Medieval Cosmology

The beginnings of astronomy in Europe were very modest, certainly by comparison with the intellectual vigor of Muslim societies. Most Greek astronomical works – including the writings of Ptolemy and Aristotle – were not available after the collapse of western half of the Roman Empire. Knowledge of Greek was minimal to nonexistent in Europe, and very few Greek works had been translated into Latin. Astronomical knowledge was for a long time limited to fragments in Latin encyclopedias, like Pliny’s Natural History and in some commentaries on the biblical story of creation from Genesis. Scholars of the early Middle Ages had only a glimpse of the achievements of Greek astronomy. They had a basic picture of cosmos: the earth was at the center of the universe, the planets, sun, moon and stars moved around the earth on concentric spheres with uniform circular motion. However, the encyclopedic sources and commentaries on Genesis did not describe the methods by which these results had been obtained, nor did they convey the mathematical form in which they had been expressed. This meant that early medieval Europeans were unable to extend or challenge the Greek achievement – either observationally or theoretically.

Europeans in the period between roughly 500 and 1100 knew LESS about the cosmos than their Muslim neighbors. But they also knew LESS about the cosmos than the ancient Greeks. Let me give you just one example of this loss of knowledge. My example is drawn from the writings of the twelfth-century saint, Hildegard of Bingen (1098 – 1179). Although Hildegard is today better known as a saint of the Catholic Church, as a woman who had mystical visions of God, and a composer of music, she was also a natural philosopher and a physician. (You can find out more about Hildegard here and here.)  She read books on these subjects and wrote two books of her own on natural philosophy and medicine, Cause et cure and Physica. Cause et cure begins with a commentary on the story of creation from Genesis and describes the make-up of the physical world. Hildegard, like almost all medieval scholars, believed that the cosmos was geocentric and that all celestial bodies moved around the earth. But let’s look at her description of lunar eclipses:

That once in a while one can see an eclipse of the moon results from the fact that now and then the elements and the storms collide, as though they were in conflict. When this occurs the moon is neither extinguished nor waning, but, rather those storms have just darkened it for a while. But the moon’s force is such that it can overcome these storms and such that it will emit its brightness, because the moon’s force is greater than the force of these storms. (Hildegard of Bingen, 29)

This is a quite startling passage, because hundreds of years before Aristotle, Greek astronomers had discovered that eclipses of the moon happen when the earth is between the sun and the moon. (Solar eclipses happen when the moon is between the sun and the earth.) Recall that one piece of evidence Aristotle used to support his argument that the earth is spherical was the shadow cast by the earth on the moon during a lunar eclipse.  Aristotle stated: “in eclipses the outline is always curved: and, since it is the interposition of the earth that makes the eclipse, the form of this line will be caused by the form of the earth’s surface, which is therefore spherical.”  Further, eclipses are not phenomena that occur “once in a while” or “now and then.” They are not random; they are highly regular and predictable. Not only does Hildegard seem unaware of this predictability, but she also has no knowledge of the true cause of eclipses.

It was certainly unusual for a woman to be as highly educated as Hildegard, and even rarer for a woman to write scientific books. However, in some other ways Hildegard is a representative example of an eleventh- and twelfth-century scholar. Most scholars, like Hildegard, were monks or clerics. Most collections of books of any size were in monasteries or other religious institutions. To the extent that literacy and learning were preserved at all in Europe after the fall of the Roman Empire it was because they were actively supported by the Catholic Church.

Europe began very gradually to become more prosperous and more urban beginning in about the ninth century. The economy recovered, population levels began to rise, old cities grew in size and new cities were established. Greater wealth and urbanization led to an increase in schools, in literacy and in scholarship. It also led to increased contact with the richer and more sophisticated societies of the Islamic Empire and the Byzantine Empire. Some of these contacts were negative, like the Crusades, but there was increased trade and travel, and some European scholars began traveling to parts of the Islamic and Byzantine territories in order to learn from scholars there.  Some of the earliest intellectual contacts were initiated in the tenth century, when a few scholars from northern Europe crossed into Spain to learn Arabic and study science in the Islamic world. Remember that for a long time Spain was part of the Islamic world, not the Latin west. By the tenth century, almost all Greek astronomical works, including those by Aristotle and Ptolemy, had been translated from Greek into Arabic. While some Europeans took the trouble to learn Arabic so they could read these works, along with the subsequent original work of Islamic astronomers, they also felt the need to make this material accessible to scholars who could read Latin but not Greek or Arabic. So one of the first results of increased contact between the Islamic and European Christian world was an extensive program of translation.  Some of this translation was done by Europeans who had to first learn Arabic.  And some of it was done by Islamic scholars who had to learn Latin.  (A prime example of this second group was Constantine the African [d. 1099], whose work we will discuss in our section on medieval medicine.)

