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Zīj-i Sulṭānī (Persian:زیجِ سلطانی) is aZij astronomical table andstar catalogue that was published byUlugh Beg in 1438–1439. It was the joint product of the work of a group ofMuslim astronomers working under the patronage ofUlugh Beg atSamarkand'sUlugh Beg Observatory. These astronomers includedJamshid al-Kashi andAli Qushji, among others.
TheZij-i-Sultani was not surpassed in accuracy until the work ofTaqi ad-Din andTycho Brahe in the 16th century.
The serious errors which Ulugh Beg found in previousZij star catalogues (many of the earlier ones were simply updates on Ptolemy's work, adding the effect ofprecession to the longitudes) induced him to redetermine the positions of 992 fixed stars, to which he added 27 stars from al-Sufi'sBook of Fixed Stars (964), which were too far south for observation from Samarkand. This catalogue, one of the most original of the Middle Ages, was edited byThomas Hyde at Oxford in 1665 under the titleJadāvil-i Mavāzi' S̱avābit, sive, Tabulae Long. ac Lat. Stellarum Fixarum ex Observatione Ulugh Beighi, byGregory Sharpe in 1767, and in 1843 byFrancis Baily in Vol. XIII of theMemoirs of the Royal Astronomical Society.
In 1437, Ulugh Beg determined the length of thesidereal year as 365.2570370...d = 365d 6h 10m 8s (an error of +58s). In his measurements over many years he used a 50 m highgnomon. This value was improved by 28s, 88 years later in 1525 byNicolaus Copernicus (1473–1543), who appealed to the estimation ofThābit ibn Qurra (826–901), which was accurate to +2s. However, Ulugh Beg later measured another more precise value as 365d 6h 9m 35s, which has an error of +25s, making it more accurate than Copernicus' estimate which had an error of +30s. Ulugh Beg also determined the Earth'saxial tilt as 23;30,17 degrees insexagesimal notation, which in decimal notation converts to 23.5047 degrees.[1]
