Islamic astronomy was born from an extraordinary translation movement initiated in the 8th century under the Abbasid Caliphate. Caliph Al-Mansur (754-775) invited scholars from diverse origins to Baghdad to translate Greek, Persian, Indian, and Babylonian scientific texts into Arabic. This endeavor reached its peak under Al-Ma'mun (813-833), who founded the Bayt al-Hikma (House of Wisdom), a true academy of sciences where translators, mathematicians, and astronomers worked.
Major works were translated: Ptolemy's Almagest, Euclid's Elements, Aristotle's treatises, and Brahmagupta's Brahmasphutasiddhanta (translated as Zij al-Sindhind). These translations were not mere transcriptions; they included critical commentaries, corrections, and improvements. Muslim astronomers thus inherited the Ptolemaic geocentric system, Greek mathematics, and the Indian numerical system with zero.
Under Al-Ma'mun, the first institutional astronomical observatory was established in Baghdad around 828 CE, followed by a second in Damascus. These institutions were not just observation sites but true research centers where instruments were made, precise measurements were taken, and astronomical tables called zij were compiled.
Baghdad astronomers undertook systematic observation programs to verify and correct Ptolemaic data. They measured the obliquity of the ecliptic (Earth's axial tilt), the precession of the equinoxes, and conducted geodetic measurements to determine the Earth's circumference. A famous expedition in the Sinjar plain measured a meridian degree and found approximately 56⅔ Arabic miles, or about 111.8 km (modern value: 111 km), demonstrating remarkable precision.
N.B.:
The zij (astronomical tables) are one of the major contributions of Islamic astronomy. These works compile the positions of celestial bodies, ephemerides, eclipse calculation methods, and cosmographic parameters. Over 200 zij have been recorded, including the famous Zij al-Sindhind by Al-Khwarizmi (c. 820), based on Indian and Greek sources, and Al-Battani's Zij al-Sabi', which significantly corrected Ptolemaic values.
Muslim astronomers improved the observation instruments inherited from the Greeks and invented new ones. The planispheric astrolabe, known since antiquity, became a high-precision instrument thanks to improvements by scholars such as Al-Fazari (8th century) and Al-Khwarizmi (c. 780-850).
New instruments emerged: the mural quadrant (for measuring the altitude of celestial bodies with great precision), the sextant, the spherical armillary (a three-dimensional representation of celestial circles), and sophisticated sundials for determining prayer times. Al-Zarqali (1029-1087) invented the universal astrolabe (al-safiha al-zarqaliyya), usable at all latitudes without changing the plate.
In the 13th century, Nasir al-Din al-Tusi built a monumental observatory in Maragha (Persia) equipped with giant instruments: a mural quadrant with a 4-meter radius and a colossal armillary, enabling measurements with a precision of about 1 arc minute.
While adopting Ptolemy's geocentric framework, Muslim astronomers identified its mathematical and physical inconsistencies. The main issue concerned the equant, a fictitious point around which a planet's angular velocity appears uniform, violating Aristotle's principle of uniform circular motion.
Ibn al-Haytham (Alhazen, 965-1040), in his Doubts on Ptolemy, sharply criticized the inconsistencies of the Ptolemaic system and proposed a more rigorous approach based on physical geometry. He emphasized that mathematical models must correspond to a coherent physical reality.
Nasir al-Din al-Tusi (1201-1274) invented the Tusi couple, an ingenious geometric mechanism that generates linear motion from two uniform circular motions. This innovation, which eliminates the need for equants, was rediscovered by Copernicus two centuries later and represents a crucial step toward heliocentrism.
Ibn al-Shatir (1304-1375), a Damascene astronomer, developed a geocentric model without equants, using only epicycles and uniform circular motions. His system, remarkable for its mathematical elegance, is mathematically equivalent to Copernicus' heliocentric model (1543), except that the Earth remains at the center.
