Who was Al-Bīrunī and what were his main achievements in India?

Al-Bīrunī was a Persian scholar from Khwarizm who accompanied Mahmud. He was a mathematician, astronomer, geographer, historian, and linguist. He learned Sanskrit and wrote an encyclopaedic survey of Indian religion, philosophy, literature, geography, and sciences. He studied works of Aryabhaṭa, Varahamihira, and Brahmagupta, and translated Patanjali's Yogasutras into Arabic.