In today’s blog we continue our exploration of the origin of heliocentrism, with Dr. Jamil Ragep, PhD from Harvard University, and Canada Research Chair in the History of Science in Islamic Societies at McGill University
David: Copernicus was hardly forthcoming with his heliocentric ideas and the Islamic astronomers that were the subject of your research lived hundreds of years earlier. Were the pressures of their time similar to those experienced by Copernicus? Could this explain why their work seems to have gone only so far in terms of the transition from geocentrism to heliocentrism?
Jamil: Actually, several of the astronomers I study lived just before, or were even contemporaries, of Copernicus. And traditional Islamic astronomy continued well after Copernicus; in fact, some texts, generally based on Ptolemy, were still being written and studied into the twentieth century. Some of these even provided descriptions of the “new” heliocentric astronomy; an Ottoman text from the nineteenth century even shows the newly discovered planets Uranus and Neptune (which it calls Herschel’s planet and Le Verrier’s, respectively; here). So it is important to understand that when we speak of Islamic science we need to see it as a millennium-long enterprise that had its own twists and turns that was in some ways parallel to, but in others distinctive, from what happened in Christian Europe.
As for the transition from geocentrism to heliocentrism: this is a complex story that I can only touch on here. One thing to keep in mind is that similar scientific traditions, or “paradigms,” can take on different trajectories within different cultural contexts. As mentioned above, the Islamic scientific tradition was quite long-lived, and a core element was Ptolemaic/Aristotelian geocentrism. Even when Aristotelian natural philosophy and metaphysics was challenged on religious grounds, and Ptolemaic models were criticized for violating physical principles (uniform, circular motion in the celestial region), the core elements remained, i.e. combinations of uniformly, rotating physical orbs that brought about the observed observations. This was also a fundamental principle for Copernicus: where the two traditions diverged was in the willingness of Copernicus to put the Earth in motion, both about its own axis and about the Sun. Why was this? I don’t think religious issues were as critical as sometimes alleged: Copernicus didn’t see what he was doing as irreligious and the anti-Copernican position of the Church came later. Given the fact that there is no “Church” per se in Islam, and the range of physical possibilities (including the Earth’s motion) was extensively discussed in Islamic theological texts, the possibility always existed in Islam that a scientist might take the Copernican plunge. That none did, at least as far as we know until much later under European influence, perhaps indicates how entrenched the Ptolemaic/Aristotelian paradigm was in Islam, even among those who criticized it or tried to reform it through alternative, geocentric models. One idea that has been advanced is that in the European middle ages up to Copernicus, this paradigm was somewhat less established, allowing for alternative ideas to seem less daring or dangerous. But this will need a great deal more research as well as scholars who take seriously a cross-cultural approach.
David: Ibn al-Shatir adopted seemingly simpler structures in his geocentric model compared to Ptolemy. Is this the first example of the valuing of the concept of simplicity/economy or even elegance, in a rigorous and quantitative physical model?
Jamil: I don’t agree that Ibn al-Shatir adopted a simpler model. Here are schematic views (showing the linked vectors) for the Mercury models: Ptolemy Mercury and Ibn al-Shatir Mercury; and the physical solid-orb versions: Ptolemy and Ibn al-Shatir. (The original article, containing these figures with labels, can be found here: https://www.cambridge.org/core/journals/arabic-sciences-and-philosophy/article/mercury-models-of-ibn-alsatir-and copernicus/4795AA8292986D0EA0139DB326A43879/share/4ba76c8b7912874ada5062a214519909a90ebd11 ; see figs. 3 and 10 for schematic; figs. 5 and 7 for solid).
Ibn al-Shatir’s goal, as also other Islamic astronomers, was not simplicity per se but rather correcting what they saw as flaws in Ptolemy’s models. Besides corrections to parameters, due mainly to Ptolemy’s faulty observational data (see here), the issue was to formulate models conforming to Ptolemy’s predictive values in which all the constituent orbs were rotating uniformly, something that Ptolemy did not do. The notions of “simplicity” or “economy” in astronomy have a long history, and they often depended on how exactly the scientist viewed the concept. Ptolemy, for example, thought of an eccentric model as being simpler than an epicyclic model, which is why he opted for the former for his solar model. In the Planetary Hypotheses, Ptolemy elected to put Venus and Mercury’s orbs below the Sun because they nearly exactly fit the space between the Sun and Moon (based on a faulty Earth-Sun distance); the justification was a kind of economy of nature, that such an empty space would be superfluous if Mercury and Venus were placed above the Sun.
In Islam, the homocentric cosmology of al-Bitruji in the 12th c. might be considered a move toward simplicity, since he rejected epicycles and eccentrics and opted only for spherical orbs with the Earth as center. I have argued that Ibn al-Shatir (and Copernicus in the Commentariolus but not De revolutionibus) had a kind of compromise homocentricity, where they accepted epicycles but not eccentrics (see my “Ibn al-Shāṭir and Copernicus: The Uppsala Notes Revisited” and The Mercury Models of Ibn al-Šāṭir and Copernicus”).
David: You see Copernicus as being the beneficiary of a reconfiguration, by Islamic astronomers, of mathematical, physical, and philosophical ideas from premodern astronomy. And you are in a unique position to evaluate the details of this in their historical context. Is it possible to produce an informed estimate of the influence of religion in fostering and/or stifling the development of this reconfiguration?
Jamil: Interesting question that doesn’t have a straightforward answer. As I have argued in several publications (see esp. Islamic Reactions to Ptolemy’s Imprecisions and Freeing Astronomy from Philosophy), the Greek approach to astronomy and cosmology seems to have had a “Platonic” influence even among those philosophers and scientists who were his nominal opponents. By this I mean that the Platonic idea of a “REAL” world beyond our senses (articulated in various places but especially in the Republic, Book VII), resonated in various ways among prominent Greek thinkers from Aristotle to Proclus. The effect of this, I have argued, is that at various crucial junctures Greek astronomers and philosophers used the idea of a world beyond our senses to somehow “fudge” their models and even observational data. As Ptolemy says in the Almagest, “we should not judge ‘simplicity’ in heavenly things from what appears to be simple on earth” (XIII.2). On the other hand, Islamic doctrine, by its insistence on a universe created by an omnipotent God without an otherworldly model or template, made it the duty of intellectuals to understand this reality through our senses. Did this cause Muslims to somehow demand more accuracy in their astronomy and models that reflected the reality of what we observe? Perhaps. But it is telling that on both the level of observations and of models that most Islamicate astronomers insisted that they were formulating the reality of the created world rather than the REAL beyond the senses.
David: Thank you Professor!