Astronomy in the medieval Islamic world

• 1. Introduction

  ›

  Astronomy in the medieval Islamic world gave us the night sky we still navigate by today. Stars named Aldebaran, Altair, and Deneb carry Arabic names still in use centuries after they were catalogued. Astronomical terms like azimuth, alidade, and nadir come from the same tradition. Yet behind these familiar words lies a vast, largely unread archive: roughly 10,000 manuscripts scattered across libraries and private collections worldwide, many never catalogued. What drove a civilization to look so obsessively at the heavens? And what did they find there that Europe's astronomers would later rely on to ignite their own revolution?

• 2. Before the Golden Age

  ›

  The pre-Islamic Arabs read the sky in a way that set them apart from every other ancient civilization. The historian Ahmad Dallal noted that unlike the Babylonians, Greeks, and Indians, who had built elaborate systems of mathematical astronomy, the pre-Islamic Arabs relied on direct observation. They tracked the rising and setting of particular stars within a tradition called Anwa'. It was only after Islamization that Arab astronomers began weaving mathematics into that indigenous sky-watching practice.

  The obligation at the heart of Islamic practice drove much of what followed. Determining the five daily prayer times and the qibla, the direction toward the Kaaba in Mecca's Sacred Mosque, made precise astronomical calculation a religious necessity, not merely an intellectual pastime.

  The first texts translated into Arabic came from India and Persia. The most significant was Zij al-Sindhind, produced by Muhammad ibn Ibrahim al-Fazari and Yaqub ibn Tariq after 770. They worked from an 8th-century Indian astronomical text, helped by Indian astronomers present at the court of Caliph al-Mansur. Among the concepts these translations introduced was the sine function, which Arab astronomers adopted from India in place of the chord-of-arc approach used in Greek trigonometry. Ptolemy's Almagest was translated into Arabic at least five times across the late 8th and 9th centuries, forming the bedrock of what would become a century of intense original work.

• 3. The House of Wisdom

  ›

  In the early 9th century in Baghdad, the Abbasid caliph al-Mamun opened an institution called the House of Wisdom to the public and funded it from the state. Al-Mamun also built the first observatory in Baghdad, and further observatories followed across Iraq and Iran.

  The first major original Muslim work of astronomy emerged from this environment. In 830, the mathematician Muhammad ibn Musa al-Khwarizmi produced the Zij al-Sindhind, containing tables for the movements of the Sun, the Moon, and the five visible planets. The work introduced Ptolemaic frameworks into Islamic science and marked the moment when translation gave way to innovation.

  Among al-Mamun's court figures was Sind ibn Ali, a Jewish convert to Islam who contributed to the Zij al-Sindhind and was credited with constructing astronomical instruments. Jewish scholars in the broader Islamic world also engaged deeply with these methods. In 931, Saadia Gaon, the head of the Sura Academy in Iraq, used a zij to calculate the positions of the Sun, Moon, and five visible planets at a specific moment, recorded in his commentary on Sefer Yetzirah. He may have studied the zij tradition partly to counter contemporaries who sought to use such calculations to determine and sanctify the new moon each month.

  The philosopher al-Farabi, who died in 950, brought his own frame to the discipline. He described astronomy in terms of mathematics, music, and optics, and drew a line between mathematical astronomy and the physical science of celestial motion, drawing on Ptolemy's Analemma to show how the Sun's position could be calculated from any fixed location.

• 4. Doubting Ptolemy

  ›

  In 850, the Abbasid astronomer al-Farghani wrote Kitab fi Jawami, a compendium of astronomical science that surveyed Ptolemaic cosmography but also corrected it. Al-Farghani revised Ptolemy's values for the obliquity of the ecliptic, the precession of the apogees of the Sun and Moon, and the circumference of the Earth. The book circulated widely through the Islamic world and was eventually translated into Latin.

  By the 10th century, independent scrutiny had produced open skepticism. The Egyptian astronomer Ibn Yunus found specific errors in Ptolemy's calculations: where Ptolemy had held that the Earth's axial precession shifted by one degree every 100 years, Ibn Yunus calculated the rate as one degree every 70 years.

