Ulugh Beg Observatory

• 1. Introduction

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  The Ulugh Beg Observatory once stood on a hill 21 meters above the plains of Samarkand, housing a sextant so large that part of it had to be buried underground to keep the building from collapsing under its own height. Built in the 1420s by a ruler who was also a serious astronomer, the observatory would eventually produce one of the most precise star catalogues the world had seen since antiquity. Who was the man behind it? What kind of intellectual life unfolded inside those cylindrical brick walls? And how did a place dedicated to measuring the heavens come to be destroyed just a few decades after it was built?

• 2. The Ruler Who Studied Stars

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  Ulugh Beg was the son of Shah Rukh and the grandson of Amir Timur, the conqueror who gave his name to the Timurid Empire. He became ruler of Samarkand in the 15th century AD, and he brought to that role an unusual set of priorities. Where other rulers built armies or pursued trade, Ulugh Beg built a madrassa and an observatory, and he filled them with the finest mathematicians and astronomers he could find.

  Jamshid al-Kashi left his native city of Kashan in north central Iran after receiving a personal invitation from Ulugh Beg. His letters home, written in Persian, became one of the main sources historians later used to reconstruct what life at the observatory was actually like. Over 60 mathematicians and astronomers were eventually invited to work there.

  Ulugh Beg's earlier visit to the Maragha Observatory appears to have shaped his ambitions in Samarkand. Al-Kashi's letters to Ulugh Beg's father record that impression directly. The decision to build in Samarkand may also have been driven by Ulugh Beg's disagreements with the observations of the earlier astronomer al-Sufi. Whatever the mix of motives, by the earlier half of the 1420s the foundation had been laid.

• 3. The Instrument Underground

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  Ulugh Beg needed an architect capable of solving an unusual problem. The sextant he wanted for the observatory had a radius of 40.04 meters, making it the largest astronomical instrument in the world at the time. A structure built entirely above ground to house such a device would have been dangerously tall. He turned to Qazizada-I Rumi, who recommended a mathematician and architect named Kashani.

  The solution was to sink part of the sextant into a ditch roughly 2 meters wide cut into the earth. This kept the building to a cylindrical shape 46 meters in diameter and between 30 and 33 meters high. The building was made of brick, which was available but not particularly stable, and the sextant occupied the center of the cylinder.

  The Fakhri sextant's divisions were precise. Each division of 70.2 centimeters represented one degree. Marks every 11.7 millimeters showed one minute, and marks spaced 1 millimeter apart represented five seconds. The observatory had three stories: the first was living quarters for the staff, and all observations were made from the second and third floors, which were lined with arches. The flat roof allowed instruments to be set up on top of the building as well.

  With this device, the observatory could measure the sun from the horizon, record the altitude of stars and planets, calculate the length of the year, track the periods of planets, and predict eclipses. Al-Kashi's letters noted that the flat roof at Samarkand gave it an advantage over Maragha, where the sextant was positioned higher but instruments could not be placed on the roof.

• 4. Life Inside the Observatory

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  A newly discovered letter from Al-Kashi to his father, translated by a scholar named Bagheri, has added considerably to what historians know about the daily atmosphere in Samarkand. The earlier letter, translated by Kennedy and Sayili in 1960, had painted only a partial picture. The new letter overlaps with it in places and fills in details about the intellectual life of the institution.

  Much of the teaching at the observatory was done from memory by teachers who had learned from astronomers in Herat. Three of the teachers Al-Kashi named were also deeply versed in Islamic jurisprudence: Qazızada, Maulana Muhammad Khani, and Maulana Abu'l-Fath. Al-Kashi noted in his letter that Ulugh Beg himself did not show up to teach class often.

  Maulana Abu'l-Fath described his own standing with characteristic modesty: "To do them justice, when there is a discussion in their meeting, I dare not intervene, all the more so because His Majesty knows this art well and therefore one cannot impudently claim competence." One of the scientists Al-Kashi studied with was Yusuf H. allaj, a man who had worked previously in Herat, Egypt, Syria, Anatolia, and other regions. His father, Maulana H. allaj, also lived in Samarkand at the same time.

  The new letter also revealed something unexpected about the intellectual culture there. A problem Al-Kashi described to his father concerned astrology, specifically the position of the Sun in Aquarius and the calculation of the ascendant. The scientific atmosphere of Samarkand may have been more oriented toward astrology than earlier scholarship had assumed. Krisciunas's paper on the legacy of Ulugh Beg had mentioned astrology only once in passing. Music was also studied and taught at the observatory. Al-Kashi himself was curious about both music and mathematics, though he noted that his colleagues urged him to keep his attention on his assigned tasks.

• 5. The Zij-i Sultani

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  In 1437, Ulugh Beg published the Zij-i Sultani, the work for which the observatory is best remembered. A Zij is a type of astronomical table that provides numerical data and explanations sufficient to calculate the positions of stars and planets and measure time. The Zij-i Sultani listed around 1,000 stars and was written in Persian.

  The project had begun in 1420, when Ulugh Beg was 26 years old. It took 17 years to compile the full catalogue. Ulugh Beg did not work alone; his collaborators included Jamshid al-Kashi and Ali Qushji, along with other Muslim astronomers. The primary instrument used to gather the data was the observatory's own quadrant.

  The Zij-i Sultani was the first star catalogue published since Ptolemy's Almagest, and it covered the same stars while providing more accurate figures. Ulugh Beg determined the length of the tropical year as 365 days, 5 hours, 49 minutes, and 15 seconds, an error of only plus 25 seconds. Nicolaus Copernicus later calculated the same figure with an error of plus 30 seconds, making Ulugh Beg's measurement the more precise of the two. Ulugh Beg also calculated the Earth's axial tilt as 23.5047 degrees in decimal notation.

  One of the most technically demanding sections of the Zij is the sine table. It runs 18 pages and records the value of sine to nine decimal places for every degree from zero to 87. From 87 to 90 degrees, the precision increases to 11 decimal places. The work was so reliable that it remained in active use until the 19th century, and hundreds of copies of the original Persian manuscript survive around the world. Translations exist in Arabic, Turkish, and Hebrew, and commentaries were written by astronomers including Qushji himself.

• 6. Destruction and Rediscovery

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  The observatory's end came from within the Timurid household. Around 1449, Ulugh Beg was assassinated by his own son, Abd al-Latif. The killing destabilized everything the observatory depended on. Dozens of talented astronomers and mathematicians were driven from Samarkand, and the building itself was destroyed.

  For centuries the site was largely forgotten. In 1908, the Russian archaeologist Vassily Vyatkin uncovered its remains. What survives today is the foundations and the buried section of the marble quadrant, the portion of the instrument that had been sunk into the earth to solve the height problem. The above-ground structure is gone.

  In 1970, the Ulug Beg Observatory Museum was built on the site to commemorate the observatory and its founder. The museum holds reproductions of the Arabic manuscripts of Ulugh Beg's star charts and the Zij-i Sultani, as well as important European printed editions of his work. Astrolabes and other instruments are on display, along with a miniature reconstruction of the observatory itself. The isolation that Al-Kashi once described in his letters, that strict adherence to Ptolemy's methods and limited awareness of other scholarly traditions, was something the diversity of invited scientists had only just begun to overcome when Ulugh Beg's assassination brought the whole enterprise to a halt.