Ulugh Beg Observatory: Astronomy in Timurid Samarkand

The Ulugh Beg Observatory was built on a hill outside Samarkand, in present-day Uzbekistan, in about 1424–1429 by the Timurid prince and astronomer Ulugh Beg. Its giant meridian instrument allowed his team of scholars to compile the Zij-i Sultani, a set of astronomical tables whose star catalogue was the most accurate of its time. It was one of the greatest observatories of the medieval world and a high point of science in Central Asia.

Ulugh Beg and the observatory tradition

Ulugh Beg, whose given name was Muhammad Taraghay, was born on 22 March 1394 at Sultaniyeh in Iran, during the campaigns of his grandfather Timur. His father, Shah Rukh, ruled the Timurid empire from Herat after Timur’s death in 1405. In 1409 he appointed the young Ulugh Beg, then about fifteen, as governor of Samarkand and Transoxiana.

Ulugh Beg governed there for some forty years, but he is remembered more as a patron of learning than as a ruler or general. He had seen the ruins of the 13th-century observatory at Maragha in north-west Iran as a boy, and he was well educated in mathematics and astronomy. Samarkand under his rule became one of the leading scientific centres of the Islamic world.

Large state observatories had a long history in the Islamic world. Abbasid caliphs sponsored astronomical observations in Baghdad and Damascus in the 9th century, building on the work of scholars linked to the House of Wisdom. After the Mongol sack of Baghdad, the Ilkhan Hulagu founded the Maragha observatory in 1259 under the scholar Nasir al-Din al-Tusi.

Ulugh Beg built on this tradition. In 1417–1420 he founded a madrasa on Samarkand’s Registan square, where astronomy and mathematics were taught alongside religious subjects. The observatory soon followed, giving his scholars an instrument large enough to improve on earlier measurements.

The building and the great arc

The observatory stood on Kohak hill, also called Chupan-Ata, north-east of the city. Accounts and excavations suggest it was a cylindrical building about 46 metres in diameter and perhaps 30 metres high, with three storeys. Its walls were decorated with glazed tiles and, according to later writers, with images of the heavens.

At its heart was the main instrument, known as the Fakhri sextant. It was a huge arc of brick and stone, set precisely along the north–south meridian line, with a radius of about 40 metres. Part of it was cut into a trench in the rock, and the upper part rose through the building. Marble-faced tracks marked with degrees and minutes allowed observers to measure the height of the Sun, Moon and planets as they crossed the meridian.

The enormous size of the arc was intended to improve accuracy, since each degree of the scale was roughly 70 centimetres long. Smaller instruments, such as armillary spheres and astrolabes, were probably also used, though they have not survived.

The scholars and the Zij-i Sultani

Ulugh Beg gathered a group of gifted astronomers and mathematicians. Jamshid al-Kashi, the first director of the observatory, was one of the finest mathematicians of the age; in 1424 he calculated pi to 16 decimal places, a record that stood for well over a century. He died in 1429, near Samarkand.

Qadi Zada al-Rumi, a scholar from Bursa in Anatolia, was Ulugh Beg’s teacher and led the work after al-Kashi’s death. When he died, the young Ali Qushji took charge and completed the observations. Ali Qushji later moved to Istanbul, where he taught under Sultan Mehmed II and carried the Samarkand tradition to the Ottoman world.

The main result of the observatory was the Zij-i Sultani, also known as the Zij-i Gurgani, written in Persian and completed around 1437, with revisions continuing into the 1440s. A zij is a handbook of astronomical tables. It contained sections on calendars and chronology, practical astronomy, the motions of the planets and the positions of fixed stars.

Its catalogue listed 1,018 stars. It was the first major star catalogue since antiquity based largely on fresh observation rather than on adjusting Ptolemy’s positions, though some faint stars that could not be seen at Samarkand’s latitude were taken from the earlier work of al-Sufi. Ulugh Beg also measured the tilt of the Earth’s axis and the length of the year with great precision.

How the observations were made

Astronomers worked without telescopes, which were not invented until the early 17th century. Instead, they relied on the sheer size of the meridian arc and on careful, repeated sightings. When the Sun crossed the meridian at noon, its light passed through an opening above the arc and fell on the graduated scale, allowing its altitude to be read off directly.

From such noon altitudes, measured across the seasons, the astronomers could work out the tilt of the Earth’s axis, the latitude of Samarkand and the length of the year. Star positions were measured with other instruments at night and then converted into celestial coordinates using spherical trigonometry, a field in which al-Kashi and his colleagues were leading experts. The results were checked against earlier tables, including those of Maragha, and corrected where they differed.

Death of Ulugh Beg and the fate of the observatory

Shah Rukh died in 1447, and Ulugh Beg became head of the Timurid dynasty. His reign was short and troubled. His eldest son, Abd al-Latif, rebelled, defeated him in 1449 and had him murdered on 27 October 1449 while he was travelling to Mecca as an exile.

After his death the observatory lost its patron and soon fell out of use. Over the following centuries the building was dismantled and its materials taken for other construction, until its location was forgotten. In 1908 the Russian archaeologist Vasily Vyatkin, using an old land document, found and excavated the underground part of the great arc, which can still be seen today.

Legacy

The Zij-i Sultani was copied widely across the Islamic world and studied in Persia, India and the Ottoman Empire. In Europe, the Oxford scholar John Greaves published parts of it in the mid-17th century, and Thomas Hyde issued the star catalogue with a Latin translation in 1665. A crater on the Moon is named Ulugh Beigh in his honour.

In Uzbekistan Ulugh Beg is regarded as a national figure. His madrasa on the Registan and the remains of the observatory form part of the Samarkand World Heritage property, and a small museum on the hill presents his work.

By the numbers

Timeline

• 1394, 22 March: Ulugh Beg is born at Sultaniyeh.
• 1409: He becomes governor of Samarkand.
• 1417–1420: The Ulugh Beg Madrasa is built on the Registan.
• c. 1424–1429: The observatory is built on Kohak hill.
• 1429: Death of Jamshid al-Kashi, the observatory’s first director.
• c. 1437: The Zij-i Sultani is completed.
• 1449, 27 October: Ulugh Beg is murdered on his son’s orders.
• 1665: Thomas Hyde publishes the star catalogue in Oxford.
• 1908: Vasily Vyatkin rediscovers the observatory’s remains.

Why it matters

The Ulugh Beg Observatory shows that the descendants of Timur, a conqueror remembered for destruction, also sponsored some of the most advanced science of the 15th century. Its measurements set a standard of precision that was not surpassed until Tycho Brahe’s work in Denmark more than a century later. It remains a key link between the astronomy of the medieval Islamic world and the scientific revolution in Europe.