  Between 1025 and 1028, the polymath Ibn al-Haytham wrote Al-Shukuk ala Batlamyus, which translates as Doubts on Ptolemy. He did not challenge the geocentric model itself, but he picked apart the internal logic of Ptolemy's theories. Other astronomers took that critique as a starting point. In 1070, Abu Ubayd al-Juzjani published Tarik al-Aflak, addressing problems specifically in Ptolemy's equant theory and proposing a solution.

  Nasir al-Din al-Tusi pressed harder still. In 1261, he published the Tadkhira, enumerating 16 fundamental problems with Ptolemaic astronomy. That list set off a chain of scholars working to solve each one: Qutb al-Din al-Shirazi, Ibn al-Shatir, and Shams al-Din al-Khafri all built new planetary models in response. Al-Tusi's own contribution was the Tusi couple, a geometric device that replaced the equant concept. Where the equant would require the Moon's distance from Earth to vary by at least a factor of two across a month, the Tusi couple allowed the Moon to orbit Earth in a way consistent with actual observation. Mu'ayyad al-Din al-Urdi developed a parallel concept called the Urdi lemma: a way to represent the epicyclical motion of planets without the Ptolemaic equant.

  At the Maragha and Samarkand observatories, the question of whether the Earth itself might rotate was examined by Najm al-Din al-Qazwini al-Katibi, who died in 1277, as well as by Tusi and Qushji. A 13th-century Arabic reference work states directly that according to engineers and geometers, the Earth is in constant circular motion, and that the apparent movement of the heavens is due to the Earth's motion rather than the stars. Despite all of this, the Maragha school never made the leap to a heliocentric model.

• 5. Observatories

  ›

  The first systematic observations in the Islamic tradition took place under al-Mamun's patronage, with meridian degree measurements performed across private observatories from Damascus to Baghdad. During the 10th century, the Buwayhid dynasty funded large-scale instruments used in observations recorded in the year 950, later preserved in the zij of astronomers including Ibn al-Alam.

  It was Malik Shah I who established the first large observatory, probably in Isfahan. Omar Khayyam worked there alongside collaborators to produce a zij and formulate the Persian Solar Calendar, known as the Jalali calendar. A modern version of that calendar, called the Solar Hijri calendar, remains in official use in Iran and Afghanistan today.

  The most consequential observatory in the tradition was founded by Hulagu Khan in the 13th century in Maragha, where Nasir al-Din al-Tusi supervised the technical construction. The facility included a library, a mosque, and resting quarters for Hulagu Khan. Top astronomers gathered there, and over roughly 50 years their collaboration produced significant modifications to the Ptolemaic system. Chinese astronomers also worked at Maragha; one was Fu Mengchi, also known as Fu Mezhai.

  In 1420, the Timurid ruler Ulugh Beg ordered the construction of an observatory in Samarkand. Ulugh Beg had studied astronomy in his youth, and the observatory produced a new set of astronomical tables. Its remains were excavated in 1908 by Russian teams.

  In Ottoman Constantinople in 1577, Taqi ad-Din Muhammad ibn Maruf founded a large observatory comparable in scale to Maragha and Samarkand. It was short-lived. Opponents who objected specifically to its use for astrology rather than to the science of astronomy itself prevailed, and the observatory was destroyed in 1580.

• 6. Instruments

  ›

  Brass astrolabes were an invention of late antiquity, but the first Islamic astronomer recorded as having built one was Muhammad al-Fazari in the late 8th century. Astrolabes became central instruments in the Islamic Golden Age, used chiefly as an aid to finding the qibla. The earliest known surviving example is dated to 927-928.

  The device functioned as a portable model of the heavens, capable of calculating the position of any body in the Solar System at any point in time, provided the observer's latitude was known. Users could swap in a secondary plate calibrated to their latitude. Al-Zarqali of Andalusia constructed one astrolabe that dispensed with latitude adjustments entirely, making it usable anywhere; in Europe, this became known as the Saphea. Eventually a Mariner's astrolabe was developed to handle the difficult conditions of the sea, where standard instruments failed under rough waters and wind.

