sources Al-Biruni (973–1052) described polar equi-azimuthal equidistant projection of the celestial sphere. Abu Nasr Mansur (960–1036) known for his work with the spherical sine law. His Book of Azimuths is no longer extant. Avicenna (980–1037) wrote on earth sciences in his Book of Healing. Ibn al-Faqih (10th century) wrote Concise Book of Lands (Mukhtasar Kitab al-Buldan). Ibn Rustah (10th century) wrote a geographical compendium known as Book of Precious Records. Further details about some of these are given below: In the early 10th century, Abū Zayd al-Balkhī, a Persian originally from Balkh, founded the "Balkhī school" of terrestrial mapping in Baghdad. The geographers of this school also wrote extensively of the peoples, products, and customs of areas in the Muslim world, with little interest in the non-Muslim realms. Suhrāb, a late 10th-century Persian geographer, accompanied a book of geographical coordinates with instructions for making a rectangular world map, with equirectangular projection or cylindrical equidistant projection. In the early 11th century, Avicenna hypothesized on the geological causes of mountains in The Book of Healing (1027). In mathematical geography, Persian Abū Rayhān al-Bīrūnī, around 1025, was the first to describe a polar equi-azimuthal equidistant projection of the celestial sphere. He was also regarded as the most skilled when it came to mapping cities and measuring the distances between them, which he did for many cities in the Middle East and western Indian subcontinent. He combined astronomical readings and mathematical equations to record degrees of latitude and longitude and to measure the heights of mountains and depths of valleys, recorded in The Chronology of the Ancient Nations. He discussed human geography and the planetary habitability of the Earth, suggesting that roughly a quarter of the Earth's surface is habitable by humans. He solved a complex geodesic equation in order to accurately compute the Earth's circumference. His estimate of 6,339.9 km for the Earth radius was only 16.8 km less than the modern value of 6,356.7 km. By the early 12th century the Normans had overthrown the Arabs in Sicily. Palermo had become a crossroads for travelers and traders from many nations and the Norman King Roger II, having great interest in geography, commissioned the creation of a book and map that would compile all this wealth of geographical information. Researchers were sent out and the collection of data took 15 years. Al-Idrisi, one of few Arabs who had ever been to France and