The Five Wandering Stars Part III: The Doubts Movement and Islamic Scholarship

Zafar Bhatti, Warwick, UK

Challenging Aristotle and Ptolemy – The Doubts (Shukuk) Movement 

We have seen that the entire Greek Astronomical tradition had been assimilated, with corrections and updates, within the first three centuries of Islam. Due to the contributions of Muslim scholars, observational astronomy had reached an unprecedented level of accuracy.

The next development was more fundamental; it was an attack on the construct of the Ptolemaic model of our universe. The Muslim scientists had assimilated the Ptolemaic method but were deeply unsatisfied by its cosmological and structural underpinnings. This movement and revolt against Greek Science is known as the Shukuk or ‘Doubts’ movement and was a direct attack on the Greek understanding of the heavens, rather than their observations. Here, we explore the founders of this movement, their contributions to the development of astronomy and their challenge to the Ptolemaic model.

Ibn al-Haitham (Al-Hazen 965-1040 CE)

The doubts movement and its critique can be summarised in the words of the great Muslim polymath Ibn al-Haitham, also nicknamed the ‘Second Ptolemy’, who declared:

‘[Ptolemy] assumed an imaginary configuration with imaginary lines and circles that could move in those motions, even though only some of those motions could indeed take place in [real] bodies that moved in those motions. He was obliged to follow that route for he could not devise another.

But if one were to assume an imaginary line, and made that line move in his imagination, it would not follow that there should be a corresponding line that would move in the heavens with the motion. Nor would it be true that if one imagined a circle in the heavens, and imagined a planet to move on that circle, that the [real] planet would [in fact] move along that imaginary circle…

The motions of the planets, however, have correct configurations in [real] existent bodies that Ptolemy did not come to understand nor could he achieve. For it is not admissible that a perceptible, perpetual and uniform motion be found without it having a correct configuration in [real] existent bodies…

The truth that leaves no room for doubt is that there are correct configurations for the movements of the planets, which exist, are consistent, and entail none of these impossibilities and contradictions. But they are different from the ones that were established by Ptolemy. And Ptolemy could not comprehend them. Nor could his imagination attain their true nature’.[1]

Here, Ibn al-Haitham quite clearly and forcefully declares that the cosmological understanding of the Universe as presented by Ptolemy does not exist and that a new system of cosmology is needed.

Al-Biruni (973-1048 CE)

An interesting text worthy of note is the correspondence between two of the great heavyweights of the Muslim Scientific era, namely Al-Biruni and Ibn Sina (980-1037 CE), who is known in the west as Avicenna. In this text we clearly see Al-Biruni openly questioning and challenging Aristotelian thought and scientific understanding, including cosmological principles.[2]

Al-Biruni’s first question was aimed directly at the very heart of Aristotelian cosmology: ‘Why did Aristotle assert that the heavenly bodies have neither levity nor gravity and why did he deny absence of motion from and to the centre?’ This lies at the heart of Aristotelian cosmology: what is holding the heavens aloft? Al-Biruni was not at all satisfied by the explanation given by Aristotle.

Their correspondence is full of objections regarding Aristotelian thought. One very interesting exchange is Al-Biruni’s objection to the reasons Aristotle provides for the circular motion of the heavens as opposed to an elliptical (lentil-shaped) motion.[3] Fascinatingly, Al-Biruni was also of the view that the orbits were circular, but he still attacked the fundamental reasons underpinning an Aristotelian Universe.

Al-Ghazali (1058-1111 CE)

The great Muslim reformer Al-Ghazali often gets unfair press in the West. He is seen as someone who halted or slowed Muslim Scientific progress in the East, but this is a gross misunderstanding of Al-Ghazali’s impact. Al-Ghazali was, in fact, emphatic and clear-cut when he stated that disbelieving in that which has been established by mathematical and observational evidence is a disservice to religion:

‘He who thinks that it is his religious duty to disbelieve such things is really unjust to religion, and weakens its cause. For these things have been established by astronomical and mathematical evidence, which leaves no room for doubt.’[4]

Thus, we must exonerate Al-Ghazali here of these misunderstood accusations. Al-Ghazali does not object to observation or mathematical certainty, but rather a particular cosmological or theoretical understanding of what physically underpins mathematics and observation. As such, his main objections are toward the understanding of the Aristotelian Universe, and overreliance on Greek philosophy. For instance, he writes:

‘Let it be known that it is our purpose to disillusion those who think too highly of the philosophers, and consider them to be infallible.’[5]

As such, Al-Ghazali was throwing down the gauntlet for a new understanding, that was more akin to a revolution in the sciences. His arguments against the immutability of the Universe, for example, became key in the time of Kepler when Aristotelian science was being overhauled.

