Up: Astronomy 9 Lecture Notes

ASTRONOMY 9: HISTORY OF COSMOLOGY

Handout #12

J. E. Baker
UC Berkeley, Spring 2000

Cosmology in the Middle Ages

1.

Early Middle Ages (approx. 200-1200 AD)

Background
- Rise in power of Christianity as state religion
- Instituted by Emperor Constantine (324 AD)
- Roman Empire divided into West (Latin) and East (Greek, Byzantine)
- Time of great turmoil (invasion, social decay, corruption ...)
- Attempts to repulse Germanic and Persian invaders fail; Rome falls in 400s, start of European ``Dark Ages''
- Catastrophic, fatalistic world-views mirrored in salvational themes of Christianity
Greek curiosity about nature and love for reason are lost
Natural philosophy no longer of interest, concern only with abstract theological questions
Very few Greek writings survive
Plato's Timaeus still widely read, also encyclopedic works of Latin compilers (Pliny, Macrobius, Capella, Chalcidius)
Literal interpretation of scripture generally the final authority on cosmology
Scholarly works all in Latin, knowledge of Greek fades
Loss of Greek knowledge near total; from about 300-1000 AD, most ``educated'' western Europeans believe in flat Earth!
Neo-Platonism generally dominates during this period
St. Augustine (354-430)
- Leads a wild, pleasure-seeking life until age 31, then adopts Christianity after trying many other philosophies
- Philosophy seen as mere handmaiden to theology, not a distinct path to truth
- Strongly influenced by Platonic ideas
- Division of world into Forms vs. imperfect, shadowy reality, parallels the Christian ``Fall'' from grace
- Plato's ideas wedded to Christian dogma
- The Confessions: amplifies Plato's distrust of knowledge gained through the senses
- Curiosity about the world, knowledge for its own sake, seen as an inherently sinful pleasure
- Ideas further amplified by Augustine's followers
St. Lactantius
- Demolishes ``pagan'' idea of spherical Earth in Divine Institutions (written 302-323)
- Earth believed flat for first time in almost 1000 years!
- Argues against possibility of the antipodes
Cosmas
- First detailed description of early medieval cosmological system, in Christian Topography (written 535-547)
- Cosmos as a giant Tabernacle
  - Much like earlier Babylonian/Hebrew views
  - Earth as flat rectangle or disc at the bottom of the universe
  - Cosmas was widely traveled, should have known better!
  - Earth surrounded by waters, also celestial waters above
  - Heavens are not spherical because Isaiah says God ``laid them out like a tent''
  - Earth is tilted so the sun can go literally ``down''
  - Tabernacle divided into two Aristotelian zones by firmament
  - Angels carry stars daily below the firmament
Philoponus (6th c.)
- Argues against the common abuse of scriptures to prove non-spherical heavens
- Ideas have little influence on dominant Church
Isidore (570-636)
- Bishop in western Church
- Also mentions spherical heavens and earth, going against the orthodoxy
Bede (approx. 673-735, English monk)
- Another unorthodox teacher of spherical Earth
- Moves Earth from cosmic bottom back to center
- Influenced by Pliny
Virgil or Fergil (8th c., Irish ecclesiastic)
- Threatened with expulsion from Church for teaching of spherical Earth
- But not much happens; condemnation of cosmological heresy not at all strict
Gerbert
- Noted scholar and astronomer, becomes Pope Sylvester II in 999 AD
- Spherical Earth now firmly reinstated
Immutability of the Heavens
- Like Aristotle, believed in perfect, unchanging celestial realm for Moon and above
- Appearance of supernovae in 1006 and 1054 barely even noted, though 1006 as bright as quarter Moon!
- As evidence of celestial change, clashed with medieval world-view
- Chinese astronomy reached high-point, left records of many celestial events including the SNe
By 1000, European knowledge roughly same as 500 BC Greece!

2.

Cosmic Innovation in Arabia (approx. 700-1250 AD)

Preservation and translation of many important Greek works (Ptolemy, Aristotle, Euclid, ...)
Significant contributions to mathematics (algebra, numerals, $\sin$ function, ...)
Re-introduction of Greek texts to Europe via Islamic Spain starting in 1200s
Sparked rebirth of scientific curiosity in Europe at end of Middle Ages
Built large, organized observatories for new tables of planetary motion

(a)

Adalusian school (Iberian peninsula)

Followers of Aristotle's physics: uniform, geocentric, circular motion is only natural possibility
Rejected Ptolemaic equant (non-uniform), epicycles and eccentric (not geocentric) as against physical principles
Reverted to Aristotelian model with concentric spheres
Predictions for planets off by up to 21 $^\circ$ !

(b)

Maragha school (Persia)

Desired to improve accuracy of Ptolemaic model with new data and mathematical tricks
al-Tusi (d. 1274) invented Tusi couple
- Combination of two circles, one smaller inside the other
- Can produce variety of motion, even linear!
Ptolemy's eccentric and equant replaced with Tusi couples
Back to uniform circular motion, with little loss of accuracy

3.

Late Middle Ages and Early Renaissance (approx. 1200-1500)

By mid 1200s, translations of Aristotle and Ptolemy come to Europe via Arabia and Spain
Philosophers comment on ancient Greeks, add few new ideas
Aristotle's influence eclipses Plato's
St. Thomas Aqunias (1227-1274)
- Marriage of Aristotelian ideas and Christian dogma
- Revelation still most important source of knowledge, but natural philosophy recognized as a distinct path to truth, followed when not directly contradicting scripture
- Aristotelian physics instituted as dogma, not challenged
- Believed Aristotle already contained all that could be known about the world, nothing left to discover!
- Problems uniting Aristotle with Christianity:
  
  (a)
  Eternal vs. created cosmos
  (b)
  Only one world--a limit on God's power?
Roger Bacon (1214-1294)
- Unlike most, did not follow Aristotle as dogma
- Stressed use of experiment as path to truth
- Establishment of scientific method
- Imprisoned for two years for promoting ``dangerous novelties''
Dante (approx. 1300)
- Divine Comedy presents dominant medieval view of cosmos
- Hierarchical chain of being, walled-in, finite cosmos
- Aristotelian lunary/sub-lunary division subdivided into many concentric shells
- Outer shell is most perfect, angelic; Lucifer at center of Earth
Nicolas of Oresme (1323-1382, France)
- Wrote Book of the Heavens and the World
- Familiar with Heraclides' idea of rotating Earth
- Notes relative nature of motion
- Argues rotation of Earth more natural than rotation of heavens!
- Refutes his own ideas at the end of the work
Nicolas of Cusa (1401-1464, France)
- Argues for unbounded, eternal universe in On Learned Ignorance
- Earth moves!
- Forced to write apology, quotes ancient philosophers
Regiomontanus (1436-1476)
- Completes Peuerbach's translation of Ptolemy's Almagest from original Greek
- Discovers errors up to 2 $^\circ$ in planetary motions, eclipse ends one hour early
Columbus (1492 trans-Atlantic voyage)
- Did not have to convince anyone of spherical Earth (fiction invented by American writer Washington Irving)
- Knew of accurate 9th c. Arabic computation of circumference
- Got value 25% too small by assuming Arabic mi = Roman mi
- Wrong Ptolemaic geography, thought Asia extended far east
- Used to justify voyage
- Europe not at cosmic center

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Up: Astronomy 9 Lecture Notes

jonathan baker
2000-02-23