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In the history of astronomy, Islamic astronomy or Arabic astronomy refers to the astronomical developments made in the Islamic world, particularly during the Islamic Golden Age (8th-16th centuries), and mostly written in the Arabic language. These developments mostly took place in the Middle East, Central Asia, Al-Andalus, North Africa, and later in China and India. It closely parallels the genesis of other Islamic sciences in its assimilation of foreign material and the amalgamation of the disparate elements of that material to create a science. These included Indian, Sassanid and Hellenistic works in particular, which were translated and built upon.^[1] In turn, Islamic astronomy later had a significant influence on Indian^[2] and European^[3] astronomy (see Latin translations of the 12th century) as well as Chinese astronomy.^[4]

A significant number of stars in the sky, such as Aldebaran and Altair, and astronomical terms such as alhidade, azimuth, and almucantar, are still today recognized with their Arabic names.^[5]

A large corpus of literature from Islamic astronomy remains today, numbering approximately 10,000 manuscripts scattered throughout the world, many of which have not been read or catalogued. Even so, a reasonably accurate picture of Islamic activity in the field of astronomy can be reconstructed.^[6]

Islam and astronomy

Islam has affected astronomy directly and indirectly. A major impetus for the flowering of astronomy in Islam came from religious observances, which presented an assortment of problems in mathematical astronomy, specifically in spherical geometry.^[1]

Background

In the 7th century, both Christians and Jews observed holy days, such as Easter and Passover, whose timing was determined by the phases of the moon. Both communities had confronted the fact that the approximately 29.5-day lunar months are not commensurable with the 365-day solar year. To solve the problem, Christians and Jews had adopted a scheme based on a discovery made in circa 430 BC by the Athenian astronomer Meton. In the 19-year Metonic cycle, there were 12 years of 12 lunar months and seven years of 13 lunar months. The periodic insertion of a 13th month kept calendar dates in step with the seasons.^[1]

On the other hand, astronomers used Ptolemy's way to calculate the place of the moon and stars. The method Ptolemy used to solve spherical triangles was a clumsy one devised late in the first century by Menelaus of Alexandria. It involved setting up two intersecting right triangles; by applying Menelaus' theorem it was possible to solve one of the six sides, but only if the other five sides were known. To tell the time from the sun's altitude, for instance, repeated applications of Menelaus' theorem were required. For medieval Islamic astronomers, there was an obvious challenge to find a simpler trigonometric method.^[1]

Islamic attitude towards astronomy

Islam advised Muslims to find ways of using the stars. The Qur'an says: "And it is He who ordained the stars for you that you may be guided thereby in the darkness of the land and the sea."^[7] On the basis of this advice Muslim began to find better observational and navigational instruments, thus most navigational stars today have Arabic names.^[1]

Other influences of the Qur'an on Islamic astronomy included its "insistence that the Universe is ruled by a single set of laws" which was "rooted in the Islamic concept of tawhîd, the unity of God", as well its "greater respect for empirical data than was common in the preceding Greek civilization" which inspired Muslims to place a greater emphasis on empirical observation,^[8] in contrast to ancient Greek philosophers such as the Platonists and Aristotelians who expressed a general distrust towards the senses and instead viewed reason alone as being sufficient to understanding nature. The Qur'an's insistence on observation, reason and contemplation ("see", "think" and "contemplate"), on the other hand, led Muslims to develop an early scientific method based on these principles, particularly empirical observation. Muhammad Iqbal writes:^[9]

“The general empirical attitude of the Qur'an which engendered in its followers a feeling of reverence for the actual, and ultimately made them the founders of modern science. It was a great point to awaken the empirical spirit in an age that renounced the visible as of no value in men's search after God.”

There are also several cosmological verses in the Qur'an (610-632) which some modern writers have interpreted as foreshadowing the expansion of the universe and possibly even the Big Bang theory:^[10]

Don't those who reject faith see that the heavens and the earth were a single entity then We ripped them apart?^[11]

And the heavens We did create with Our Hands, and We do cause it to expand.Qur'an 51:47

Several hadiths attributed to Muhammad also show that he was generally opposed to astrology as well as superstition in general. An example of this is when an eclipse occurred during his son Ibrahim ibn Muhammad's death, and rumours began spreading about this being God's personal condolence. Muhammad is said to have replied:^[12]

"An eclipse is a phenomenon of nature. It is foolish to attribute such things to the death or birth of a human being."

Islamic rules

There are several rules in Islam which lead Muslims to use better astronomical calculations and observations.

The first issue is the Islamic calendar. The Qur'an says: "The number of months in the sight of Allah is twelve (in a year) so ordained by Him the day He created the heavens and the earth; of them four are sacred; that is the straight usage."^[13][1] Therefore Muslims could not follow the Christian or Hebrew calendars and they thus had to develop a new one.