Correcting Ptolemy’s Cosmological Model

We have seen how Islamic science first integrated the Almagest, then commented and corrected astronomical values, following which a critique of the Greek Cosmological model began. The next phase of Muslim astronomical endeavours would be to take up the challenge issued by these great Muslim thinkers, and begin trying to resolve the inconsistencies of the Greek Cosmological model. However, it must be noted that what proceeded only attempted to resolve the Ptolemaic model in line with Aristotelian cosmology, rather than an attempt to resolve both incorrect models; a mistake that potentially cost Islam the renaissance. We shall examine this in two separate parts – the Muslims of the East and the Muslims of the West.

Muslims of the East – The Maragha Astronomers

After an intense period of scholarship at all levels, it seemed as if, for some time, there was a decline in Muslim Scientific advancement in the East. However, during the last century, a number of European academics such as Kennedy, Hartner, Neugebauer, Roberts, and others made a number of startling discoveries demonstrating the pioneering efforts of Eastern Muslim astronomers.[6] In fact, the historian George Saliba deems this effort in Eastern Islam to be the golden age of Muslim astronomy. Three giants stand out during this period of astronomy in the East: Mu’ayyad al-Din al-Urdi (d. 1266), Nasir al-Din al-Tusi (d.1274) and Ibn al-Shatir (d.1375).

They focused on tackling the first anomaly of the ‘wandering stars’ (as to why they seem to move differently and even recede) and eventually resolved it, harmonising Ptolemy’s model with Aristotle’s cosmological model of uniform circular motion, thereby removing the fictitious equant and eccentrics which had plagued Ptolemy’s model [see part 1 of this series, published in April 2025.] To do this, they produced a number of mathematical innovations to produce a consistent and uniform model, reaching its epitome in Ibn al-Shatir’s work in the 14^th century.

The astronomers involved in this work are now called the ‘Maragha Astronomers’. Their name is based on the observatory established under the patronage of Hulagu Khan in 1259 by the great Persian-Muslim Scientist and Astronomer, Nasir al-Din al-Tusi. Located in Maragha, Iran, it was 22 metres in diameter.⁷ The uniqueness of these astronomers is that they took up the mantle laid down by Ibn al-Haitham, which severely criticised the consistency of Ptolemy’s work and called for new astronomical models. These ‘Maragha Astronomers’ went about conducting exhaustive measurements, all the while constructing new mathematical models to correct Ptolemy’s work.

Image credit: Wikimedia Commons | Nasir al-Din al-Tusi. Original source: Biblioteca Apostolica Vaticana, Vat. Arabic ms 319, fol. 28 verso.

Figure 1 – Tusi Couple – 13th century CE sketch by Nasir al-Din al-Tusi. Generates a linear motion as a sum of two circular motions. Invented for Tusi’s planetary model.

These astronomers made three significant steps to resolve the first anomaly of Ptolemy and harmonise the model:

1. Al-Urdi invented the Urdi Lemma, a means of transposing angles across parallel lines.

2. Tusi invented the ‘Tusi couple’, a mathematical model that couples two circles together to create linear motion, used to resolve the equant problem.[8] A Tusi Couple (which is also used in a number of engineering solutions) works in this instance by the ‘wandering star’ being located on a rotating smaller circle, which is itself rotating along a larger outer circle, thereby resulting in linear movement of the ‘wandering star’. In layman’s terms, because the smaller circle is rotating around this bigger circle, it would appear as if the ‘wandering star’ is moving backwards.

3. Ibn al-Shatir used both of the above principles to harmonise the Ptolemaic model and create a consistent model of the Universe, removing equants and eccentricities and, to a large extent, resolving one of the primary issues that plagued the Ptolemaic theory.[9] Thus, he succeeded in describing the motion of all the wandering stars in terms of uniform circles, which met the criteria for Aristotelian cosmology.

This resolution of the first anomaly of Ptolemy’s work is a landmark in the evolution of the Universe’s cosmological model and, as will be demonstrated later, is work that Copernicus would copy without attribution in his magnum opus the De Revolutionibus Orbium Coelestium, or ‘On the Revolutions of the Heavenly Spheres’.  

Muslims of the West – The Andalusian Astronomers

At a similar time to the Maragha astronomers, we find Islamic scientists who would challenge, correct and present alternative models to the Ptolemaic model in Andalusia, present-day Spain. Figures such as Al-Zarqali (Azarqueil, 1029-1087), Ibn Bajjah (Avempace, 1085-1138), Jabir ibn Aflah (Geber, 1100-1250),  Ibn Tufail (Abubacer, 1105 – 1185), Nur al-Din al-Bitruji (Alpetragius, d. 1204) and the great philosopher Ibn Rushd (Averroes 1126-1198) would carry the torch of Islamic science. We will look at some of these scholars and their work below, and how they tried resolving the motion of the heavens and the enigma of the ‘wandering stars’.

Ibn Rushd (Averroes)

The works of Ibn Rushd, known in the West as Averroes, would be of profound significance in the early part of the European renaissance. It seems that his particular goal – one which was shared by some of the Andalusian scientists – was to revolutionise and overhaul the Ptolemaic view of the Universe. These scientists recognised and emphasised that the Ptolemaic structure, with more than 50 different circles and mechanisms with which to describe the heavens, did not represent our physical reality. 

