Ulugh Beg’s observatory and radiation laws

Mirzo Ulugh Beg (1394–1449), a grandson of Amir Timur, governor of Samarkand and ruler of Mawarannahr, had an observatory built on Kohak hill in the 1420s (construction roughly 1424–1429). Its main instrument was the Fakhri sextant: a huge arc of a circle of radius ≈ 40 m set in the meridian plane (1/6 of a circle, i.e. 60°); it was not a telescope – it was made about 180 years before telescopes were invented, and observations were made by eye with sighting slits. One degree of the arc was ≈ 70 cm long, so angles could be measured very precisely. With Qadi Zada al-Rumi, Ghiyath al-Din Jamshid al-Kashi and Ali Qushchi, Ulugh Beg produced the Zij-i Sultani (its main part completed around 1437 and refined later), containing the coordinates of about 1018 stars, tables of planetary motion and calendars. The sidereal year was found to be ≈ 365 days 6 hours 10 minutes (an error of about a minute) and the obliquity of the ecliptic ≈ 23°30′. Ulugh Beg was killed in 1449, the observatory fell into ruin later, and the underground remains of the sextant were rediscovered in 1908 by V. L. Vyatkin.