This wave of translations, which went on throughout the twelfth and thirteenth centuries, made the works of the ancient Greeks accessible to Europeans for the first time ever. This is sometimes referred to as the “recovery” of the Greeks by Europeans. In reality, Europeans gained access to the writings of the ancient Greeks for the very first time in this period. They also gained access to several centuries of original astronomical, mathematical, philosophical and other scientific work written in Arabic. Almost all of Aristotle’s works were translated into Latin by 1200, along with Arabic commentaries and work based on Aristotle (for example, the work of Averroes, 1126 – 1198). Europeans now had a complete picture of the Aristotelian cosmos. The terrestrial realm was made of earth, air, fire and water, which moved in particular ways according to their natures and natural places. The celestial realm was made up of a series of concentric spheres, each moving with uniform, circular motion. The celestial spheres were nested perfectly inside each other; there was no void space. And all the spheres had a common center, which was the earth’s center. Ptolemy’s Almagest was translated into Latin as well. The first translation was directly from Greek in about 1160. A later, and much more widely used translation was made from the Arabic in about 1175, by Gerard of Cremona (ca. 1114–1187), and Italian translator working in Toledo, Spain. The text’s title, the Almagest, is its Arabic title.  With the translations of this text, and of Arabic works of mathematics and mathematical astronomy, European scholars finally had a comprehensive exposition of what Ptolemy and his predecessors, and their Islamic successors, had achieved in mathematical astronomy. They could finally understand how celestial phenomena could be predicted using geometrical models. And they could grasp how the numerical parameters of the Ptolemaic models had been derived from observations. Europeans also became away of the additions, critiques and corrections Islamic scholars had made to the ancient Greeks.

As part of the general rise in the educational and intellectual level of Europe in the eleventh and twelfth centuries, the very first universities in the West were founded. The oldest European university is the University of Bologna in Italy, established in 1088. Some other very old universities are Padua (1222) in Italy, Paris (1160) and Montpellier (1289) in France, Oxford (1096) and Cambridge (1209) in England, and Vienna (1365) in Austria.

All of these universities shared certain features. The language of instruction was exclusively Latin, and the curriculum was similar throughout Europe. The undergraduate curriculum consisted of moral philosophy, natural philosophy, and metaphysics. This was all heavily based on Aristotle. The three higher (or graduate) faculties were theology, law and medicine (in order of their prestige). Not all universities had all three higher faculties (most Italian universities just had law and medicine, and the medical faculties at Oxford and Cambridge were very small). The fact that the language of instruction and curriculum were the same throughout Europe meant that students and professors could easily move from university to university. And in fact, it was far more common to study at multiple universities than it is today. Universities only admitted men, and with rare exceptions, Christians. Natural philosophy – that is, what we would call science – was part of the basic undergraduate curriculum. All university-educated men learned the basics of the Aristotelian cosmos. Only a few went on to learn the complex mathematical models of Ptolemy and his Islamic successors. But everyone learned about earth, air, fire, water, quintessence and the geocentric universe.

Here is a picture of the cosmos from the most widely used university astronomy textbook of the Middle Ages and early modern period, Johannes de Sacrobosco’s On the Sphere, composed in around 1230.  (The image is from a later, printed version.)  Note the concentric spheres of earth (the dark circle at the center), water (Aqua), air (Aer), fire (Ignis), then the sphere of the moon (Sphera Lune), the sphere of Mercury (Spera Mercurii), the sphere of Venus (Spera Veneris), the sphere of the sun (Spera Solis), the sphere of Mars (Spera Martis), the sphere of Jupiter (Spera Jovis), and the sphere of Saturn (Spera Saturni).  Outside the sphere of Saturn is the “firmament or the heaven of the fioxed stars (Firmamentum sive celum stellarum fixarum).  The outermost sphere is the “last heaven or the Prime Mover” (Celum ultimum sive primum mobile).  For medieval university students and professors, the Prime Mover was generally identified as God.

