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 Khujandī). On the scale of this instrument, which partially lay in an underground slit with a width of half a meter, 70 cm corresponded to 1° of arc, so that the solar position could be read off with a precision of 5".
The Fakhri sextant determines basic constants in astronomy: the inclination of the ecliptic to the equator, the point of the vernal equinox, the length of the tropical year, and other constants arising from observation of the sun. It was built chiefly for solar observations in general, and for those of the moon and the planets, too. for transit over meridian transit
The main reason behind the sextant’s success was the accuracy it gave due to its large size. On the arc of the sextant, divisions of 70.2 cm represented one degree, while marks separated by 11.7mm corresponded to one minute, while marks spaced only 1mm apart represented five seconds.
Abu-l-Husan Abd al-Rahman ibn Omar al-Fufi al-Razi (al-Sufi). known as Azophi and Azophi Arabus, Born in Ray 903, died 986. One of the greatest Muslim astronomers. Friend and teacher of the Buwayhid sultan Adud al-dawla. His main work is the "Book of the Fixed Stars" illustrated with figures "Kitab al-kawakib al-thabita al-musawwar", one of the three masterpieces of Muslim observational astronomy (the two others being due to Ibn Yunus, first half of the eleventh century, and Ulugh Beg, first half of the fifteenth century).
Mcgill
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