Abd al-Rahman al-Sufi

• Name — ʿAbd al-Raḥmān al-Ṣūfī • Native Name — عبدالرحمن الصوفي • Born — 903 12 7 • Birthplace — Rey, Jibal, Abbasid Caliphate • Died — 986 5 25 • Occupation — Astronomer, mathematician • Era — Islamic Golden Age • Notable Works — Kitāb ṣuwar al-kawākib ("The Book of Fixed Stars")

ʿAbd al-Raḥmān al-Ṣūfī was one of the nine famous Muslim astronomers. He lived at the court of Emir 'Adud al-Dawla in Isfahan and worked on translating and expanding ancient Greek astronomical works, especially the Almagest of Ptolemy. He made corrections to Ptolemy's star list, and his estimations of star brightness and magnitude deviated from those of Ptolemy; just over half of al-Ṣūfī's magnitudes were identical to Ptolemy's. A Persian, al-Ṣūfī wrote in Arabic, the lingua franca of the scientific Muslim world. Al-Ṣūfī was a major contributor to the translation into Arabic of Hellenistic astronomy, which had been centered in Alexandria, Egypt. He was among the first to attempt to relate Greek star names with the traditional Arabic names and constellations, which were largely unrelated and overlapped in complex ways.

Al-Ṣūfī made his astronomical observations at a latitude of 32.7° N in Isfahan. It has been claimed that he identified the Large Magellanic Cloud, but this appears to be a misunderstanding of a reference to some stars south of Canopus, which he admits he did not observe. He also made the earliest recorded observation of the Andromeda Galaxy in 964, describing it as a "small cloud". This was the first galaxy other than the Milky Way to be mentioned in writing.[3]

Al-Ṣūfī also wrote about the astrolabe, identifying numerous additional uses for it. According to American Near Eastern scholar Adam L. Bean, al-Ṣūfī's work describes over 1,000 different uses in areas as diverse as astronomy, astrology and horoscopes, navigation, surveying, timekeeping, determining the Qibla, and performing Salat prayer.[4][5]

• Illustration — “Sign of Sagittarius” by al-Sufi in his book Ṣuwar al-kawākib al-thābita, Artuqid Mardin, 1131 CE.[6] Al-Ṣūfī published Kitāb ṣuwar al-kawākib ("The Book of Fixed Stars") in 964, and dedicated it to Adud al-Dawla, the ruler of Buwayhid at the time. This book describes 48 constellations and the stars within them. Al-Ṣūfī compared Greek constellations and stars as described in Ptolemy’s Almagest with Arabic ones,[7] linking those that were the same. He included two illustrations of each constellation, one showing the orientation of the stars from the perspective outside the celestial globe, and the other from the perspective of looking at the sky while standing on the Earth. He separated them into three groups; 21 seen from the north, 15 seen from the south, and the 12 zodiac constellations. He included a complete set of star charts, that included the names and numbers of the individual stars in each of the 48 constellations, and each star's longitudinal and latitudinal coordinates, magnitude, and location north or south of the ecliptic. Scribal errors within the 35 surviving copies of The Book of Fixed Stars have caused the value of the magnitude for a particular star to vary from manuscript to manuscript. Al-Ṣūfī organized the stars in each of his drawings into two groups: those that form the image depicted, and others that are in close proximity to the image. He identified and described stars not included by Ptolemy, but he did not include them in his own star charts. Stating that his charts were modelled after Ptolemy, he left the stars excluded in Ptolemy's catalogue out of his charts as well. To allow for the longitudinal placement of the stars within constellations having changed over the eight centuries since the Almagest was written, Al-Ṣūfī added 12° 42' to all the longitudes values provided by Ptolemy. Al-Ṣūfī differed from Ptolemy by having a three level scale to measure the magnitude of stars instead of a two level scale. This extra level increased the precision of his measurements. His methodology for determining these magnitude measurements cannot be found in any of his extant texts. Despite the importance of The Book of Fixed Stars in the history of astronomy, it took more than 1000 years until the first partial English translation of the book was published in 2010.[8]