Reception of Hellenistic astronomy Much of 9th century astronomy in the Islamic world revolved around the dissemination of the astronomical work of
Ptolemy, primarily through his
Almagest. Translations of it were produced, and summaries and commentaries of it were also written. In 850,
al-Farghani wrote his own summary of the work, titled
Kitab fi Jawani Ilm al-Nujum ("
A compendium of the science of stars"), a summary of Ptolemic
cosmography. The primary purpose of the work was to help explicate Ptolemy's, but it also included some corrections based on the findings of earlier Arab astronomers. Al-Farghani gave revised values for the
obliquity of the
ecliptic, the
precessional movement of the
apogees of the Sun and the Moon, and the circumference of the Earth. The books were widely circulated through the Muslim world, and even translated into
Latin. Under the caliph
Al-Ma'mun, an astronomical program was instituted in Baghdad and Damascus with the stated intention of verifying Ptolemy's observations by comparing the predictions made from his models with new observations. The findings were compiled into a book called al-Zij al-Mumtahan ("The verified tables"), which is widely quoted in later astronomers but itself no longer extant.
Galaxy observation The Arab astronomer
Ibn Haytham (965–1040) "determined that because the Milky Way had no
parallax, it was very remote from the
earth and did not belong to the atmosphere." The
Persian astronomer
Abū Rayhān al-Bīrūnī (973–1048) proposed the Milky Way
galaxy to be "a collection of countless fragments of the nature of nebulous stars." The
Andalusian astronomer
Ibn Bajjah ("Avempace", d. 1138) proposed that the Milky Way was made up of many stars which almost touched one another and appeared to be a continuous image due to the effect of
refraction from sublunary material, citing his observation of the
conjunction of Jupiter and Mars on 500
AH (1106/1107 AD) as evidence.
Ibn Qayyim Al-Jawziyya (1292–1350) proposed the Milky Way galaxy to be "a myriad of tiny stars packed together in the sphere of the fixed stars". Al-Sufi also identified the
Large Magellanic Cloud, which is visible from
Yemen, though not from
Isfahan; it was not seen by Europeans until
Magellan's voyage in the 16th century. These were the first galaxies other than the Milky Way to be observed from Earth. Al-Sufi published his findings in his
Book of Fixed Stars in 964.
Early heliocentric models 's, Sharh al-Tadhkirah, a manuscript copy, beginning of 17th Century The
Hellenistic Greek astronomer Seleucus of Seleucia, who advocated a
heliocentric model in the 2nd century BC, wrote a work that was later translated into Arabic. A fragment of his work has survived only in Arabic translation, which was later referred to by the
Persian philosopher Muhammad ibn Zakariya al-Razi (865–925). In the late ninth century,
Ja'far ibn Muhammad Abu Ma'shar al-Balkhi (Albumasar) developed a planetary model which some have interpreted as a
heliocentric model. This is due to his
orbital revolutions of the planets being given as heliocentric revolutions rather than
geocentric revolutions, and the only known planetary theory in which this occurs is in the heliocentric theory. His work on planetary theory has not survived, but his astronomical data was later recorded by
al-Hashimi,
Abū Rayhān al-Bīrūnī and
al-Sijzi. In the early eleventh century,
al-Biruni had met several Indian scholars who believed in a rotating Earth. In his
Indica, he discusses the theories on the
Earth's rotation supported by
Brahmagupta and other
Indian astronomers, while in his
Canon Masudicus, al-Biruni writes that
Aryabhata's followers assigned the first movement from east to west to the Earth and a second movement from west to east to the fixed stars. Al-Biruni also wrote that
al-Sijzi also believed the Earth was moving and invented an
astrolabe called the "Zuraqi" based on this idea: In his
Indica, al-Biruni briefly refers to his work on the refutation of heliocentrism, the
Key of Astronomy, which is now lost:
Early ''Hay'a'' program showing both
mathematics and
astronomy.