N.B.:
The Tusi couple transforms two circular motions into an oscillatory linear motion according to the formula: \( x(t) = r[\cos(\omega t) - \cos(2\omega t)] \) where a small circle of radius \(r\) rolls inside a large circle of radius \(2r\). This geometric device anticipates some concepts of vector calculus and demonstrates the mathematical sophistication of Islamic astronomy.
| Name | Period | Major Contributions | Main Works |
|---|---|---|---|
| Al-Fazari | died c. 796 CE | First Muslim astronomer; translation of Zij al-Sindhind; development of the Arabic astrolabe. | Zij al-Sindhind (adapted) |
| Al-Khwarizmi | c. 780 – 850 CE | Astronomical tables based on Indian and Greek sources; algebra; decimal system; trigonometric calculations. | Zij al-Sindhind, Al-Jabr |
| Al-Farghani (Alfraganus) | c. 800 – 870 CE | Estimation of Earth's circumference; widely disseminated astronomy treatises in Europe; improvement of planetary parameters. | Kitab fi Jawami Ilm al-Nujum |
| Al-Battani (Albatenius) | 858 – 929 CE | Precise observations over 40 years; improved precession (54.5" per year); tropical year accurate to 2 seconds; trigonometric tables. | Kitab al-Zij al-Sabi |
| Al-Sufi (Azophi) | 903 – 986 CE | Catalog of 1018 stars with magnitudes; first mention of the Andromeda galaxy; precise descriptions of constellations. | Kitab Suwar al-Kawakib al-Thabita (964) |
| Ibn al-Haytham (Alhazen) | 965 – 1040 CE | Critique of the Ptolemaic system; physical model of celestial spheres; founder of modern optics; experimental method. | Al-Shukuk ala Batlamyus, Kitab al-Manazir |
| Al-Biruni | 973 – 1048 CE | Measurement of Earth's radius by triangulation; discussion of Earth's rotation; astronomical encyclopedia; comparative calendar studies. | Al-Qanun al-Mas'udi, Kitab al-Tafhim |
| Omar Khayyam | 1048 – 1131 CE | Reform of the Persian calendar (Jalali calendar); tropical year calculated at 365.2424 days (precision: 1 day/5000 years). | Zij Malikshahi |
| Al-Zarqali (Arzachel) | 1029 – 1087 CE | Universal astrolabe; solar apogee motion; influential Toledan tables in Europe. | Almanach, Al-Safiha al-Zarqaliyya |
| Nasir al-Din al-Tusi | 1201 – 1274 CE | Tusi couple (linear motion by circles); Maragha Observatory; complete revision of the Almagest. | Al-Tadhkira fi Ilm al-Hay'a, Zij-i Ilkhani |
| Ibn al-Shatir | 1304 – 1375 CE | Geocentric model without equants; mathematically equivalent to Copernicus' model; muwaqqit (astronomical clockmaker) of the Umayyad Mosque. | Kitab Nihayat al-Sul |
| Ulugh Beg | 1394 – 1449 CE | Samarkand Observatory; catalog of 1018 stars (precision 1'); sultanic tables; scientific patronage. | Zij-i Sultani (1437) |
| Al-Kashi | c. 1380 – 1429 CE | Calculation of π to 16 decimal places; improved astronomical instruments; advanced trigonometry. | Zij-i Khaqani, Miftah al-Hisab |
| Taqi al-Din | 1526 – 1585 CE | Istanbul Observatory; precise mechanical clocks; comet observations; revised star catalogs. | Sidrat Muntaha al-Afkar |
N.B.:
Al-Battani determined the tropical year to be 365 days, 5 hours, 46 minutes, and 24 seconds, an error of only 2 seconds compared to the modern value (365.2422 days). This extraordinary precision, achieved through 40 years of continuous observations, was not surpassed until the 16th century by Tycho Brahe.
Islamic astronomy relied on major mathematical developments. Spherical trigonometry, essential for astronomical calculations, was systematized and perfected. Al-Khwarizmi compiled the first sine tables, while Abu al-Wafa (940-998) introduced the tangent function and established fundamental trigonometric formulas such as: \( \sin(a \pm b) = \sin a \cos b \pm \cos a \sin b \)
In the 15th century, Al-Kashi calculated π with 16 decimal places using regular polygons with \(3 \times 2^{28}\) sides, a computational feat. He also developed numerical approximation methods that anticipated modern analysis techniques.