The other issue is moon sighting. Islamic months do not begin at the astronomical new moon, defined as the time when the moon has the same celestial longitude as the sun and is therefore invisible; instead they begin when the thin crescent moon is first sighted in the western evening sky.^[1] The Qur'an says: "They ask you about the waxing and waning phases of the crescent moons, say they are to mark fixed times for mankind and Hajj."^[14][15] This led Muslims to find the phases of the moon in the sky, and their efforts led to new mathematical calculations and observational instruments, as well as a special science being formed specifically for moon sighting.^[16]

Muslims are also expected to pray towards the Kaaba in Mecca and orient their mosques in that direction. Thus they need to determine the direction of Mecca from a given location.^[17][18] Another problem is the time of Salah. Muslims need to determine from celestial bodies the proper times for the prayers at sunrise, at midday, in the afternoon, at sunset, and in the evening.^[1][19]

Necessity of spherical geometry

See also: Islamic mathematics

Predicting just when the crescent moon would become visible is a special challenge to Islamic mathematical astronomers. Although Ptolemy's theory of the complex lunar motion was tolerably accurate near the time of the new moon, it specified the moon's path only with respect to the ecliptic. To predict the first visibility of the moon, it was necessary to describe its motion with respect to the horizon, and this problem demands fairly sophisticated spherical geometry. Finding the direction of Mecca and the time of Salah are the reasons which led to Muslims developing spherical geometry. Solving any of these problems involves finding the unknown sides or angles of a triangle on the celestial sphere from the known sides and angles. A way of finding the time of day, for example, is to construct a triangle whose vertices are the zenith, the north celestial pole, and the sun's position. The observer must know the altitude of the sun and that of the pole; the former can be observed, and the latter is equal to the observer's latitude. The time is then given by the angle at the intersection of the meridian (the arc through the zenith and the pole) and the sun's hour circle (the arc through the sun and the pole).^[1][19]

History

Pre-Islamic Arabian knowledge of stars was empirical; their knowledge was what they observed regarding the rising and setting of stars. The rise of Islam is claimed to have provoked increased Arab thought in this field.^[20] The foundations of Islamic astronomy closely parallels the genesis of other Islamic sciences in its assimilation of foreign material and the amalgamation of the disparate elements of that material to create a science that was essentially Islamic. These include Indian, Sassanid and Hellenistic works which were translated and built upon.

The science historian Donald Routledge Hill has divided the history of Islamic astronomy into the four following distinct time periods in its history:^[20]

• Assimilation and syncretization of earlier Hellenistic, Indian and Sassanid astronomy (700—825 AD)
• Vigorous investigation, and acceptance and modification to the Ptolemaic system (825—1025 AD)
• Flourishing of a distinctive Islamic system of astronomy (1025—1450 AD)
• Stagnation, where few significant contributions were made (1450—1900 AD)

610-700

From the beginning, Muslim community in Medina sight new moon to determine the lunar months especially Ramadan and holy days.

In approximately 638 A.D, Caliph Umar introduced a new lunar calendar which is known as lunar calendar was made on the basis of Islamic view point. This calendar has twelve lunar months, the beginnings of which are determined by the sighting of the crescent moon. This calendar is about 11 days shorter than the solar year. This calendar is still in use for religious purposes among Muslims.^[1][21]

700-825

This period was most notably the period of assimilation and syncretization of earlier Hellenistic, Indian and Sassanid astronomy occurred during the eighth and early ninth centuries.

Impetus

Historians point out several factors that fostered the growth of Islamic astronomy. The first was the proximity of the Muslim world to the world of ancient learning. Much of the ancient Greek, Sanskrit and Middle Persian texts were translated into Arabic during the ninth century. This process was enhanced by the tolerance towards scholars of other religions.^[1]

Another impetus came from Islamic religious observances, which presented a host of problems in mathematical astronomy. In solving these religious problems the Islamic scholars went far beyond the Greek mathematical methods.^[1]

Ancient influences and translation movement

During this period, a number of Sanskrit and Middle Persian texts were first translated into Arabic. The most notable of the texts was Zij al-Sindhind,^[22] based on the Surya Siddhanta and the works of Brahmagupta, and translated by Muhammad al-Fazari and Yaqūb ibn Tāriq in 777. Sources indicate that the text was translated after an Indian astronomer visited the court of Caliph Al-Mansur in 770. The most notable Middle Persian text translated was the Zij al-Shah, a collection of astronomical tables compiled in Sassanid Persia over two centuries.

Fragments of text during this period indicate that Arabs adopted the sine function (inherited from Indian trigonometry) instead of the chords of arc used in Hellenistic mathematics.^[20] Another Indian influence was an approximate formula used for timekeeping by Muslim astronomers.^[23]

A page from Ptolemy's Almagest.

Islamic interest in astronomy ran parallel to the interest in mathematics. Especially noteworthy in this regard was the Almagest (c. 150) of the astronomer Ptolemy (c. 100-178). The Almagest was a landmark work in its field, assembling, as Euclid's Elements had previously done with geometrical works, all extant knowledge in the field of astronomy that was known to the author. This work was originally known as The Mathematical Composition, but after it had come to be used as a text in astronomy, it was called The Great Astronomer. The Islamic world called it The Greatest prefixing the Greek work megiste (greatest) with the article al- and it has since been known to the world as Al-megiste or, after popular use in Western translation, Almagest.^[24] though much of the Almagest was incorrect, even in premise, it remained a standard astronomical text in both the Islamic world and Europe until the Maragha Revolution and Copernican Revolution.^[25] Ptolemy also produced other works, such as Optics, Harmonica, and some suggest he also wrote Tetrabiblon.