This brings me to a crucial feature of medieval scientific endeavor: the reconciliation of Greek natural philosophy (and in particular Aristotelian natural philosophy) with Christianity. There are a number of features of Aristotle’s work that were directly contradictory to Christian teachings. First, Aristotle posited that the world was eternal, that is, it had no beginning and would have no end. Christians believed that the world was created by God, and would ultimately be destroyed by God. Second, Aristotle asserted that the soul was mortal and that there was no afterlife. Christians believed in an afterlife that included heaven, hell and (eventually) purgatory. Christians flatly rejected these two aspects of Aristotle’s thought. Note that these points were problems with Aristotelianism that Muslim scholars had already had to deal with. They did not end up rejecting Aristotle because he was a pagan or because some aspects of his natural philosophy were contradictory to their religion, and neither did Christians.  This is a measure of how powerful and persuasive Aristotle’s thought was.

A number of other aspects of Aristotle’s thought were not necessarily incompatible with Christian doctrines, but they were (arguably) incompatible with a strictly literal reading of the Bible. Let me first explain the concept of biblical literalism, specifically how this concept was understood in pre-modern period (that is, until the eighteenth century). A literal interpretation of the Bible treated passages of the Bible as descriptions of events that actually happened in the past. The literal meaning of many, if not most, biblical passages was neither simple nor obvious. It could not be arrived at simply by picking up the text and reading it. It required a highly educated, sophisticated reader – a trained theologian – who brought the full resources of a long and rigorous training to bear on the text to produce a literal interpretation. Almost all theologians (both Catholics and later Protestants) would have vehemently denied that an uneducated, untrained reader could interpret the Bible. In modern America, biblical literalism has connotations of a simple, naive, even anti-intellectual approach to the Bible. Biblical literalism had none of these connotations in the past.

Bearing this definition of biblical literalism in mind, let’s consider some of the tenets of Aristotelian natural philosophy that we have studied thus far and consider how they appeared to medieval Christians.

1) The terrestrial realm is made of the four elements earth, water, air and fire, and the celestial realm is made of the quintessence. The Bible doesn’t state explicitly that God made five elements and then created the cosmos out of these elements. However, nothing in the Bible contradicts this. There is no statement in Genesis (or anywhere else) that there are more or less than these five elements. And Aristotle’s physics had a great deal of explanatory power, so there was every reason to accept it.

2) The cosmos as a whole is a sphere. Again, nowhere in the Bible is the universe described as a sphere. Is there any passage of the Bible that says the universe is NOT a sphere? Here the situation is a little more complex. Arguably, there ARE passages from the Bible that contradict the notion that the universe is spherical. For example, in Isaiah 40:22, God is described as He, “that stretcheth out the heavens as a curtain, and spreadeth them out as a tent to dwell in.” Curtains and tents are not spherical. Does describing the heavens as a tent stretched out over the earth suggest that the heavens are NOT spherical? By and large, European Christians decided that this passage was not meant to be taken literally. It was not a description of the actual physical world, but a poetic metaphor.

3) The earth is a sphere at the center of the cosmos. There is no passage of the Bible that describes the earth as either spherical or central. Further, these propositions too are at odds with some biblical passages. In Job 9.6, God is described as He “Which shaketh the earth out of her place, and the pillars thereof tremble.” If the earth rests on pillars, it clearly can’t be a sphere. And it clearly can’t be in the center of a spherical cosmos. (What the pillars rest on is a whole other question.) Again, most educated Christians agreed that this passage (and others like it) was not an actual physical description of the world.

4) The earth is stationary. This Aristotelian idea actually finds positive support in the Bible. To take just one passage that seems to confirm that the earth is stationary (and that will have particular importance to debates over the Copernican system):

Then spake Joshua to the Lord in the day when the Lord delivered up the Amorites before the children of Israel, and he said in the sight of Israel, Sun, stand thou still upon Gibeon; and thou, Moon, in the valley of Ajalon. And the sun stood still, and the moon stayed, until the people had avenged themselves upon their enemies. Is not this written in the book of Jasher? So the sun stood still in the midst of heaven, and hasted not to go down about a whole day. (Joshua 10:12-13)

This passage certainly suggests that in the normal course of events, the sun and moon move across the sky. In this miracle, God makes these celestial bodies stand still so that the Israelites can have enough time to win a decisive battle.

As these examples suggest, Aristotle’s natural philosophy is largely incompatible with a strictly literal reading of the Bible.  Here is a modern reconstruction of the Hebrew cosmos based on a literal reading of the Bible.  Doesn’t look anything like the lovely spherical Aristotelian cosmos above, does it?  Why then were Aristotelian ideas so widely accepted in medieval Europe?

