The Great Contribution of Islamic Astronomers

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http://en.wikipedia.org/wiki/Islamic_astronomy









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.  The first recorded mention of the Large Magellanic Cloud was also given by Abd Al-Rahman al-Sufi.

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.

Abu-Mahmud al-Khujandi relatively accurately computed the axial tilt to be 23°32'19" (23.53°),. 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.

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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.

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:

"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:

"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."

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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:




"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:

"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.

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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.

In his Astral Motion and The Force of Attraction, Muhammad ibn Musa also proposed that there is a force of attraction between heavenly bodies, foreshadowing Newton's law of universal gravitation.

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".  This led to the use of exacting empirical observations and experimental techniques by Muslim astronomers from the eleventh century onwards.

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."

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.

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."

Ibn al-Haytham refuted this by making the first attempt at observing and measuring the Milky Way's parallax, 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."

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.  He discovered the Milky Way galaxy to be a collection of numerous nebulous stars.

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

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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.  Most of these criticisms, however, continued to follow the Ptolemaic astronomical paradigm, remaining within the geocentric framework.


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."

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.

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.

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Refutations of 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.

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.  He recognized that the stars
are much larger than the planets, and thus argued:

"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."

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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.

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

"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.

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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.











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,
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."

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."
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.

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.

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.

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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.

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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.

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.  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 considered heliocentrism to be a philosophical problem.  He remarked that if the Earth rotates on its axis and moves around the Sun, it would remain consistent with his astronomical parameters:

"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), 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.

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.

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.

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Averroes rejected the eccentric deferents introduced
by Ptolemy.

He rejected the Ptolemaic model and instead argued
for a strictly concentric model of the universe.










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.

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".

In the late 11th century, al-Zarqali (Latinized as Arzachel) discovered that the orbits of the planets are elliptic orbits and not circular orbits,  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:

"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."

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. Al-Betrugi was also the first to discover that the planets are self-luminous.

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, mainly because they followed Aristotle's notion of perfect circular motion.

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                  Nasīr al-Dīn al-Tūsī









Maragha Revolution



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.

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).

 


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 "MaraghaRevolution",
"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.

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, 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.

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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, the separation of natural philosophy from astronomy by Ibn al-Shatir and al-Qushji, the rejection of the Ptolemaic model on empirical rather than philosophical grounds by Ibn al-Shatir, and the development of a non-Ptolemaic model by Ibn al-Shatir that was mathematically identical to the heliocentric Copernical model.

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".  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, and conceived a plausible model for elliptical orbits.

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.

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Medieval Manuscript
by Qutb al-din al-Shirazi depicting an eptcyclic planetary model

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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, and was also the first that permitted empirical testing.

His work thus marked a turning point in astronomy, which may be considered a "Scientific Revolution before the Renaissance". His r

ectified model was later adapted into a heliocentric model by Copernicus, which was mathematically achieved by reversing the direction of the last vector connecting the Earth to the Sun.

In the published version of his masterwork, De revolutionibus orbium coelestium, Copernicus also cites the theories of al-Battani, Arzachel and Averroes as influences, 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.