page_title: Ulugh Beg
section_title: Astronomy
hierarchical_section_title: Ulugh Beg / Science / Astronomy
caption_attribution_description: Observatory built by Ulugh Beg in Samarkand, the Timurid Empire (15th century) — present day Uzbekistan
context_page_description: Mīrzā Muhammad Tāraghay bin Shāhrukh, better known as Ulugh Beg, was a Timurid sultan, as well as an astronomer and mathematician.
  Ulugh Beg was notable for his work in astronomy-related mathematics, such as trigonometry and spherical geometry, as well as his general interests in the arts and intellectual activities. It is thought that he spoke five languages: Arabic, Persian, Turkic, Mongolian, and a small amount of Chinese. During his rule the Timurid Empire achieved the cultural peak of the Timurid Renaissance through his attention and patronage. Samarkand was captured and given to Ulugh Beg by his father Shah Rukh.
  He built the great Ulugh Beg Observatory in Samarkand between 1424 and 1429. It was considered by scholars to have been one of the finest observatories in the Islamic world at the time and the largest in Central Asia. Ulugh Beg was subsequently recognized as the most important observational astronomer from the 15th century by many scholars. He also built the Ulugh Beg Madrasah in Samarkand and Bukhara, transforming the cities into cultural centers of learning in Central Asia.
  However, Ulugh Beg's scientific expertise was not matched by his skills in governance.
context_section_description: Astronomy piqued Ulugh Beg's interest when he visited the Maragheh Observatory at a young age. This observatory, located in Maragheh,  Iran, is where the well-known astronomer Nasir al-Din al-Tusi practised.
  In 1428, Ulugh Beg built an enormous observatory, similar to Tycho Brahe's later Uraniborg as well as Taqi al-Din's observatory in Constantinople. 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 about 36 meters (118 feet) and the optical separability of 180" (seconds of arc). The Fakhri sextant was the largest instrument at the observatory in Samarkand (an image of the sextant is on the side of this article). There were many other astronomical instruments located at the observatory, but the Fakhri sextant is the most well-known instrument there. The purpose of the Fakhri sextant was to measure the transit altitudes of the stars. This was a measurement of the maximum altitude above the horizon of the stars. It was only possible to use this device to measure the declination of celestial objects. The image, which can be found in this article, shows the remaining portion of the instrument, which consists of the underground, lower portion of the instrument that was not destroyed. The observatory built by Ulugh Beg was the most pervasive and well-known observatory throughout the Islamic world.
  With the instruments located in the observatory in Samarkand, Ulugh Beg composed a star catalogue consisting of 1018 stars, which is eleven less stars than are present in the star catalogue of Ptolemy. Ulugh Beg utilized dimensions from al-Sufi and based his star catalogue on a new analysis that was autonomous from the data used by Ptolemy. Throughout his life as an astronomer, Ulugh Beg came to realize that there were multiple mistakes in the work and subsequent data of Ptolemy that had been in use for many years.
  Using it, he compiled the 1437 Zij-i-Sultani of 994 stars, generally considered the greatest star catalogue between those of Ptolemy and Tycho Brahe, a work that stands 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 the year 964, which were too far south for observation from Samarkand. This catalogue, one of the most original of the Middle Ages, was first edited by Thomas Hyde at Oxford in 1665 under the title Tabulae longitudinis et latitudinis stellarum fixarum ex observatione Ulugbeighi and reprinted in 1767 by G. Sharpe. More recent editions are those by Francis Baily in 1843 in vol. xiii of the Memoirs of the Royal Astronomical Society and by Edward Ball Knobel in Ulugh Beg's Catalogue of Stars, Revised from all Persian Manuscripts Existing in Great Britain, with a Vocabulary of Persian and Arabic Words (1917).
  In 1437, Ulugh Beg determined the length of the sidereal year as 365.2570370...ᵈ = 365ᵈ 6ʰ 10ᵐ 8ˢ (an error of +58 seconds). In his measurements over the course of many years he used a 50 m high gnomon. This value was improved by 28 seconds in 1525 by Nicolaus Copernicus, who appealed to the estimation of Thabit ibn Qurra (826–901), which had an error of +2 seconds. However, Ulugh Beg later measured another more precise value of the tropical year as 365ᵈ 5ʰ 49ᵐ 15ˢ, which has an error of +25 seconds, making it more accurate than Copernicus's estimate which had an error of +30 seconds. Ulugh Beg also determined the Earth's axial tilt as 23°30'17" in the sexagesimal system of degrees, minutes and seconds of arc, which in decimal notation converts to 23.5047°.
caption: Ulugh Beg Observatory in Samarkand. In Ulugh Beg's time, the walls were lined with polished marble.
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