  Celestial globes served a different function: solving problems in celestial astronomy by locating objects relative to the observer's meridian. Of the 126 such instruments still surviving worldwide, the oldest dates from the 11th century. The initial blueprint for a portable celestial globe came from the Spanish Muslim astronomer Jabir ibn Aflah, who died in 1145. Abd al-Rahman al-Sufi, born in 903, wrote a treatise called the Book of Fixed Stars describing how to design constellation images on a globe and how to use it; in Iraq in the 10th century, al-Battani extended the globe's function to recording celestial data rather than only using it as an observational instrument, plotting coordinates for 1,022 stars.

  Abu Rayhan Biruni, born in 973, designed what he called the Box of the Moon, a mechanical lunisolar calendar driven by a gear train with eight gear-wheels. The Syrian astronomer Abu Bakr ibn al-Sarah al-Hamawi, who died in 1329, invented a quadrant he named al-muqantarat al-yusra and wrote several books on its applications, including Treatise on Operations with the Hidden Quadrant and Rare Pearls on Operations with the Circle for Finding Sines. The muwaqqit of the Umayyad Mosque in Damascus, Ibn al-Shatir, built one of the most notable sundials in the tradition in the 14th century, placed on the mosque to determine prayer times.

• 7. Reaching Europe and China

  ›

  Al-Battani, who died in 929, wrote Kitab az-Zij, a book of astronomical tables that would be cited repeatedly by European astronomers for centuries. Nicolaus Copernicus mentioned al-Battani no fewer than 23 times in De revolutionibus orbium coelestium, the book that initiated the Copernican Revolution, and cited him again in the Commentariolus. Tycho Brahe, Johannes Kepler, Giovanni Battista Riccioli, and Galileo Galilei all referenced his work. His observational data is still used in geophysics.

  Around 1190, al-Bitruji published an alternative geocentric system that spread through much of Europe during the 13th century. In 1217, Michael Scot completed a Latin translation of al-Bitruji's Book of Cosmology, and it became a recognized alternative to Ptolemy's Almagest in scholastic circles. Writers including Albertus Magnus and Roger Bacon discussed it in detail.

  The Urdi lemma and the Tusi couple have drawn sustained scholarly attention regarding their relationship to Copernican astronomy. Some historians maintain that these mathematical devices, developed at the Maragha observatory, influenced Renaissance-era European astronomy. The exact replacement of the equant by two epicycles that Copernicus used in the Commentariolus was found in an earlier work by Ibn al-Shatir of Damascus, who died around 1375. Copernicus's lunar and Mercury models are also identical to Ibn al-Shatir's. One possible route for these ideas to have reached Copernicus was through Byzantine science, which translated some of al-Tusi's works from Arabic into Byzantine Greek; several Byzantine Greek manuscripts containing the Tusi couple survive in Italy today. Otto E. Neugebauer postulated direct Maragha influence in 1957, but the question remains open: Copernicus explicitly cites al-Battani, Ibn Rushd, Thabit ibn Qurra, al-Zarqali, and al-Bitruji in De revolutionibus, but shows no awareness of the later Maragha school astronomers.

  In China, Islamic influence on astronomy was first recorded during the Song dynasty, when a Hui Muslim astronomer named Ma Yize introduced the seven-day week. During the Mongol Empire and the Yuan dynasty, Islamic astronomers were brought to China for calendar-making. In 1267, the Persian astronomer Jamal ad-Din, who had worked at Maragha, presented Kublai Khan with seven Persian astronomical instruments including a terrestrial globe and an armillary sphere, along with an almanac known in China as the Wannian Li, or Ten Thousand Year Calendar. In 1271, Jamal ad-Din was appointed first director of the Islamic Astronomical Bureau in Beijing, which operated alongside the Chinese Astronomical Bureau for four centuries.

  Around 1384, Hongwu Emperor of the Ming dynasty ordered the translation of Islamic astronomical tables into Chinese, a task carried out by the Muslim astronomer Mashayihei and the Chinese scholar-official Wu Bozong. These became the Huihui Lifa, or Muslim System of Calendrical Astronomy, published in China repeatedly until the early 18th century. The Qing dynasty officially ended the tradition of Chinese-Islamic astronomy in 1659. In Korea, a translation of the Huihui Lifa was studied during the reign of Sejong the Great in the 15th century, where the Islamic calendar served as the basis for calendar reform because it was more accurate than the existing Chinese-based systems.