The Almagest was a particularly unifying work for its exhaustive lists of sidereal phenomena. He drew up a list of chronological tables of Assyrian, Persian, Greek, and Roman kings for use in reckoning the lapse of time between known astronomical events and fixed dates. In addition to its relevance to calculating accurate calendars, it linked far and foreign cultures together by a common interest in the stars and astrology. The work of Ptolemy was replicated and refined over the years under Arab, Persian and other Muslim astronomers and astrologers.

825-1025

The period throughout the ninth, tenth and early eleventh centuries was one of vigorous investigation, in which the superiority of the Ptolemaic system of astronomy was accepted and significant contributions made to it. Astronomical research was greatly supported by the Abbasid caliph al-Mamun. Baghdad and Damascus became the centers of such activity. The caliphs not only supported this work financially, but endowed the work with formal prestige.^[26]

Observational astronomy

Al-Khwarizmi, the father of algebra and algorithms, wrote the Zij al-Sindh, the first original Zij in Islamic astronomy.

In observational astronomy, the first major original Muslim work of astronomy was Zij al-Sindh by al-Khwarizimi in 830. The work contains tables for the movements of the sun, the moon and the five planets known at the time. The work is significant as it introduced Indian and Ptolemaic concepts into Islamic sciences. This work also marked the turning point in Islamic astronomy. Hitherto, Muslim astronomers had adopted a primarily research approach to the field, translating works of others and learning already discovered knowledge. Al-Khwarizmi's work marked the beginning of non-traditional methods of study and calculations.^[27]

In 850, al-Farghani wrote Kitab fi Jawani ("A compendium of the science of stars"). The book primarily gave a summary of Ptolemic cosmography. However, it also corrected Ptolemy's Almagest based on findings of earlier Iranian 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.^[28]

Muhammad ibn Jābir al-Harrānī al-Battānī (Albatenius) (853-929) discovered that the direction of the Sun's eccentric was changing, which in modern astronomy is equivalent to the Earth moving in an elliptical orbit around the Sun.^[29] His times for the new moon, lengths for the solar year and sidereal year, prediction of eclipses, and work on the phenomenon of parallax, carried astronomers "to the verge of relativity and the space age."^[30] Around the same time, Yahya Ibn Abi Mansour carried out extensive observations and tests, and wrote the Al-Zij al-Mumtahan, in which he completely revised the Almagest values.^[31]

Azophi's Book of Fixed Stars, which described more than a thousand stars in detail and gave the first descriptions on the Andromeda Galaxy and Large Magellanic Cloud. The constellation pictured here is Sagittarius.

In the 10th century, Abd al-Rahman al-Sufi (Azophi) carried out observations on the stars and described their positions, magnitudes, brightness, and colour and drawings for each constellation in his Book of Fixed Stars (964). He also gave the first descriptions and pictures of "A Little Cloud" now known as the Andromeda Galaxy. He mentions it as lying before the mouth of a Big Fish, an Arabic constellation. This "cloud" was apparently commonly known to the Isfahan astronomers, very probably before 905 AD.^[32] The first recorded mention of the Large Magellanic Cloud was also given by Abd Al-Rahman al-Sufi.^[33][34]

Ibn Yunus observed more than 10,000 entries for the sun's position for many years using a large astrolabe with a diameter of nearly 1.4 meters. His observations on eclipses were still used centuries later in Simon Newcomb's investigations on the motion of the moon, while his other observations inspired Laplace's Obliquity of the Ecliptic and Inequalities of Jupiter and Saturn's.^[35]

Abu-Mahmud al-Khujandi relatively accurately computed the axial tilt to be 23°32'19" (23.53°),^[36]. In 1006, the Egyptian astronomer Ali ibn Ridwan observed SN 1006, the brightest supernova in recorded history, and left a detailed description of the temporary star. He says that the object was two to three times as large as the disc of Venus and about one-quarter the brightness of the Moon, and that the star was low on the southern horizon. Monks at the Benedictine abbey at St. Gall later corroborated bin Ridwan's observations as to magnitude and location in the sky.

Early heliocentric models

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.^[37]

In the early eleventh century, al-Biruni had met several Indian scholars who believed in a heliocentric system. 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:^[38]

"I have seen the astrolabe called Zuraqi invented by Abu Sa'id Sijzi. I liked it very much and praised him a great deal, as it is based on the idea entertained by some to the effect that the motion we see is due to the Earth's movement and not to that of the sky. By my life, it is a problem difficult of solution and refutation. [...] For it is the same whether you take it that the Earth is in motion or the sky. For, in both cases, it does not affect the Astronomical Science. It is just for the physicist to see if it is possible to refute it."

In his Indica, al-Biruni briefly refers to his work on the refutation of heliocentrism, the Key of Astronomy, which is now lost:^[38]

"The most prominent of both modern and ancient astronomers have deeply studied the question of the moving earth, and tried to refute it. We, too, have composed a book on the subject called Miftah 'ilm al-hai'ah (Key of Astronomy), in which we think we have surpassed our predecessors, if not in the words, at all events in the matter."

