In 349 AH (960–61 AD) the Persian astronomer ‘Abd al-Rahmān al-Sūfī was visiting the court of Būyid caliph ‘Adud al-Dawlah in Isfahan. Shortly after his arrival, another learned gentleman known to be well-read in astronomical matters stopped by. When night fell, the caliph asked his two visitors to identify a star that should be visible above the eastern horizon.

When the other astronomer struggled to answer, al-Sūfī saw that his colleague may have memorized a few astral charts, but he had spent little time using them to read the night sky. He explained to the caliph that a calculation error in Ptolemy’s Almagest — a cosmological almanac that had been translated into Arabic the previous century — meant the stars had shifted from the positions plotted by the Alexandrian eight hundred years before. In order to read the stars now, you had to add twelve longitudinal degrees. There are leading scientists, al-Sūfī bemoaned, who have never confirmed the books they read with direct observation.

The caliph hired Al-Sūfī as his personal tutor and put him in charge of a new imperial observatory in Shiraz. Three years later al-Sūfī presented the caliph with the Kitāb al-Kawākib al-Thābitah Musawwar (Illustrated Book of Fixed Stars), an expansion on the Almagest that became the greatest astronomical work of its age. Portable and illustrated, it was not meant simply as a display of learning but as an aid in direct observation. Each constellation is represented twice: once reflecting how the starry forms would look to an observer looking down at a celestial globe, a second time reflecting how they appear while gazing up at the night sky.

Ptolemy’s Almagest contained tables identifying names and positions of over a thousand stars that could be seen with the naked eye. The Kitāb al-Kawākib al-Thābitah Musawwar lists several bodies not mentioned by Ptolemy and includes traditional Arabic names for certain constellations and stars. To Arab stargazers, for example, the six stars that form the head of Cetus (the Whale) are known as al-kaff al-jadhmā — “the mutilated hand”. The five in the central body are called al-na’āmāt — “the ostriches” — and the star on the tip of the tail is called al-difdi’ al-thānī: “the second frog”.

Meanwhile, in the area where Greeks found the constellation Andromeda, Arabs saw two fish. Al-Sūfī combined the images, so that the fish bisected Andromeda’s body. A star in one creature’s mouth, overlapping with Andromeda’s belt, is particularly large. In his accompanying notes al-Sūfī calls it, descriptively, al-latkhā al-sahābiya — “the nebulous smear”. This turns out to be the Andromeda galaxy, and al-Sūfī’s reference to it is the first ever written mention of a galaxy other than the Milky Way.

During the Mongol invasion of Baghdad in 656 AH (1258 AD), a copy of the Illustrated Book of Fixed Stars was saved by astronomer Nasīr al-Dīn al Tūsī, who offered it to Hulagu Khan and convinced him to build an observatory. By the fifteenth century, this distressed volume found its way to the Timurid sultan Ulugh Bey in Samarkand, who commissioned a new version and, like al-Sūfī, became the greatest astronomer of his age.

The pages shown here are from Ulugh Bey’s manuscript, now held at the Bibliotheque Nationale de France. Other versions can be found in New York’s Metropolitan Museum of Art and Oxford’s Bodleian Library.