Dynastic History | Ulugh Beg Observatory — The Sextant in the Trench, and the Succession That Buried It

On the hill northeast of old Samarkand, Ulugh Beg had a three-storey observatory built around a single instrument: the Fakhri sextant, a meridian arc of about forty metres radius whose lower portion ran in a trench cut into the rock. Size was the point, since a larger arc carries finer graduations. Published dates for the building run from 1421 to about 1428, and its completion is still argued over.

The work was institutional rather than princely. Ghiyath al-Din Jamshid al-Kashi and Qadi Zada al-Rumi are recorded working here, and Ali Qushji was among those who saw the results into final form. The output, the Zij-i Sultani, fixed the positions of roughly a thousand stars, nearly all newly observed rather than carried over from Ptolemy as earlier tables had been, and gave a length for the sidereal year within about a minute of the modern figure. Editions differ over whether it was finished in 1437 or in 1438–39.

Ulugh Beg succeeded his father as sultan on 13 March 1447 and lost the throne to his own son Abd al-Latif two years later. He was killed on 27 October 1449. The observatory was wrecked after his death and its astronomers dispersed. The site itself was lost until the archaeologist Vasily Vyatkin located and excavated the sextant trench in 1908.

The trench is what there is to see, roofed over, with a museum built beside it in 1970.

📸 Cover: The excavated underground arc of the Fakhri sextant, 2011, Bobyrr, CC BY-SA 4.0 via Wikimedia Commons