One of the most important measurements carried out by Ulugh Beg's astronomers was the obliquity of the ecliptic. The ecliptic is the circular path described by the the sun in the course of a year, and its obliquity is the angle at which it cuts the equator. Establishing this precisely is important for a variety of other astronomical measurements and calendar calculations. The astronomers in the classical world had errors on the order of 7'-10'. Arab astronomers achieved for the most part much greater precision; in the case of Ulugh Beg, the error was only -0'32". The most remarkable instrument in Ulugh Beg’s there was the huge Fakhri sextant, or underground mural quadrant which boasted a radius of 40m, making it, at the time, the largest astronomical instrument in the world of that type. in Samarkand 撒馬爾罕 ( 乌兹别克斯坦 Uzbekistan) The north–south axis of the main building was occupied by a huge sextant with a radius of 40 m (called Fakhrī sextant after that of Khu...