Cosmology

See also: Early Islamic philosophy

In contrast to ancient Greek philosophers who believed that the universe had an infinite past with no beginning, medieval philosophers and theologians developed the concept of the universe having a finite past with a beginning (see Temporal finitism). This view was inspired by the creation myth shared by the three Abrahamic religions: Judaism, Christianity and Islam. The Christian philosopher, John Philoponus, presented the first such argument against the ancient Greek notion of an infinite past. However, the most sophisticated medieval arguments against an infinite past were developed by the early Muslim philosopher, Al-Kindi (Alkindus); the Jewish philosopher, Saadia Gaon (Saadia ben Joseph); and the Muslim theologian, Al-Ghazali (Algazel). They developed two logical arguments against an infinite past, the first being the "argument from the impossibility of the existence of an actual infinite", which states:^[39]

"An actual infinite cannot exist."

"An infinite temporal regress of events is an actual infinite."

".•. An infinite temporal regress of events cannot exist."

The second argument, the "argument from the impossibility of completing an actual infinite by successive addition", states:^[39]

"An actual infinite cannot be completed by successive addition."

"The temporal series of past events has been completed by successive addition."

".•. The temporal series of past events cannot be an actual infinite."

Both arguments were adopted by later Christian philosophers and theologians, and the second argument in particular became more famous after it was adopted by Immanuel Kant in his thesis of the first antimony concerning time.^[39]

Experimental astronomy, astrophysics and celestial mechanics

In the 9th century, the eldest Banū Mūsā brother, Ja'far Muhammad ibn Mūsā ibn Shākir, made significant contributions to astrophysics and celestial mechanics. He was the first to hypothesize that the heavenly bodies and celestial spheres are subject to the same laws of physics as Earth, unlike the ancients who believed that the celestial spheres followed their own set of physical laws different from that of Earth.^[40] In his Astral Motion and The Force of Attraction, Muhammad ibn Musa also proposed that there is a force of attraction between heavenly bodies,^[41] foreshadowing Newton's law of universal gravitation.^[42]

In the 10th century, Muhammad ibn Jābir al-Harrānī al-Battānī (Albatenius) (853-929) introduced the idea of testing "past observations by means of new ones".^[43] This led to the use of exacting empirical observations and experimental techniques by Muslim astronomers from the eleventh century onwards.^[44]

In the early 11th century, Ibn al-Haytham (Alhazen) wrote the Maqala fi daw al-qamar (On the Light of the Moon) some time before 1021. This was the first attempt successful at combining mathematical astronomy with physics and the earliest attempt at applying the experimental method to astronomy and astrophysics. He disproved the universally held opinion that the moon reflects sunlight like a mirror and correctly concluded that it "emits light from those portions of its surface which the sun's light strikes." In order to prove that "light is emitted from every point of the moon's illuminated surface," he built an "ingenious experimental device." Ibn al-Haytham had "formulated a clear conception of the relationship between an ideal mathematical model and the complex of observable phenomena; in particular, he was the first to make a systematic use of the method of varying the experimental conditions in a constant and uniform manner, in an experiment showing that the intensity of the light-spot formed by the projection of the moonlight through two small apertures onto a screen diminishes constantly as one of the apertures is gradually blocked up."^[45]

Ibn al-Haytham, in his Book of Optics (1021), was also the first to discover that the celestial spheres do not consist of solid matter, and he also discovered that the heavens are less dense than the air. These views were later repeated by Witelo and had a significant influence on the Copernican and Tychonic systems of astronomy.^[46]

Ibn al-Haytham also refuted Aristotle's view on the Milky Way galaxy. Aristotle believed the Milky Way to be caused by "the ignition of the fiery exhalation of some stars which were large, numerous and close together" and that the "ignition takes place in the upper part of the atmosphere, in the region of the world which is continuous with the heavenly motions."^[47] Ibn al-Haytham refuted this by making the first attempt at observing and measuring the Milky Way's parallax,^[48] and he thus "determined that because the Milky Way had no parallax, it was very remote from the earth and did not belong to the atmosphere."^[49]

Also in the early 11th century, Abū Rayhān al-Bīrūnī introduced the experimental method into astronomy and was the first to conduct elaborate experiments related to astronomical phenomena.^[50] He discovered the Milky Way galaxy to be a collection of numerous nebulous stars.^[51] In Afghanistan, he observed and described the solar eclipse on April 8, 1019, and the lunar eclipse on September 17, 1019, in detail, and gave the exact latitudes of the stars during the lunar eclipse.^[50]

1025-1450

During this period, a distinctive Islamic system of astronomy flourished. Within the Greek tradition and its successors it was traditional to separate mathematical astronomy (as typified by Ptolemy) from philosophical cosmology (as typified by Aristotle). Muslim scholars developed a program of seeking a physically real configuration (hay'a) of the universe, that would be consistent with both mathematical and physical principles. Within the context of this hay'a tradition, Muslim astronomers began questioning technical details of the Ptolemaic system of astronomy.^[52] Most of these criticisms, however, continued to follow the Ptolemaic astronomical paradigm, remaining within the geocentric framework.^[53] As the historian of astronomy, A. I. Sabra, noted:

