Ulugh Beg (1394-October 27, 1449) was a Timurid astronomer, mathematician and sultan. His name also appears as Uluğ Bey, (bey in Turkish means "chieftain"), Ulugh Bek and Ulug Bek.
The grandson of the conqueror Timur the Lame (1336 - 1405) and the oldest son of Shah Rukh, both of whom came from the Turkic Barlas tribe of Transoxiana (present Uzbekistan), Ulugh Beg was born in Sultaniyya in modern-day Iran. As a child he wandered through a substantial chunk of the Middle East and India as his grandfather expanded his conquests in those areas. With Timur's death, however, and the accession of Ulugh's father to much of the Timurid Empire, he settled in Samarkand which had been Timur's capital. After Shah Rukh moved the capital to Herat (in modern Afghanistan), sixteen year-old Ulugh Beg became the shah's governor in Samarkand in 1409. In 1411 he became a sovereign of the whole Mavarannhar khanate.
The teenaged ruler set out to turn the city into an intellectual center for the empire. In 1417 - 1420 he built a madrasa ("university" or "institute") on Rigestan Square in Samarkand, and invited numerous Islamic astronomers and mathematicians to study there. Ulugh Beg's most famous pupil in mathematics was Ghiyath al-Kashi (circa 1370 - 1429).
His own particular interests concentrated on astronomy, and in 1428 he built an enormous observatory, called the Gurkhani Zij, similar to Tycho Brahe's later Uraniborg. Lacking telescopes to work with, he increased his accuracy by increasing the length of his sextant; the so-called Fakhri Sextant had a radius of circa 36 meters and the optical separability of 180" (seconds of arc). Using it he compiled the 1437 Zij-i Sultani of 994 stars, generally considered the greatest of star catalogues between those of Ptolemy and Brahe. The serious errors which he found in the Arabian star catalogues (the authors had simply copied from Ptolemy, adding the effect of precession to the longitudes) induced him to redetermine the positions of 992 fixed stars, to which he added 27 stars from Al Sufi's catalogue from 964, which were too far south for observation from Samarkand. This catalogue, the first original one since Ptolemy, was edited by Thomas Hyde at Oxford in 1665 (Tabulae longitudinis et latitudinis stellarum fixarum ex observatione Ulugbeighi), by G. Sharpe in 1767, and in 1843 by Francis Baily in vol. xiii. of the Memoirs of the Royal Astronomical Society.
In 1437 Ulugh Beg determined the length of the sidereal year as 365.2570370...d = 365d 6h 10m 8s (an error +58s). In his measurements within many years he used a 50 m high gnomon. This value was improved by 28s 88 years later in 1525 by Nicolaus Copernicus (1473-1543), who appealed to the estimation of Thabit ibn Qurra (826-901), which was accurate to +2s.
Ulugh Beg was also notable for his work in astronomy-related mathematics, such as trigonometry and spherical geometry.
Unfortunately for Ulug he was the heir of Shah Rukh, and when his father died in 1447 he became ruler of the Timurid Empire. Within two years he was beheaded by his own eldest son, 'Abd al-Latif, while on his way to Mecca. Eventually, however, he was rehabilitated by his grandson Babur, founder of the Mogul Empire, who placed Ulugh Beg's remains in the tomb of Timur in Samarkand. There archeologists found them in 1941.
In honour of his achievements, a crater on the Moon has been named for him, Ulug Beigh (the spelling used by the German astronomer Johann Heinrich von Mädler (1794-1874) who proposed the name in his 1830 map of the Moon).