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Medieval Astronomy and Cosmology

Page history last edited by lindsey 15 years, 9 months ago



     One must take care to distinguish between cosmology, astronomy, and astrology, especially as they were studied in the Middle Ages. Cosmology, the study of the nature of the cosmos, was primarily derived from Aristotle's On the Heavens. Aristotle's cosmology was adapted to take biblical references, such as the separation of the firmament and the waters into account. Astronomy was an observational approach to the heavens that seeked to use empirical results to devise adequate predictive tools. The source of most Middle Age knowledge on astronomy comes from Ptolemy's Almagest, in translations from both Greek and Arabic. Astrology in turn, focuses on the power of the celestial bodies to influence events in the terrestial realm. This originates in the philosophy of microcosm and macrocosm in which the events on earth mirror the developments in the heavens.

     In the 12th century, a flood of new knowledge reached Medieval scholars. They absorbed and developed the new ideas in two forms, questions and commentaries. Commentaries would include a specific text taken directly from another work, which the author proceeded to clarify and argue. Questiones were a form in which the author posed a series of yes or no questions and then argued both for the negative and positive finally supporting only one case.  The downside was no authoritative text explaining the whole of medieval thought in a subject emerged.


Medieval Cosmology: 

     Initially, medieval philosophy concerning the cosmos was Platonic. One of the important representatives of medieval cosmological studies was Robert Grosseteste, who wrote a number of studies in the first part of the thirteenth century. A central idea in Grosseteste's cosmology was light. The cosmos was created by God when he created a dimensionless point of light. The point immediately diffused itself into a large sphere, drawing matter with it and giving rise to the corporeal cosmos. Subsequent radiation and differentiation gave rise to celestial orbs and the characteristic features of the sublunar region. Much of Grosseteste's work involves a theme of microcosm (earth) and macrocosm (heavens). He, along with his medieval bretheren, believed that humanity was God's greatest creation, and thus simultaneously mirrored the divine nature and the structural principles of the created cosmos. Grosseteste also shared an early medieval belief in a homogeneous cosmos. In his cosmology, the heavens are made of finer stuff than the terrestrial substances, but the difference is quantitative rather than qualitative.

     Later medieval cosmology became based primarily on Aristotle's On the Heavens. However, Aristotle's propositions did not always agree with the biblical descriptions of the universe. Several issues concerning the cosmos arose such as the eternity of the world, the Unmoved movers, the waters seperated by the firmament and limits placed by Aristotle on God's powers. Scholars such as Moses Maimonodes, Nicole Oresme, and Jean Buridan addressed these issues in their commentaries. For instance, Oresme said that it would be plausible to have any number of concentric worlds, each following world placed within the sphere of the earth of the former. He dismissed this idea though as unlikely and claimed there is only universe. The arguments were made in order to satisfy the condtion that God is omnipotent and could create many worlds if he so desired.

     Another point considered was the possibility of God moving the universe. This meant that a place had to exist outside the universe, to which it could be moved. Some scholars claimed God would simply create the space as He moved the universe. Others like Thomas Bradwardine and Nicole Oresme considered the void space to exist. Bradwardine associated the void with God's omnipresence. The argument for a void outside of the universe was based on the Stoic argument of thrusting an object outside the periphery.

     Since God is omnipotent, the motion of the fixed stars versus a rotating Earth also required discussion. Jean Buridan was convinced the rotating Earth or rotating sphere of stars were equivalent and an observer would not be able to tell the difference. He did, however, decide that the argument of an arrow shot into the air discussed by Ptolemy among others, decided in favor of rest. Nicole Oresme disagreed, and claimed the arrow shared the circular motion of the Earth, as did the observer and so it appeared to have only a vertical component of motion. He gave other reasons for a rotating Earth as well. With the Aristotelian principle of rest being nobler to motion, it would be fitting that God would be at rest. There was also no requirement for a ninth sphere that would give the rotation and no excessive speeds were necessary. The Earth's motion would be fastest, and the motion of each planet as it approaches the fixed stars progressively slower. In the end Oresme decided in favor of rest, however, citing biblical passages.

     The cosmological model thus remained largely as Aristotle had formed it, although a few changes were made. Without concern for astronomical details, the number of spheres was reduced to 9 or 10. In order from the Earth these included one for each of the planets: Moon, Mercury, Venus, Sun, Mars, Jupiter, and Saturn, followed by the firmament with fixed stars, a crystalline sphere (waters above the firmament) and an empyreum (abode of the angles). The spheres were continous, in perfect contact, rotated frictionlessly and were composed of aether, with the planetary orbs being areas of greater density. Whether the spheres were fluid or solid was seldom discussed but both alternatives found supporters. As for the Unmoved Movers of Aristotle various replacements were found. The Prime Mover was considered to be God. The other movers were thought of as angels or some intelligences without bodies. Jean Buridan, through his impetus theory, argued that no movers were required at all.



