Editing Celestial spheres (section)

===Middle Ages===
[[File:Mappa Mundi 2 from Bede, De natura rerum.jpg|thumb|left|The Earth within seven celestial spheres, from [[Bede]], ''De natura rerum'', late 11th century]]

====Astronomical discussions====
A series of astronomers, beginning with the Muslim astronomer [[Ahmad ibn Muhammad ibn Kathīr al-Farghānī|al-Farghānī]], used the Ptolemaic model of nesting spheres to compute distances to the stars and planetary spheres. Al-Farghānī's distance to the stars was 20,110 Earth radii which, on the assumption that the radius of the Earth was {{convert|3,250|mi|km|abbr=off}}, came to {{convert|65,357,500|mi|km|abbr=off}}.<ref>Van Helden, ''Measuring the Universe'', pp. 29–31.</ref> An introduction to Ptolemy's ''Almagest'', the ''Tashil al-Majisti'', believed to be written by [[Thābit ibn Qurra]], presented minor variations of Ptolemy's distances to the celestial spheres.<ref>Van Helden, ''Measuring the Universe'', p. 31.</ref> In his ''[[Zij]]'', [[Muḥammad ibn Jābir al-Ḥarrānī al-Battānī|Al-Battānī]] presented independent calculations of the distances to the planets on the model of nesting spheres, which he thought was due to scholars writing after Ptolemy. His calculations yielded a distance of 19,000 Earth radii to the stars.<ref name="Van Helden pp. 31-2">Van Helden, ''Measuring the Universe'', pp. 31–2.</ref>

Around the turn of the millennium, the [[Astronomy in medieval Islam|Arabic astronomer]] and polymath [[Ibn al-Haytham#Astronomy|Ibn al-Haytham (Alhacen)]] presented a development of Ptolemy's [[geocentric model]]s in terms of nested spheres. Despite the similarity of this concept to that of Ptolemy's ''Planetary Hypotheses'', al-Haytham's presentation differs in sufficient detail that it has been argued that it reflects an independent development of the concept.<ref>{{cite book|first=Y. Tzvi|last=Langermann|year=1990|title=Ibn al Haytham's on the Configuration of the World|pages=11–25|location=New York|publisher=Garland Publishing}}</ref> In chapters 15–16 of his ''[[Book of Optics]]'', Ibn al-Haytham also said that the celestial spheres do not consist of [[solid]] matter.<ref>{{cite journal|first=Edward|last=Rosen|year=1985|title=The Dissolution of the Solid Celestial Spheres|journal=Journal of the History of Ideas|volume=46|issue=1|pages=13–31 [19–20, 21]|doi=10.2307/2709773|jstor=2709773}}.</ref>

Near the end of the twelfth century, the [[Al-Andalus|Spanish Muslim]] astronomer [[Nur Ed-Din Al Betrugi|al-Bitrūjī (Alpetragius)]] sought to explain the complex motions of the planets without Ptolemy's epicycles and eccentrics, using an Aristotelian framework of purely concentric spheres that moved with differing speeds from east to west. This model was much less accurate as a predictive astronomical model,<ref>{{cite book|first=Bernard R.|last=Goldstein|title=Al-Bitrūjī: On the Principles of Astronomy|location=New Haven|publisher=Yale University Press|year=1971|volume=1|pages=40–5}}</ref> but it was discussed by later European astronomers and philosophers.<ref>Goldstein, ''Al-Bitrūjī'', p. 6.</ref><ref>Grant, ''Planets, Stars, and Orbs,'' pp. 563–6.</ref>

In the thirteenth century the astronomer [[Mu'ayyad al-Din al-'Urdi|al-'Urḍi]] proposed a radical change to Ptolemy's system of nesting spheres. In his ''Kitāb al-Hayáh'', he recalculated the distance of the planets using parameters which he redetermined. Taking the distance of the Sun as 1,266 Earth radii, he was forced to place the sphere of Venus above the sphere of the Sun; as a further refinement, he added the planet's diameters to the thickness of their spheres. As a consequence, his version of the nesting spheres model had the sphere of the stars at a distance of 140,177 Earth radii.<ref name="Van Helden pp. 31-2"/>

About the same time, scholars in European [[medieval universities|universities]] began to address the implications of the rediscovered philosophy of Aristotle and astronomy of Ptolemy. Both astronomical scholars and popular writers considered the implications of the nested sphere model for the dimensions of the universe.<ref>Grant, ''Planets, Stars, and Orbs'', pp. 433–43.</ref> [[Campanus of Novara]]'s introductory astronomical text, the ''Theorica planetarum'', used the model of nesting spheres to compute the distances of the various planets from the Earth, which he gave as 22,612 Earth radii or {{convert|73387747+100/660|mi|km}}.<ref>Grant, ''Planets, Stars, and Orbs'', pp. 434–8.</ref><ref>Van Helden, ''Measuring the Universe'', pp. 33–4.</ref> In his ''[[Opus Majus]]'', [[Roger Bacon]] cited Al-Farghānī's distance to the stars of 20,110 Earth radii, or {{convert|65357700|mi|km}}, from which he computed the circumference of the universe to be {{convert|410818517+3/7|mi}}.<ref>Van Helden, ''Measuring the Universe'', p. 36.</ref> Clear evidence that this model was thought to represent physical reality is the accounts found in Bacon's ''Opus Majus'' of the time needed to walk to the Moon<ref>Van Helden, ''Measuring the Universe'', p. 35.</ref> and in the popular [[Middle English]] ''[[South English Legendary]]'', that it would take 8,000 years to reach the highest starry heaven.<ref>Lewis, ''The Discarded Image'', pp. 97–8.</ref><ref>Van Helden, ''Measuring the Universe'', p. 38.</ref> General understanding of the dimensions of the universe derived from the nested sphere model reached wider audiences through the presentations in Hebrew by [[Moses Maimonides]], in French by Gossuin of Metz, and in Italian by [[Dante Alighieri]].<ref>Van Helden, ''Measuring the Universe'', pp. 37–9.</ref>

====Philosophical and theological discussions====

Philosophers were less concerned with such mathematical calculations than with the nature of the celestial spheres, their relation to revealed accounts of created nature, and the causes of their motion.