Ibn al-Haytham (
Latinized as Alhazen) wrote a work in the ''hay'a
tradition of Islamic astronomy known as Al-Shukūk ‛alà Baṭlamiyūs
(Doubts on Ptolemy''). He criticized
Ptolemy's astronomical system on theoretical grounds but also sought reconciliation with it. Ibn al-Haytham developed a physical structure of the Ptolemaic system in his
Treatise on the configuration of the World, or
Maqâlah fî hay'at
al-‛âlam, which became an influential work in the ''hay'a
tradition. In his Epitome of Astronomy'', he insisted that the heavenly bodies "were accountable to the
laws of physics." In 1038, Ibn al-Haytham described the first non-Ptolemaic configuration in
The Model of the Motions. His reform was not concerned with
cosmology, as he developed a systematic study of celestial
kinematics that was completely
geometric. This in turn led to innovative developments in
infinitesimal geometry. His reformed model was the first to reject the
equant and
eccentrics, separate
natural philosophy from astronomy, free celestial kinematics from cosmology, and reduce physical entities to geometrical entities. The model also propounded the
Earth's rotation about its axis, and the centres of motion were geometrical points without any physical significance, like
Johannes Kepler's model centuries later. In 1030,
Abū al-Rayhān al-Bīrūnī discussed the
Indian planetary theories of
Aryabhata,
Brahmagupta and
Varahamihira in his ''Ta'rikh al-Hind
(Latinized as Indica''). Al-Biruni agreed with the
Earth's rotation about its own axis, and while he was initially neutral regarding the
heliocentric and
geocentric models, he eventually came to reject heliocentrism towards the end of his life. He remarked that if the Earth rotates on its axis and moves around the Sun, it would remain consistent with his astronomical parameters:
Andalusian Revolt rejected the
eccentric deferents introduced by
Ptolemy. He rejected the
Ptolemaic model and instead argued for a strictly
concentric model of the universe. In 1984, Abdelhamid Sabra coined the term "Andalusian Revolt" to describe an event beginning among twelfth century astronomers in
al-Andalus where mounting discomfort over the conflicts between theory and observation resulted in astronomers transitioning away from the unquestioned authority of Ptolemy to rejecting his theory in favor of radically different solutions.
Nur ad-Din al-Bitruji (d. 1204) rejected the existence of eccentrics and epicycles. Instead, the motions of the planets would be explained by concentric spheres, as explained in his (only extant) work
al-Murtaʿish fī ʾl-hayʾa ("The Revolutionary Book on Astronomy"). The major figures of this "revolt" were, among the astronomers,
Ibn al-Zarqālluh,
Jābir b. al-Aflaḥ, and
al-Biṭrūjī, and among the philosophers,
Ibn Bājja (Avempace),
Ibn Ṭufayl (the teacher of al-Biṭrūjī),
Averroes, and
Maimonides. In the 12th century,
Averroes rejected the
eccentric deferents introduced by
Ptolemy. He rejected the
Ptolemaic model and instead argued for a strictly
concentric model of the universe. He wrote the following criticism on the Ptolemaic model of planetary motion: Averroes' contemporary,
Maimonides, wrote the following on the planetary model proposed by
Ibn Bajjah (Avempace): Ibn Bajjah also proposed the
Milky Way galaxy to be made up of many stars but that it appears to be a continuous image due to the effect of
refraction in the
Earth's atmosphere. Later in the 12th century, his successors
Ibn Tufail and
Nur Ed-Din Al Betrugi (Alpetragius) were the first to propose planetary models without any
equant,
epicycles or eccentrics. Their configurations, however, were not accepted due to the numerical predictions of the planetary positions in their models being less accurate than that of the Ptolemaic model, Ibn al-Haytham, in his
Book of Optics (1021) also suggested that heaven, the location of the fixed stars, was less dense than air. In the 12th century,
Fakhr al-Din al-Razi participated in the debate among Islamic scholars over whether the
celestial spheres or orbits (
falak) are "to be considered as real, concrete physical bodies" or "merely the abstract circles in the heavens traced out year in and year out by the various stars and planets." He points out that many astronomers prefer to see them as solid spheres "on which the stars turn," while others, such as the Islamic scholar Dahhak, view the celestial sphere as "not a body but merely the abstract orbit traced by the stars." Al-Razi himself remains "undecided as to which
