Christopher Columbus tried to sail to Asia using a measurement of the Earth’s circumference via meridian arc length commissioned by this ruler, but forgot to convert Arabic miles to Roman miles. A qayna named for this ruler lived into her 90s and sang at the court of five caliphs. Bābak led the Khurramites in revolt against this ruler before he could centralize power away from the Sahlids. This patron of al-Farghānī sent his general Ṭāhir to govern his base of Khorāsān. This ruler tried to mend the (*) Sunni–Shīʿite schism and initiated the miḥna to promote Muʿtazilite (10[1])doctrine. This ruler supported al-Khwārizmī’s treatise on Algebra while presiding over the House of Wisdom’s (10[1]-5[2])heyday. (-5[3])For 10 points, (-5[1])name this Abbāsid (-5[1])caliph (-5[1])who wrested power from his brother al-Amīn after the death of his father, Hārūn ar-Rashīd. ■END■ (10[2]0[5])