"All Islamic astronomers from Thabit ibn Qurra in the ninth century to Ibn al-Shatir in the fourteenth, and all natural philosophers from al-Kindi to Averroes and later, are known to have accepted what Kuhn has called the "two-sphere universe" ...—the Greek picture of the world as consisting of two spheres of which one, the celestial sphere made up of a special element called aether, concentrically envelops the other, where the four elements of earth, water, air, and fire reside."^[54]

Some Muslim astronomers, however, most notably Abū Rayhān al-Bīrūnī and Nasīr al-Dīn al-Tūsī, discussed whether the Earth moved and considered how this might be consistent with astronomical computations and physical systems.^[55] Several other Muslim astronomers, most notably those following the Maragha school of thought, developed non-Ptolemaic planetary models within a geocentric context that were later adapted in the Copernican model in a heliocentric context.

Refutations of astrology

See also: Islamic astrology

The first semantic distinction between astronomy and astrology was given by the Persian astronomer Abu Rayhan al-Biruni in the 11th century,^[56] though he himself refuted astrology in another work. The study of astrology was also refuted by other Muslim astronomers at the time, including al-Farabi, Ibn al-Haytham, Avicenna and Averroes. Their reasons for refuting astrology were often due to both scientific (the methods used by astrologers being conjectural rather than empirical) and religious (conflicts with orthodox Islamic scholars) reasons.^[57]

Ibn Qayyim Al-Jawziyya (1292-1350), in his Miftah Dar al-SaCadah, used empirical arguments in astronomy in order to refute the practice of astrology and divination.^[58] He recognized that the stars are much larger than the planets, and thus argued:^[59]

"And if you astrologers answer that it is precisely because of this distance and smallness that their influences are negligible, then why is it that you claim a great influence for the smallest heavenly body, Mercury? Why is it that you have given an influence to al-Ra's and al-Dhanab, which are two imaginary points [ascending and descending nodes]?"

Al-Jawziyya also recognized the Milky Way galaxy as "a myriad of tiny stars packed together in the sphere of the fixed stars" and thus argued that "it is certainly impossible to have knowledge of their influences."^[59]

Astrophysics and celestial mechanics

In astrophysics and celestial mechanics, Abū Rayhān al-Bīrūnī described the Earth's gravitation as:^[60]

"The attraction of all things towards the centre of the earth."

Al-Biruni also discovered that gravity exists within the heavenly bodies and celestial spheres, and he criticized the Aristotelian views of them not having any levity or gravity and of circular motion being an innate property of the heavenly bodies.^[61]

In 1121, al-Khazini, in his treatise The Book of the Balance of Wisdom, states:^[62]

"For each heavy body of a known weight positioned at a certain distance from the centre of the universe, its gravity depends on the remoteness from the centre of the universe. For that reason, the gravities of bodies relate as their distances from the centre of the universe."

Al-Khazini was thus the first to propose the theory that the gravities of bodies vary depending on their distances from the centre of the Earth. This phenomenon was not proven until Newton's law of universal gravitation in the 18th century.^[62]

Beginning of hay'a tradition

Ibn al-Haytham (Alhacen) was a pioneer of the Muslim haya tradition of astronomy, presented the first critique and reform of Ptolemy's model, introduced experimentation to astrophysics, and laid the theoretical foundations for modern telescopic astronomy.

Between 1025 and 1028, Ibn al-Haytham (Latinized as Alhazen), began the hay'a tradition of Islamic astronomy with his Al-Shuku ala Batlamyus (Doubts on Ptolemy). While maintaining the physical reality of the geocentric model, he was the first to criticize Ptolemy's astronomical system, which he criticized on empirical, observational and experimental grounds,^[63] and for relating actual physical motions to imaginary mathematical points, lines and circles:

"Ptolemy assumed an arrangement that cannot exist, and the fact that this arrangement produces in his imagination the motions that belong to the planets does not free him from the error he committed in his assumed arrangement, for the existing motions of the planets cannot be the result of an arrangement that is impossible to exist."^[64]

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.^[65] In his Epitome of Astronomy, he insisted that the heavenly bodies "were accountable to the laws of physics."^[66] The foundations of telescopic astronomy can also be traced back to Ibn al-Haytham, due to the influence of his optical studies on the later development of the modern telescope.^[67]

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.^[68] His reformed model was the first to reject the equant^[69] and eccentrics,^[70] 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,^[71] and the centres of motion were geometrical points without any physical significance, like Johannes Kepler's model centuries later.^[72] Ibn al-Haytham also describes an early version of Occam's razor, where he employs only minimal hypotheses regarding the properties that characterize astronomical motions, as he attempts to eliminate from his planetary model the cosmological hypotheses that cannot be observed from Earth.^[73]

Al-Biruni was the first to conduct elaborate experiments related to astronomical phenomena, and he introduced the analysis of the acceleration of planets, discovered that the motions of the solar apogee and precession are not identical, discussed the possibility of heliocentrism, and suggested that the Earth's rotation on its axis would be consistent with his astronomical parameters.

