Gorky: Ulueg Beg's sextant

The sextant of Ulueg Beg in Samarkand 

 

In Samarkand, the enormous sextant of Ulueg Beg, grandson of Timurlane

[aka Timur], by which he took sun and star positions and accurately calculated the true length of a year. This could only be possible with a deep Arab learning about the stars, assisted by the Ptolemaic version of the heavenly spheres, a strong practical need for the knowledge (astrology and religious festivals), the Arabic (Hindu) numerals and the decimal system and a very strong Arabic & Khurasani development of abstract mathematics, trigonometry and large numbers. Preceded the Jantar Mantar in Jaipur and Delhi by 150 yrs.

  

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Mural sextants were built into a wall, oriented to lie precisely along a meridian (north-south line), with the arc marked in degrees, or fractions of degrees. The first known mural sextant was built in Iran, to a new design called the al-Fakhri sextant, in 994, and had a radius of about 20 metres. 


Ulugh Beg (which means "Great Ruler" or similar) set out to turn Samarkand into an intellectual centre of the Timurid Empire. Between 1417 and 1420, he built a madrasa (~university) there, a building that survives to this day. In 1428 he built an enormous observatory, called the Gurkhani Zij, incorporating a Fakhri sextant with a radius of 40 metres (reports differ). Although destroyed within a few generations of Ulugh Beg's death, the foundations and lower part of the sextant have since been excavated. A model reconstruction of the observatory is in the small museum at the site. The sextant extended three stories above ground.

 

Working two centuries before the invention of telescopes, this sextant allowed the calculation of the obliquity of the ecliptic (critical for an number of other astronomical measurements and calendrical calculations) to within 0'32". Contemporary classical (ie European) astronomers were only within 7'-10' of the true figure. He was able to measure the length of the sidereal year to within 25s of the current figure, more accurate than Copernicus' estimate a century later. Beg's calculation of the Earth's axial tilt remains the most accurate measurement to date, precisely matching the currently accepted value. He also compiled a catalogue of 1018 stars and their locations, the only such undertaking carried out between Claudius Ptolemy (ca 170AD) and Tycho Brahe (ca 1600).