Adi Setia describes the debate among Islamic scholars in the twelfth century, based on the commentary of [[Fakhr al-Din al-Razi]] about whether the celestial spheres are real, concrete physical bodies or "merely the abstract circles in the heavens traced out… by the various stars and planets." Setia points out that most of the learned, and the astronomers, said they were solid spheres "on which the stars turn… and this view is closer to the apparent sense of the Qur'anic verses regarding the celestial orbits." However, al-Razi mentions that some, such as the Islamic scholar Dahhak, considered them to be abstract. Al-Razi himself, was undecided, he said: "In truth, there is no way to ascertain the characteristics of the heavens except by authority [of divine revelation or prophetic traditions]." Setia concludes: "Thus it seems that for al-Razi (and for others before and after him), astronomical models, whatever their utility or lack thereof for ordering the heavens, are not founded on sound rational proofs, and so no intellectual commitment can be made to them insofar as description and explanation of celestial realities are concerned."<ref name=Setia>{{citation|title=Fakhr Al-Din Al-Razi on Physics and the Nature of the Physical World: A Preliminary Survey|author=Adi Setia|journal=Islam & Science|volume=2|year=2004|url=http://findarticles.com/p/articles/mi_m0QYQ/is_2_2/ai_n9532826/|access-date=2010-03-02}}</ref>

Christian and Muslim philosophers modified Ptolemy's system to include an unmoved outermost region, the [[empyrean]] heaven, which came to be identified as the dwelling place of [[God]] and all the elect.<ref>Grant, ''Planets, Stars, and Orbs,'' pp. 382–3.</ref> Medieval Christians identified the sphere of stars with the Biblical [[firmament]] and sometimes posited an invisible layer of water above the firmament, to accord with [[Book of Genesis|Genesis]].<ref>Lindberg, ''Beginnings of Western Science'', pp. 249–50.</ref> An outer sphere, inhabited by [[angels]], appeared in some accounts.<ref>Lindberg, ''Beginnings of Western Science'', p. 250.</ref>

[[Edward Grant]], a historian of science, has provided evidence that medieval scholastic philosophers generally considered the celestial spheres to be solid in the sense of three-dimensional or continuous, but most did not consider them solid in the sense of hard. The consensus was that the celestial spheres were made of some kind of continuous fluid.<ref>Grant, ''Planets, Stars, and Orbs,'' pp. 328–30.</ref>

Later in the century, the [[mutakallim]] Adud al-Din al-Iji (1281–1355) rejected the principle of uniform and circular motion, following the [[Ash'ari]] doctrine of [[atomism]], which maintained that all physical effects were caused directly by God's will rather than by natural causes.<ref name=Huff-175>{{Cite book |first=Toby |last=Huff |year=2003 |title=The Rise of Early Modern Science: Islam, China, and the West |url=https://archive.org/details/riseearlymoderns00huff |url-access=limited |publisher=[[Cambridge University Press]] |isbn=978-0-521-52994-5 |page=[https://archive.org/details/riseearlymoderns00huff/page/n195 175]}}</ref> He maintained that the celestial spheres were "imaginary things" and "more tenuous than a spider's web".<ref name=Ragep>{{Cite journal| doi = 10.1086/649338| issn = 0369-7827| last1 = Ragep| first1 = F. Jamil| last2 = Al-Qushji| year = 2001| first2 = Ali| title = Freeing Astronomy from Philosophy: An Aspect of Islamic Influence on Science| journal = Osiris| series = 2nd Series| volume = 16| issue = Science in Theistic Contexts: Cognitive Dimensions |pages = 55–57 |jstor = 301979| bibcode=2001Osir...16...49R| s2cid = 142586786| url = https://escholarship.mcgill.ca/concern/articles/m326m556v?locale=en}}</ref> His views were challenged by [[Ali ibn Mohammed al-Jurjani|al-Jurjani]] (1339–1413), who maintained that even if the celestial spheres "do not have an external reality, yet they are things that are correctly imagined and correspond to what [exists] in actuality".<ref name=Ragep />

Medieval astronomers and philosophers developed diverse theories about the causes of the celestial spheres' motions. They attempted to explain the spheres' motions in terms of the materials of which they were thought to be made, external movers such as celestial intelligences, and internal movers such as motive souls or impressed forces. Most of these models were qualitative, although a few incorporated quantitative analyses that related speed, motive force and resistance.<ref>Grant, ''Planets, Stars, and Orbs,'' p. 541.</ref> By the end of the Middle Ages, the common opinion in Europe was that celestial bodies were moved by external intelligences, identified with the [[angel]]s of [[revelation]].<ref>Grant, ''Planets, Stars, and Orbs,'' p. 527.</ref> The [[Primum Mobile|outermost moving sphere]], which moved with the daily motion affecting all subordinate spheres, was moved by an [[unmoved mover]], the [[Cosmological argument#History|Prime Mover]], who was identified with God. Each of the lower spheres was moved by a subordinate spiritual mover (a replacement for Aristotle's multiple divine movers), called an intelligence.<ref>Grant, ''Planets, Stars, and Orbs,'' pp. 526–45.</ref>