Early alternative models

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). Biruni stated that Brahmagupta and others consider that the earth rotates on its axis and Biruni noted that this does not create any mathematical problems.^[74]

Abu Said al-Sijzi, a contemporary of al-Biruni, suggested the possible heliocentric movement of the Earth around the Sun, which al-Biruni did not reject.^[75] Al-Biruni agreed with the Earth's rotation about its own axis, and while he was initially neutral regarding the heliocentric and geocentric models,^[76] he considered heliocentrism to be a philosophical problem.^[3] He remarked that if the Earth rotates on its axis and moves around the Sun, it would remain consistent with his astronomical parameters:^[60][77]

"Rotation of the earth would in no way invalidate astronomical calculations, for all the astronomical data are as explicable in terms of the one theory as of the other. The problem is thus difficult of solution."

In 1031, al-Biruni completed his extensive astronomical encyclopaedia Kitab al-Qanun al-Mas'udi (Latinized as Canon Mas’udicus),^[78] in which he recorded his astronomical findings and formulated astronomical tables. In it he presented a geocentric model, tabulating the distance of all the celestial spheres from the central Earth, computed according to the principles of Ptolemy's Almagest.^[79] The book introduces the mathematical technique of analysing the acceleration of the planets, and first states that the motions of the solar apogee and the precession are not identical. Al-Biruni also discovered that the distance between the Earth and the Sun is larger than Ptolemy's estimate, on the basis that Ptolemy disregarded the annual solar eclipses.^[60][80]

In 1070, Abu Ubayd al-Juzjani, a pupil of Avicenna, proposed a non-Ptolemaic configuration in his Tarik al-Aflak. In his work, he indicated the so-called "equant" problem of the Ptolemic model, and proposed a solution for the problem. He claimed that his teacher Avicenna had also worked out the equant problem.^[81]

Andalusian Revolt

Averroes rejected the eccentric deferents introduced by Ptolemy. He rejected the Ptolemaic model and instead argued for a strictly concentric model of the universe.

In the 11th-12th centuries, astronomers in al-Andalus took up the challenge earlier posed by Ibn al-Haytham, namely to develop an alternate non-Ptolemaic configuration that evaded the errors found in the Ptolemaic model.^[82] Like Ibn al-Haytham's critique, the anonymous Andalusian work, al-Istidrak ala Batlamyus (Recapitulation regarding Ptolemy), included a list of objections to Ptolemic astronomy. This marked the beginning of the Andalusian school's revolt against Ptolemaic astronomy, otherwise known as the "Andalusian Revolt".^[83]

In the late 11th century, al-Zarqali (Latinized as Arzachel) discovered that the orbits of the planets are elliptic orbits and not circular orbits,^[84] though he still followed the Ptolemaic model.

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:^[1]

"To assert the existence of an eccentric sphere or an epicyclic sphere is contrary to nature. [...] The astronomy of our time offers no truth, but only agrees with the calculations and not with what exists."

Averroes' contemporary, Maimonides, wrote the following on the planetary model proposed by Ibn Bajjah (Avempace):

"I have heard that Abu Bakr [Ibn Bajja] discovered a system in which no epicycles occur, but eccentric spheres are not excluded by him. I have not heard it from his pupils; and even if it be correct that he discovered such a system, he has not gained much by it, for eccentricity is likewise contrary to the principles laid down by Aristotle.... I have explained to you that these difficulties do not concern the astronomer, for he does not profess to tell us the existing properties of the spheres, but to suggest, whether correctly or not, a theory in which the motion of the stars and planets is uniform and circular, and in agreement with observation."^[85]

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.^[47] 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. Al-Betrugi was also the first to discover that the planets are self-luminous.^[86] 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,^[87] mainly because they followed Aristotle's notion of perfect circular motion.

Maragha Revolution

Main article: Maragheh observatory

The "Maragha Revolution" refers to the Maragheh school's revolution against Ptolemaic astronomy. The "Maragha school" was an astronomical tradition beginning in the Maragheh observatory and continuing with astronomers from Damascus and Samarkand. Like their Andalusian predecessors, the Maragha astronomers attempted to solve the equant problem and produce alternative configurations to the Ptolemaic model. They were more successful than their Andalusian predecessors in producing non-Ptolemaic configurations which eliminated the equant and eccentrics, were more accurate than the Ptolemaic model in numerically predicting planetary positions, and were in better agreement with empirical observations.^[88] The most important of the Maragha astronomers included Mo'ayyeduddin Urdi (d. 1266), Nasīr al-Dīn al-Tūsī (1201-1274), 'Umar al-Katibi al-Qazwini (d. 1277), Qutb al-Din al-Shirazi (1236-1311), Sadr al-Sharia al-Bukhari (c. 1347), Ibn al-Shatir (1304-1375), Ali al-Qushji (c. 1474), al-Birjandi (d. 1525) and Shams al-Din al-Khafri (d. 1550).^[89]

Nasīr al-Dīn al-Tūsī resolved significant problems in the Ptolemaic system with the Tusi-couple, which later played an important role in the Copernican model.