Algebra, founded by Al-Khwarizmi in his treatise Al-Jabr wa'l-Muqabala (from which the word "algebra" derives), became an indispensable tool for solving complex astronomical problems, such as calculating planetary positions and eclipses.
Islam imposes ritual obligations that require precise astronomical knowledge: determining the direction of Mecca (qibla) for prayer, calculating the times of the five daily prayers, fixing the beginning of lunar months (especially Ramadan), and establishing calendars.
This practical dimension stimulated the development of an extremely sophisticated science of time (ilm al-miqat). Specialized mathematicians, the muwaqqit, were attached to major mosques and produced tables for calculating prayer times based on latitude and season.
Determining the qibla became a complex problem in spherical trigonometry. For two locations with coordinates \((\lambda_1, \phi_1)\) and \((\lambda_2, \phi_2)\), the qibla azimuth \(q\) is given by: \( \tan q = \frac{\sin(\lambda_2 - \lambda_1)}{\cos \phi_1 \tan \phi_2 - \sin \phi_1 \cos(\lambda_2 - \lambda_1)} \) Specialized instruments such as the qibla compass and oriented sundials were developed to facilitate this task.
In 1420, the Timurid prince Ulugh Beg, himself an accomplished astronomer, founded a monumental observatory in Samarkand that surpassed all its predecessors. Equipped with a giant sextant with a 40-meter radius embedded in an underground trench, this observatory enabled measurements of unparalleled precision: about 1 arc minute.
With his collaborator Al-Kashi, Ulugh Beg compiled a new star catalog of 1018 stars (the Zij-i Sultani, 1437), the most precise since Hipparchus and Ptolemy. Their measurements were not surpassed until the 16th century by Tycho Brahe in Europe, who benefited from sighting telescopes.
The observatory became a training center where a radiant astronomical school developed. Unfortunately, after Ulugh Beg's assassination in 1449, the observatory was abandoned and gradually fell into ruin, symbolizing the decline of the Islamic scientific golden age.
From the 11th century, Islamic astronomical knowledge entered Europe through three main routes: Andalusian Spain, Sicily, and the Crusades.
Toledo, reconquered in 1085, became a major center for translating Arabic into Latin. Scholars such as Gerard of Cremona (1114-1187) translated over 80 works, including Ptolemy's Almagest (via Arabic), the works of Al-Khwarizmi, Al-Farghani, and Al-Zarqali's Toledan tables.
These translations introduced to Europe:
Arabic terms became embedded in astronomical vocabulary: zenith (samt al-ra's), nadir (nazir), azimuth (al-sumut), as well as many star names: Aldebaran, Altair, Betelgeuse, Rigel, Vega.
Islamic astronomy represents much more than a simple transmission of ancient knowledge: it constitutes a true scientific revolution characterized by systematic observation, rational critique, instrumental innovation, and mathematical refinement.
Major contributions include:
Without this Islamic mediation, the European Scientific Renaissance would have been significantly delayed. The works of Copernicus, Kepler, and Galileo directly built on the observations, critiques, and innovations developed during the Islamic golden age. Modern astronomy is thus the fruit of a transcultural intellectual collaboration spanning over two millennia.
References:
– Saliba, G., Islamic Science and the Making of the European Renaissance, MIT Press (2007).
– King, D.A., In Synchrony with the Heavens: Studies in Astronomical Timekeeping in Medieval Islamic Civilization, Brill (2004-2005).
– Ragep, F.J., Nasir al-Din al-Tusi's Memoir on Astronomy, Springer (1993).
– Kennedy, E.S., Astronomy and Astrology in the Medieval Islamic World, Ashgate (1998).
– Berggren, J.L., Episodes in the Mathematics of Medieval Islam, Springer (2003).
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