More than just removing equants and eccentricities, it seems they wished to remove all epicycles as well, and move to one single centre of motion around which all bodies moved, known as a concentric model of the Universe. Interestingly, even though the corrected Ptolemaic model was an accurate mathematical model (in fact with modern observational methods and techniques it would be even more accurate), this group of scientists were not objecting to its observational accuracy, but rather to its elegance. Or rather, the lack of elegance thereof.

These scientists were in search of a mathematical model that described the movement of the heavenly bodies with both observational accuracy and elegant simplicity – a pursuit we might today describe as adhering to ‘Occam’s Razor’, which states that where a phenomenon can be explained by two different causes the simplest cause should be adopted.[10] This quest is captured by the great Ibn Rushd, who was damning in his critique of Ptolemaic cosmology:

‘The epicycle sphere is in principle impossible, for the body that moves in a circular motion has to move around the centre of the universe and not outside of it.’ [11] In another place, he states, ‘The astronomy of our time offers no truth, but only agrees with the calculations and not with what exists.’[12]

‘The Oratio [13] reiterates the goal of constructing a concentric astronomy as a worthy one – Averroes (Ibn Rushd) would have earned glory if he had achieved it’.[14]

Ibn Rushd was a dominating figure in the enlightenment period in Europe, with his philosophies and works pervading the intellectual schools of the Renaissance period. It is certainly his damning verdicts on Ptolemaic Astronomy that may have driven the European scientists during this period towards their goal in ultimately overhauling the Ptolemaic system and creating a concentric astronomy, as alluded to in the above quote regarding the Oratio, a lecture delivered by Regiomontanus, whose works, some believe, led to Copernicus’s theory of Heliocentricity.

Nur al-Din al-Bitruji

One of the scientists who would pick up the gauntlet and offer an alternative to the Ptolemaic system of epicycles and deferents would be Al-Bitruji, who would attempt to present a model of the heavens with only one centre (concentric), removing all epicycles and eccentrics. This would be a further development of the models proposed by the Andulsian Scientists Ibn Bajjah (Avempace) and Ibn Tufail (Abubacer). However, this model was unable to successfully replace the Ptolemaic system, as its own predictions were not as accurate as the amended Ptolemaic system. Note also that similar attempts at a concentric theory were made before by Greeks such as Exodus, and are more in alignment with Aristotle’s own views. However, this aim to replace all epicycles and pursue a homocentric, concentric model, would prove to be one of the driving forces for the reformulation of the motion of the wandering stars.

Al-Zarqali (Azarqueil)

Al-Zarqali was a remarkable astronomer who made a number of important discoveries and observations.

Solar apsidal precession

Image credit: Wikimedia Commons | Mpfiz

Figure 2 – Solar apsidal precession, demonstrating the change in the Sun’s orbit over time.

This theory, first discovered by Al-Zarqali, is the rotation of the elliptical orbit over time. [15] Note that this has no bearing (as far as this author is aware) on the derivation of heliocentricity, however the enigma of precessions and how they precisely appeared was unresolved up until the 20^th century.

Elliptical Orbits

Figure 3 – Diagram showing the deferents of the ‘wandering stars’ drawn under the instructions of alZarqali, Source: Libros del Saber de Astronomia 1276-1279

Up until this point we have discussed the heavenly bodies conforming to uniform circular motion as described earlier. However, Figure 3 is one of the first ever depictions of the heavenly body illustrated as an elliptical deferent. This startling image is taken directly from page 282 the book Libros del Saber de Astronomía, or ‘Book of the Wisdom of Astronomy’ commissioned by Alfonso X of Castille, between the year 1276 & 1279. It depicts an image of the Universe, which looks very similar to the manner in which we view our solar system now, but was created by Al-Zarqali in the 11^th century!

According to scientist and polymath Willy Hartner, the elliptical orbit that has been drawn is deliberate, and in this manner, the image ‘precedes the first European mentioning by nearly 400 years’.[16] To understand the significance of this, one must realise that it would be the Kepler’s formulation of the wandering stars in an elliptical motion as opposed to a circular motion, which would finally resolve the enigma of the ‘wandering stars’ and confirm the Heliocentric model. However, it should be clearly stated that the circle in the centre is not the Sun according to Hartner, but just denotes the centre of the orbit.

The importance of Al-Zarqali’s depiction cannot be underestimated. Unfortunately, out of ignorance of the original work, so-called experts have commented on this work without understanding it. As such, we have extreme divergence of opinion; some have said Al-Zarqali proposed the elliptical orbits of the planets around the Sun, and yet others have dismissed Al-Zarqali’s work as just based on Ptolemy’s planetary deferent for Mercury. Undoubtedly, we see that Al-Zarqali built upon Ptolemy’s work, but the depiction of an elliptical orbit of the deferent is certainly deliberate. Hartner’s work in this area is that of an expert analysis and not a commentary of a commentary. As such, his opinion in this regard is of the highest order. We will later see how this may have potentially influenced the work of latter-day astronomers.

Toledan Tables (c 1080)