celestial models, concrete or abstract, most conform with external reality," and notes that "there is no way to ascertain the characteristics of the heavens," whether by "observable" evidence or by authority (
al-khabar) of "
divine revelation or
prophetic traditions." He concludes that "astronomical models, whatever their utility or lack thereof for ordering the heavens, are not founded on sound rational proofs, and so no intellectual commitment can be made to them insofar as description and explanation of celestial realities are concerned." The theologian Adud al-Din al-Iji (1281–1355), under the influence of the
Ash'ari doctrine of
occasionalism, which maintained that all physical effects were caused directly by God's will rather than by natural causes, rejected the
Aristotelian principle of an innate principle of circular motion in the heavenly bodies, and maintained that the celestial spheres were "imaginary things" and "more tenuous than a spider's web". His views were challenged by
al-Jurjani (1339–1413), who argued that even if the celestial spheres "do not have an external reality, yet they are things that are correctly imagined and correspond to what [exists] in actuality".
Astronomical physics and Earth's motion provided
empirical evidence for the
Earth's motion and developed an
astronomical physics independent from
Aristotelian physics and
natural philosophy. The work of
Ali Qushji (d. 1474), who worked at
Samarkand and then
Istanbul, is seen as a late example of innovation in Islamic theoretical astronomy and it is believed he may have possibly had some influence on
Nicolaus Copernicus due to similar arguments concerning the
Earth's rotation. Before Qushji, the only astronomer to present
empirical evidence for the Earth's rotation was
Nasīr al-Dīn al-Tūsī (d. 1274), who used the phenomena of
comets to refute
Ptolemy's claim that a stationary Earth can be determined through observation. Al-Tusi, however, eventually accepted that the Earth was stationary on the basis of
Aristotelian cosmology and
natural philosophy. By the 15th century, the influence of
Aristotelian physics and natural philosophy was declining due to religious opposition from
Islamic theologians such as
Al-Ghazali who opposed to the interference of
Aristotelianism in astronomy, opening up possibilities for an astronomy unrestrained by philosophy. Under this influence, Qushji, in his
Concerning the Supposed Dependence of Astronomy upon Philosophy, rejected Aristotelian physics and completely separated natural philosophy from astronomy, allowing astronomy to become a purely
empirical and mathematical science. This allowed him to explore alternatives to the Aristotelian notion of a stationary Earth, as he explored the idea of a moving Earth. He also observed comets and elaborated on al-Tusi's argument. He took it a step further and concluded, on the basis of empirical evidence rather than speculative philosophy, that the moving Earth theory is just as likely to be true as the stationary Earth theory and that it is not possible to empirically deduce which theory is true. His work was an important step away from Aristotelian physics and towards an independent
astronomical physics. Despite the similarity in their discussions regarding the Earth's motion, there is uncertainty over whether Qushji had any influence on Copernicus. However, it is likely that they both may have arrived at similar conclusions due to using the earlier work of al-Tusi as a basis. This is more of a possibility considering "the remarkable coincidence between a passage in
De revolutionibus (I.8) and one in Ṭūsī's
Tadhkira (II.1[6]) in which Copernicus follows Ṭūsī's objection to Ptolemy's "proofs" of the Earth's immobility." This can be considered as evidence that not only was Copernicus influenced by the mathematical models of Islamic astronomers, but may have also been influenced by the astronomical physics they began developing and their views on the Earth's motion. In the 16th century, the debate on the Earth's motion was continued by
al-Birjandi (d. 1528), who in his analysis of what might occur if the Earth were moving, develops a hypothesis similar to
Galileo Galilei's notion of "circular
inertia", which he described in the following observational test (as a response to one of
Qutb al-Din al-Shirazi's arguments): ==See also==