How it began
Ulugh Beg, grandson of Timur and ruler of Samarkand, was a trained mathematician and astronomer. He founded an observatory on a hill outside the city in the 1420s. The foundation was laid early in the decade and the building is thought to have been finished around 1428, though the completion date is debated.
It followed the model of the earlier Maragha observatory designed by Nasir al-Din al-Tusi. Ulugh Beg invited more than 60 mathematicians and astronomers to work there. Jamshid al-Kashi was the first director, followed by Qadi Zada al-Rumi and then Ali Qushji.
How it works
The main instrument was a huge meridian arc, known as the Fakhri sextant, with a radius of about 40 meters. It was the largest astronomical instrument of its time. A brick building tall enough to hold it would have been unstable, so the lower part of the arc was cut into a trench in the rock about 2 meters wide. The building above was a cylinder about 46 meters across.
The scale was fine enough to read small angles directly. One degree took up about 70 centimeters of the arc, and one minute about 11.7 millimeters. Observers used it to measure the altitude of the Sun, stars and planets as they crossed the meridian, and from those readings worked out the length of the year, planetary periods and eclipses.
What changed first
The observatory's main product was the Zij-i Sultani, a set of astronomical tables with a star catalogue, written in Persian and published in 1437. It gave positions for about a thousand stars. It was the first star catalogue based on new observation since Ptolemy's Almagest and gave more accurate figures for the same stars. Its sine table runs to 18 pages with values to nine decimal places.
Ulugh Beg measured the tropical year as 365 days, 5 hours, 49 minutes and 15 seconds, an error of about 25 seconds. The work stayed within Ptolemy's planetary model and did not propose changes to it.
What lasted
Ulugh Beg was killed in 1449 on the orders of his son Abd al-Latif. The observatory was destroyed and its astronomers scattered. The tables outlived the building. The Zij was copied in hundreds of manuscripts, translated into Arabic, Turkish and Hebrew, and remained in use until the 19th century. Ulugh Beg's year length was slightly more accurate than the figure Copernicus later used.
Where it stands today
The site was lost until 1908, when the Russian archaeologist Vassily Vyatkin uncovered the remains. The foundations and the buried section of the marble-lined arc are all that survive of the original structure. A museum built on the site in 1970 displays reproductions of the star catalogue manuscripts. The trench with the lower part of the arc is one of the main historic sights of Samarkand.