     Medieval astronomy was distinct from cosmology largely by pedigree: cosmology was based on the writings and traditions of Aristotle, and commentators of his works, while mathematical astronomy stemmed from Ptolemy's Almagest and conserned itself with movement of celestial bodies as seen against the sphere of fixed stars. Early medieval scholars had little to no access to any of the mathematical astronomy produced by Hipparchus, Ptolemy and others. Knowledge of astronomy was thus limited until the tenth and eleventh centuries when contact with Islam increased. People such as Gerbert of Aurillac, who traveled to Spain, contributed greatly to the resources available on astronomy. Translations and commentaries floated into the Latin world as well as useful instruments like the astrolabe. Furthermore, major works of astronomy such as the Almagest and Al-Farghani's The Rudiments of Astronomy became available. In the universities, teachers began to devise their own accounts of important astronomical works through which knowledge of astronomy (usually basic) would be passed down. The Theorica Planetarum and The Sphere of Sacrobosco became the chief works. Scholars did believe that the physical and mathematical descriptions of the universe should be integrated, but there was no consensus as to how this could be done. Some scholars, such as Roger Bacon, resurrected the idea of solid spheres representing Ptolemy's epicycles put forward by Ibn al-Haytham. From this consideration, with each sphere allowed just enough thickness to include the epicycles, an estimate of the size of the universe could be made.  



Medieval astrology drew largely from Ptolemy's Tetrabiblos, which was translated in 1130.  Astrology was regarded as important and true, but not deterministic, as this would have conflicted with the Christian doctrine of free will.  Another primary text on the subject was Albumasar's Introduction to the Science of Astrology which drew largely on the Tetrabiblos. It placed much stock in the conjunctions of planets - when two or more planets lined up. These moments were seen as turning points in history, the rarer the conjunction, the greater the meaning. Albumasar drew also from Platonic ideas of the world soul. Medieval astrology was also Aristotelianized, with the adoption of Aristotle's metaphysics of matter, form, and substance, and also his notion of celestial bodies being the source of all motion (Lindberg 275).  Astrology also became entwined with medicine and was considered by any able doctor in his works.



Primary Sources



Key Terms and Definitions


Tetrabiblos:  Ptolomy's famous and influential discussion of astrology

conjunctions: important turning points in astrology correlating to the special arrangements and alignments in the planets.

Toledan Tables:  Islamic Astronomical tables influential in western astronomy in the middle ages.

Alfonsine Tables:  The first set of astronomical tables calculated in the west.  They were also very widely used and influential in the west in the middle ages.

Impetus:  A force applied to a moving body that keeps it in motion.  First put forth by John Buridan in an explanation of the motion of the heavens.  God's impetus at the beginning and creation of the universe keeps the celestial spheres moving because there is no impedence to their motion.

Questiones:  The dominant form of Cosmological writings in the 14th through the 16th centuries.  The writings are in the form of questions that generally have yes or no answers, and then an answer is stated with an explaination. 

conjunction: when two or more planets form a straight line.



Relevant Links


Comments (8)

liz mastroianni said

at 12:35 pm on Oct 8, 2008

didn't have any info to add just changed the wording around a little

jgm829@... said

at 1:54 pm on Oct 8, 2008

I did a little editing and added some information on Robert Grosseteste.

Grant Berry said

at 3:07 pm on Oct 9, 2008

I added a differentiation between astronomy, cosmology, and astrology, as well as major influences.

Grant Berry said

at 3:07 pm on Oct 9, 2008

I just realized I added some of the same stuff on Grosseteste. I don't know where it best fits. Somebody move stuff around.

jonathan stutte said

at 7:40 pm on Oct 12, 2008

added a lengthy albeit poorly written account of astronomy

Garrett McCormack said

at 9:03 pm on Oct 13, 2008

I added the definitions

Kristy Carey said

at 12:20 am on Oct 14, 2008

I added the astrology portion and did some editing.

Alyson Collins said

at 12:49 pm on Oct 14, 2008

Made some basic changes in grammar, sentence structure, verb tenses, citation and misspelling. The system at Truman was down so long I can't really add any more info now though I think there are still some gaps that could be filled if anyone still needs to make some changes/additions.

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