In the roughly 14 hundred years from Ptolemy writing The Almagest in the 2nd century until the invention of the telescope made more accurate astronomical observations possible at the beginning of the 1600s, many transcripts and transcripts of transcripts were made of his star catalogue by astronomers of the mediaeval East and Europe, but only a few of them contributed to the original work with additions or corrections based on their own observations. One of these astronomers was Mīrzā Muhammad Tāraghay bin Shāhrukh, better known as Ulugh Beg, Timurid ruler of the region of Samarkand in the 15th century.
Ulugh Beg became governor of Samarkand in 1409, when his father Shah Rukh finally managed to take possession of the eastern portion of the empire established by his father Timur (Tamerlane) and moved his capital to Herat. Coming from an educated family – his parents were patrons of art and science, his mother’s efforts had even made the education of woman acceptable for a short time – his intension was to turn the city into an intellectual center.
In 1417 he began to build a madrasah – an institute for higher education – in Samarkand for studying different disciplines of secular science from astronomy and mathematics to medicine and poetry.
The two-story building-complex arranged around a rectangular courtyard with an imposing entrance portal and two minarets at the front facade included dormitory cells and rooms for the scholars and students, four lecture rooms at the four corners and a mosque at the west wall of the courtyard. Ulugh Beg invited the best scholars he could find as lecturers, and the Ulugh Beg Madrasah soon became the leading center of Islamic education in Central Asia. Today the building is still standing at the Registan, the center of Samarkand, with two other madrasahs built in the 17th century on two other sides of the square.
Although educated and active on many fields, Ulugh Beg’s main interests – inspired probably by a childhood visit to the remnants of the Marāgha Observatory, where Persian astronomer al-Tusi and his team completed around 1270 the star catalogue called the Ilkhanic tables (Zīj-i Īlkhānī) – had always been mathematics and astronomy. In 1420 he started to build the enormous observatory Gurkhani Zīj near Samarkand, with huge structures used as instruments to increase the accuracy of astronomical observations and measurements to the maximum. The observatory was unparalleled in its time.
‘Its circular main building, beautifully decorated with glazed tiles and marble plates, had a diameter of about 46 m and three stories reaching a height of approximately 30 m above ground level. The north–south axis of the main building was occupied by a huge sextant with a radius of 40 m (called Fakhrī sextant after that of Khujandī). On the scale of this instrument, which partially lay in an underground slit with a width of half a meter, 70 cm corresponded to 1° of arc, so that the solar position could be read off with a precision of 5″. On the flat roof of the main building various smaller instruments could be placed, such as an armillary sphere, a parallactic ruler, and a triquetrum. Among other instruments known to have been used in Samarqand are astrolabes, quadrants, and sine and versed sine instruments.’
Around this time Ulugh Beg was already working together with numerous scholars he invited to join his madrasah, among them renowned scientist Qāḍī Zāda al-Rūmī and the excellent Islamic astronomer and mathematician Jamshid al-Kāshī, author of an updated star catalogue called Khaqani Zīj. With a team of about 60-70 fellow scientists, based on systematical observations and computations carried out between 1420 and 1437 they completed an astronomical handbook with tables called Zīj-i-Sultani.
The catalogue includes the coordinates of 1018 stars, the listing is based on Abd al-Rahman al-Sufi‘s Kitab al-Kawakib al-Thabita – The Book of Fixed Stars from the late 10th century, an augmented and improved version of Ptolemy’s Almagest.
As Ulugh Beg explains in the preface:
“Determination of the Places of the Fixed Stars in Longitude and Latitude.
Before the time of Ptolemy 1,022 fixed stars had been observed. Ptolemy has given them in a catalogue in the Almagest. The stars are distributed in six magnitudes; the largest are of the first and the smallest of the sixth magnitude. Each magnitude is divided into thirds, and in order to recognize the stars, 48 figures or constellations have been imagined, of which 21 are north of the ecliptic, 12 in the Zodiac, and 15 south of the ecliptic. The larger number of the stars are within the figures, the others are in the neighborhood, and are designated as unformed stars of the constellation.
Abd Al Rahman Sufi composed a treatise on the stars which all learned men have received with gratitude. Before determining by our own observations the position of these stars, we have laid them down on a sphere according to this treatise, and we have found that the greater part of them are situated differently from their appearance in the heavens. This determined us to observe them ourselves with the assistance of Divine Providence, and we have found that they were advanced from the epoch at which Sufi’s work was written, so that on giving them, according to this general observation, their absolute positions, we no longer found any difference from their appearance to the eye.
It is on this principle that we have reobserved all the stars already determined, with the exception of 27 which are too far to the south to be visible at the latitude of Samarkand (…) and we have taken these 27 stars from the work of Abd Al Rahman Sufi, taking account of the difference of epoch.
Besides these there are 8 stars mentioned by Abd Al Rahman Sufi in his book, of which Ptolemy gives the positions, but which Abd Al Rahman Sufi could not find, and which notwithstanding all our researches, we have been unable to discover. For that reason we do not indicate those stars in the present catalogue.(…)
In our catalogue we have given the position of the stars for the beginning of the year 841 of the Hegira, so that at any time we may be able to find the place of any stars on the supposition that they advance one degree in seventy solar years.”
The preface to the tables consists of four parts. The first three parts contain methodical descriptions of different astronomical measurements and calculations e.g. determining the length and initial days of years and months, the equation of the Sun and Moon, positions and characteristics of the stars and the seven planets, calculating eclipses, Moon phases and determining the position of the twelve celestial houses: the Zodiac signs. The fourth part of the preface is about astrology: horoscopes and nativities. Ulugh Beg, like many other Persian rulers, believed in astrology and fortune‐telling.
The catalogue also includes sheets of the motions and eclipses of the Sun, the Moon and the planets, longitude and latitude of several cities in various countries, trigonometric tables etc.
In 1449 Ulugh Beg was killed by his oldest son Abdal-Latif Mirza on his way to Mecca, his observatory was demolished by religious extremists, the library was looted, the scholars were forced to flee. Ali Qushji, a former student and leading astronomer at the observatory managed to take a copy of the original manuscript with him. This manuscript was translated from the original Farsi to Arabic and later to Latin, French and English – today, all known versions of the Zij-i-Sultani are translations.
The observatory buildings were excavated in 1908 by Russian archaeologist V. L. Vyatkin. Foundations of the three story cylindrical structure and underground sections of the giant marble sextant can be seen on site.
The work of Ulugh Beg and his colleague astronomers may not be ‘revolutionary’ in the sense of creating something absolutely new, but the accuracy of collecting, reobserving, recalculating, updating and correcting the data of existing catalogues, and listing altogether 1018 stars make Zij-i-Sultani one of the most important star inventories in the history and a highly influential work of Persian astronomy and astrology, that continued to be used in the Islamic world until the 19th century.