One answer to this question is that medieval Christian intellectuals believed that God had endowed human beings with reason, and He expected them to use this reason to understand the created world. There was clear sensory and empirical evidence that the earth was a sphere, for example (reread the sections on Aristotle if you can’t recall what this is). To deny this evidence merely because a few passages of the Bible seem to imply that the world is flat would be to refuse to use one’s God-given capacity to sense, experience and reason about the natural world. Another answer is that medieval scholars saw in Aristotle (and other Greeks like Ptolemy) and comprehensive and convincing explanation of a wide range of natural phenomena. And Christian intellectuals, like Muslim and Greek intellectuals, strongly preferred naturalistic explanations to supernatural explanations. It was all very well to acknowledge that God could perform miracles (like stopping the sun), but they refused to explain ordinary events like eclipses and storms and earthquakes by attributing them to the wrath of God.

One of the most prominent medieval intellectuals was the theologian Thomas Aquinas (1225 –1274). Aquinas was crucial in making Aristotelian natural philosophy acceptable to Christians. He asserted that philosophy and theology were compatible. Philosophy used the natural (God-given!) faculties of the human senses and reason. Theology, by contrast, was about divine revelation. Both roads lead to the truth – they might lead to different truths, but they never contradicted each other. Aquinas used the metaphor of the “two books.” God revealed Himself to human beings through the Book of the Bible, but He also revealed Himself through his other creation, the “Book” of Nature. Aquinas certainly believed that theology was superior to philosophy, but he believed, and convinced many other prominent intellectuals, that philosophy could be useful to theology.

One of the ways Aristotelian philosophy was useful to theology was that it could be used to help interpret the Bible. Recall that I said producing a literal reading of the Bible required training and erudition. Thomas believed that theologians could use the findings of Aristotelian natural philosophy to produce better LITERAL interpretations of the Bible. To see what this looked like in practice, let’s examine Aquinas’ interpretation of the story of creation from Genesis. The account of creation in Genesis has God creating “the heaven” on the first day (“In the beginning God created the heaven and the earth.” Gen. 1:1) and the “firmament” on the second day (“And God said, Let there be a firmament in the midst of the waters, and let it divide the waters from the waters.” Gen. 1:6) Also, Genesis describes God separating the waters above the firmament from the waters below the firmament (“And God made the firmament, and divided the waters which were under the firmament from the waters which were above the firmament: and it was so.” Gen. 1:7). None of this sounds particularly Aristotelian – it sounds like there are two celestial realms (the heavens and the firmament) and water (a terrestrial element) in the celestial realm. Aquinas proposed an Aristotelian interpretation of this story, one that used Aristotelian natural philosophy to explain the story of creation.

Aquinas distinguished three heavens in the celestial realm: the Empyrean; the crystalline, or aqueous; and the firmament, which consisted of the traditional 7 planetary spheres and the sphere of the fixed stars. Aquinas had ten separate celestial spheres distributed over the three heavens. The Empyrean was the heaven brought into being on the first day. It was the place where angels were created. The firmament was the heaven created on second day. It was the visible heaven with the stars, sun, moon and planets. The crystalline or aqueous sphere was the water above the firmament, referred to in Genesis. This sphere was solid but transparent, and it was different from elemental water in terrestrial realm. Aquinas’ Christianized Aristotelian view of the cosmos became the dominant one in the Middle Ages and early modern period. Here is a picture of it from a popular sixteenth-century text.

However, some theologians were concerned about the growing dominance of Aristotle in the university curriculum. In particular, they were troubled by the claims of some natural philosophers that there were certain things God COULD NOT do because they were physically impossible (that is, they would violate Aristotelian physics). Two examples of these things were 1) God COULD NOT make a moving earth, and 2) God COULD NOT have created more than one world. Now, the Bible does not claim that God DID make a moving earth, or that He created more than one universe. So why should it be a problem to say that these things were impossible according to Aristotle? The problem, for some theologians, was that these arguments put limits on the power of God. In 1277, the Bishop of Paris, Stephen Tempier, issued a list of 219 forbidden propositions, most having to do with Aristotelian philosophy. These are known as the Condemnations of 1277. One very important category of these forbidden propositions was things that God allegedly could not do, because Aristotelian philosophy had demonstrated their impossibility. The Bishop asserted that philosophers could not put limits on divine omnipotence. The Condemnations were only ever in force at the University in Paris, but they had a significant influence on the development of science in Europe. They stimulated (or forced) natural philosophers and theologians to engage with Aristotle in new ways.

All Bible quotes are from the King James Version