How did al-Idrisi map the world for a Norman king, with south at the top?

Al-Idrisi was a Muslim geographer who spent about 15 years building a world atlas at the court of a Norman king. Around 1138, the Norman King of Sicily, Roger II, invited him to Palermo. The atlas, commissioned in 1138 and completed in 1154, compiled 70 maps of the known world, with a commentary by al-Idrisi on each location. It is known as the Tabula Rogeriana.

His method was to ask. Al-Idrisi interviewed experienced travellers and kept only what they agreed on and seemed credible, and he also sent agents to fact-check what they said. His main model was Arabic translations of Ptolemy's Geography. The book is divided into seven climatic zones, each cut into ten sections, and each section has its own map. Each map used a coordinate system that kept a consistent scale from one map to the next, though it was inaccurate by modern standards.

The maps look upside down to modern eyes: north is at the bottom and south at the top, with Mecca in the middle. The atlas also came with something that was not a page at all: a silver disc of 300 lb (140 kg) engraved with the whole world map. The atlas calculated Earth's circumference as 37,000 kilometres, an error of less than 10 percent.

The silver disc and the original atlas were destroyed in a rebellion in 1160, but copies survive: ten, five of them complete. Areas close to Roger's kingdom are the most accurate, while farther areas such as Southeast Asia are less detailed.