---
title: "Ulugh Beg"
chunk: 2/4
source: "https://en.wikipedia.org/wiki/Ulugh_Beg"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T04:07:23.755017+00:00"
instance: "kb-cron"
---

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 fewer 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 Jadāvil-i Mavāzi' S̱avābit, sive, Tabulae Long. ac Lat. Stellarum Fixarum ex Observatione Ulugh Beighi 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°.

=== Mathematics ===
In mathematics, Ulugh Beg wrote accurate trigonometric tables of sine and tangent values correct to at least eight decimal places.

== Foreign relations ==
Once Ulugh Beg became governor of Samarqand, he fostered diplomatic relations with the Yongle emperor of the Ming dynasty. In 1416, Ming envoys Chen Cheng and Lu An presented silk and silver stuffs to Ulugh Beg on behalf of the Yongle emperor. In 1419, The Timurid sent his own emissaries, Sultan-Shah and Muhammad Bakhshi, to the Ming court. Ulugh Beg's emissaries came across Ghiyāth al-dīn Naqqāsh and other envoys representing Shah Rukh, Prince Baysunghur, and other Timurid authorities in Beijing; however, they stayed at separate hostelries. Ghiyāth al-dīn Naqqāsh even saw the Yongle emperor riding a black horse with white feet which had been gifted by Ulugh Beg. 