Some have described their achievements in the 13th and 14th centuries as a "Maragha Revolution", "Maragha School Revolution", or "Scientific Revolution before the Renaissance". An important aspect of this revolution included the realization that astronomy should aim to describe the behavior of physical bodies in mathematical language, and should not remain a mathematical hypothesis, which would only save the phenomena. The Maragha astronomers also realized that the Aristotelian view of motion in the universe being only circular or linear was not true, as the Tusi-couple showed that linear motion could also be produced by applying circular motions only.^[90]

Unlike the ancient Greek and Hellenistic astronomers who were not concerned with the coherence between the mathematical and physical principles of a planetary theory, Islamic astronomers insisted on the need to match the mathematics with the real world surrounding them,^[91] which gradually evolved from a reality based on Aristotelian physics to one based on an empirical and mathematical physics after the work of Ibn al-Shatir. The Maragha Revolution was thus characterized by a shift away from the philosophical foundations of Aristotelian cosmology and Ptolemaic astronomy and towards a greater emphasis on the empirical observation and mathematization of astronomy and of nature in general, as exemplified in the works of Ibn al-Shatir, al-Qushji, al-Birjandi and al-Khafri.^[92][93][94]

Ibn al-Shatir's model for the appearances of Mercury, showing the multiplication of epicycles using the Tusi-couple, thus eliminating the Ptolemaic eccentrics and equant.

Other achievements of the Maragha school include the first empirical observational evidence for the Earth's rotation on its axis by al-Tusi and al-Qushji,^[95] the separation of natural philosophy from astronomy by Ibn al-Shatir and al-Qushji,^[96] the rejection of the Ptolemaic model on empirical rather than philosophical grounds by Ibn al-Shatir,^[88] and the development of a non-Ptolemaic model by Ibn al-Shatir that was mathematically identical to the heliocentric Copernical model.^[97]

Mo'ayyeduddin Urdi (d. 1266) was the first of the Maragheh astronomers to develop a non-Ptolemaic model, and he proposed a new theorem, the "Urdi lemma".^[98] Nasīr al-Dīn al-Tūsī (1201-1274) resolved significant problems in the Ptolemaic system by developing the Tusi-couple as an alternative to the physically problematic equant introduced by Ptolemy,^[99] and conceived a plausible model for elliptical orbits.^[78] Tusi's student Qutb al-Din al-Shirazi (1236-1311), in his The Limit of Accomplishment concerning Knowledge of the Heavens, discussed the possibility of heliocentrism. 'Umar al-Katibi al-Qazwini (d. 1277), who also worked at the Maragheh observatory, in his Hikmat al-'Ain, wrote an argument for a heliocentric model, though he later abandoned the idea.^[75]

Medieval manuscript by Qutb al-Din al-Shirazi depicting an epicyclic planetary model.

Ibn al-Shatir (1304–1375) of Damascus, in A Final Inquiry Concerning the Rectification of Planetary Theory, incorporated the Urdi lemma, and eliminated the need for an equant by introducing an extra epicycle (the Tusi-couple), departing from the Ptolemaic system in a way that was mathematically identical to what Nicolaus Copernicus did in the 16th century. Unlike previous astronomers before him, Ibn al-Shatir was not concerned with adhering to the theoretical principles of natural philosophy or Aristotelian cosmology, but rather to produce a model that was more consistent with empirical observations. For example, it was Ibn al-Shatir's concern for observational accuracy which led him to eliminate the epicycle in the Ptolemaic solar model and all the eccentrics, epicycles and equant in the Ptolemaic lunar model. His model was thus in better agreement with empirical observations than any previous model,^[88] and was also the first that permitted empirical testing.^[100] His work thus marked a turning point in astronomy, which may be considered a "Scientific Revolution before the Renaissance".^[88] His rectified model was later adapted into a heliocentric model by Copernicus,^[99] which was mathematically achieved by reversing the direction of the last vector connecting the Earth to the Sun.^[3] In the published version of his masterwork, De revolutionibus orbium coelestium, Copernicus also cites the theories of al-Battani, Arzachel and Averroes as influences,^[78] while the works of Ibn al-Haytham and al-Biruni were also known in Europe at the time.

An area of active discussion in the Maragheh school, and later the Samarkand and Istanbul observatories, was the possibility of the Earth's rotation. Supporters of this theory included Nasīr al-Dīn al-Tūsī, Nizam al-Din al-Nisaburi (c. 1311), al-Sayyid al-Sharif al-Jurjani (1339-1413), Ali al-Qushji (d. 1474), and Abd al-Ali al-Birjandi (d. 1525). Al-Tusi was the first to present empirical observational evidence of the Earth's rotation, using the location of comets relevant to the Earth as evidence, which al-Qushji elaborated on with further empirical observations while rejecting Aristotelian natural philosophy altogether. Both of their arguments were similar to the arguments later used by Nicolaus Copernicus in 1543 to explain the Earth's rotation.^[95]

Islamic astronomy in China

See also: Chinese astronomy

Muslim astronomers were brought to China work on calendar making and astronomy during the Yuan Dynasty. Kublai Khan brought Iranians to Beijing to construct an observatory and an institution for astronomical studies.^[101] Jamal ad-Din, a Persian astronomer, presented Kublai Khan with seven Persian astronomical instruments, including a Persian globe and an armillary sphere, in 1267.^[102] Several Chinese astronomers also worked at the Maragheh observatory in Persia.

