoa, Republic of Genoa. His name in Ligurian is Cristoffa Corombo. In one of his writings, he says he went to sea at the age of 10. In 1473, Columbus began his apprenticeship as business agent for the important Centurione, Di Negro and Spinola families of Genoa. Later, he allegedly made a trip to Chios, an Aegean island then ruled by Genoa. Columbus based himself in Lisbon from 1477 to 1485. Between 1482 and 1485, Columbus traded along the coasts of West Africa, reaching the Portuguese trading post of Elmina at the Guinea coast (in present-day Ghana). Ambitious, Columbus eventually learned Latin, Portuguese, and Castilian. He read widely about astronomy, geography, and history, including the works of Claudius Ptolemy, Cardinal Pierre d'Ailly's Imago Mundi, the travels of Marco Polo and Sir John Mandeville, Pliny's Natural History, and Pope Pius II's Historia Rerum Ubique Gestarum. Between 1492 and 1503, Columbus completed four round-trip voyages between Spain and the Americas, each voyage being sponsored by the Crown of Castile. On his first voyage, he independently discovered the Americas and magnetic declination. These voyages marked the beginning of the European exploration and colonization of the American continents, and are thus of enormous significance in Western history. Columbus always insisted, in the face of mounting evidence to the contrary, that the lands that he visited during those voyages were part of the Asian continent, as previously described by Marco Polo and other European travelers.