Al-Biruni calculates Earth's radius through an original trigonometric method: he measures the angle of depression of the horizon from a mountaintop of known height, without needing to travel to two distant points as Eratosthenes did. He obtains a value of ~6,339.6 km (modern value: 6,371 km), with an error under 1%. The Canon Masudicus (1031), dedicated to Sultan Masud of Ghazni, systematizes mathematical astronomy, geodesy, and cartography with 600 corrected geographic coordinates. Al-Biruni spends 13 years in India (1017-1030) producing the first systematic ethnographic description of a non-Islamic civilization. Note: the title Qānūn al-Masʿūdī has no relation to the historian Al-Masudi (10th century): 'Masʿūdī' in the title is the adjective indicating belonging to Sultan Masʿud ibn Mahmud of Ghazni, Al-Biruni's patron.