1450-1900

This period was considered the period of stagnation, when the traditional system of astronomy continued to be practised with enthusiasm, but with decreasing innovation.^[20] It was believed there was no innovation of major significance during this period, but this view has been rejected by historians of astronomy in recent times, who argue that Muslim astronomers continued to make significant advances in astronomy through to the 16th century and possibly after this as well.^[96][103] After the 16th century, there appears to have been little concern for theoretical astronomy, but observational astronomy in the Islamic tradition continued in the three Muslim gunpowder empires: the Ottoman Empire, the Safavid dynasty of Persia, and the Mughal Empire of India.

Earth's motion

Ali al-Qushji provided empirical evidence for the Earth's motion and completely separated astronomy from natural philosophy.

The work of Ali al-Qushji (d. 1474), who worked at Samarkand and then Istanbul, is seen as a late example of innovation in Islamic astronomy and it is believed he may have had an influence on Nicolaus Copernicus due to similar arguments concerning the Earth's rotation. Before al-Qushji, the only astronomer to present an empirical argument 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 stationery Earth can be determined through observation alone. Al-Tusi, however, accepted that the Earth was stationery on the basis of natural philosophy instead, particularly Aristotelian cosmology. In the 15th century, the influence of Aristotelian physics and natural philosophy was declining due to religious opposition. Al-Qushji, in his Concerning the Supposed Dependence of Astronomy upon Philosophy, thus 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 stationery Earth, as he explored the idea of a moving Earth. He elaborated on al-Tusi's argument and concluded, on the basis of empiricism 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.^{[95][96][104]}

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 rotating, develops a hypothesis similar to Galileo Galilei's notion of "circular inertia",^[105] which he described in the following observational test (as a response to one of Qutb al-Din al-Shirazi's arguments):

"The small or large rock will fall to the Earth along the path of a line that is perpendicular to the plane (sath) of the horizon; this is witnessed by experience (tajriba). And this perpendicular is away from the tangent point of the Earth’s sphere and the plane of the perceived (hissi) horizon. This point moves with the motion of the Earth and thus there will be no difference in place of fall of the two rocks."^[106]

Theoretical astronomy

It was traditionally believed that Islamic astronomers made no more advances in planetary theory after the work of Ibn al-Shatir in the 14th century, but recent studies have shown that there were several significant advances in planetary theory through to the 16th century, after George Saliba studied the works of a 16th century astronomer, Shams al-Din al-Khafri (d. 1550), a Safavid commentator on earlier Maragha astronomers. Saliba wrote the following on al-Khafri's work:

"By his sheer insight into the role of mathematics in describing natural phenomena, this astronomer managed to bring the hay'a tradition to such unparalleled heights that could not be matched anywhere else in the world at that time neither mathematically nor astronomically. By working on the alternative mathematical models that could replace those of Ptolemy, and by scrutinizing the works of his predecessors who were all searching for unique mathematical models that could describe the physical phenomena consistently, this astronomer finally realized that all mathematical modeling had no physical truth by itself and was simply another language with which one could describe the physical observed reality. He also realized that the specific phenomena that were being described by the Ptolemaic models did not have unique mathematical solutions that were subject to the same restraints. Rather there were several mathematical models that could account for the Ptolemaic observations, yield identical predictive results at the same critical points used by Ptolemy to construct his own models (thus accounting for the observations as perfectly as Ptolemy could) and still meet the consistency requirement that was imposed by the Aristotelian cosmology which was adopted by the writers in the hay'a tradition."^[103]

Ali al-Qushji also improved on al-Tusi's planetary model and presented an alternative planetary model for Mercury.^[107]

A model of the heliocentric system attributed to Nicolaus Copernicus.

Ottoman observational astronomy

Another notable 16th century Muslim astronomer was the Ottoman astronomer Taqi al-Din, who built the Istanbul observatory of al-Din in 1577, where he carried out astronomical observations until 1580. He produced a Zij (named Unbored Pearl) and astronomical catalogues that were more accurate than those of his contemporaries, Tycho Brahe and Nicolaus Copernicus. Al-Din was also the first astronomer to employ a decimal point notation in his observations rather than the sexagesimal fractions used by his contemporaries and predecessors.^[108] He also invented a variety of astronomical instruments, including accurate mechanical astronomical clocks from 1556 to 1580 and a rudimentary telescope some time before 1574.^[109]

Earlier in 1574, al-Din used astrophysics to explain the intromission model of vision. He stated since the stars are millions of kilometers away from the Earth and that the speed of light is constant, that if light had come from the eye, it would take too long for light "to travel to the star and come back to the eye. But this is not the case, since we see the star as soon as we open our eyes. Therefore the light must emerge from the object not from the eyes."^[109]

After the destruction of the Istanbul observatory of al-Din in 1580, astronomical activity stagnated in the Ottoman Empire, until the introduction of Copernican heliocentrism in 1660, when the Ottoman scholar Ibrahim Efendi al-Zigetvari Tezkireci translated Noël Duret's French astronomical work (written in 1637) into Arabic.^[110]