March 22, 1394 – October 27, 1449
Ulugh Beg was a Timurid ruler as well as an astronomer, mathematician and sultan. His commonly known name is not truly a personal name, but rather a moniker, which can be loosely translated as "Great Ruler" or "Patriarch Ruler." Ulugh Beg was also notable for his work in astronomy-related mathematics, such as trigonometry and spherical geometry.
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 Registan Square in Samarkand, and invited numerous Islamic astronomers and mathematicians to study there. The madrasa building still survives. 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 as well as Taqi al-Din's Istanbul observatory of al-Din. 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 (118 ft) 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, alongside Abd al-Rahman al-Sufi's Book of Fixed Stars. The serious errors which he found in previous Arabian star catalogues (many of which had simply updated Ptolemy's work, 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 Abd al-Rahman al-Sufi's catalogue Book of Fixed Stars from 964, which were too far south for observation from Samarkand.
This catalogue, one of the most original of the Middle Ages, was edited by Thomas Hyde at Oxford in 1665 under the title 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.
The above-ground portion of Ulugh Beg's observatory was destroyed shortly after his death; the surviving underground chamber was excavated in 1908 by primary school teacher and amateur archaeologist Vladimir Viyatkin, later Samarkand's director of antiquities.
In mathematics, Ulugh Beg (14th century) wrote accurate trigonometric tables of sine and tangent values correct to 8 decimal places.
The crater Ulugh Beigh on the Moon was named after him by the German astronomer Johann Heinrich von Mädler in his 1830 map of the Moon.