Likewise in geography, Muslims were able to extend their horizons far beyond the world of Ptolemy. As a result of travel over land and by sea and the facile exchange of ideas made possible by the unified structure of the Islamic world and the hajj which enables pilgrims from all over the Islamic world to gather and exchange ideas in addition to visiting the House of God, a vast amount of knowledge of areas from the Pacific to the Atlantic was assembled. The Muslim geographers starting with al-Khwarazmi, who laid the foundation of this science among Muslims in the 9th century, began to study the geography of practically the whole globe minus the Americas, dividing the earth into the traditional seven climes each of which they studied carefully from both a geographical and climactic point of view. They also began to draw maps some of which reveal with remarkable accuracy many features such as the origin of the Nile, not discovered in the West until much later. The foremost among Muslim geographers was Abu 'Abdallah al-Idrisi, who worked at the court of Roger II in Sicily and who dedicated his famous book, Kitab al-rujari (The Book of Roger) to him. His maps are among the great achievements of Islamic science. It was in fact with the help of Muslim geographers and navigators that Magellan crossed the Cape of Good Hope into the Indian Ocean. Even Columbus made use of their knowledge in his discovery of America.