Short History of Cosmology - Illinois State Universityhmb/phy206/Class_History.pdf · Toward...
Transcript of Short History of Cosmology - Illinois State Universityhmb/phy206/Class_History.pdf · Toward...
Building a Cosmology
Imagine yourself observing the night sky (600 BC) You observe:
• The Sun: travels in arc overhead • The Moon: travels in arc; has phases • Stars: constellations rotate together, nightly
and seasonally • Planets: complex arcs with different periods
Cultural Worldview
Regular geometric shapes are the most perfect
Circles and spheres are true perfection
Simple ratios yield perfect harmonies
Human beings are special
Make Sense of Observations in Worldview Context
All objects seem to travel across our sky Simple Model #1:
• The Earth must be at the center • The Sun: circular orbit, once per day • The Moon: circular orbit, once per day • Stars: attached to sphere which rotates • Planets: Venus, Mars, Jupiter, Saturn have circular orbits with periods in simple ratios
Model 1: Test
Model 1 does explain:
• Nightly motion of Sun and Moon across sky • Nightly rotation of constellations
Well, so far so good...
Model 1: Test (continued)
Model 1 does not explain: • The phases of the Moon (28 day period) • The seasonal change in constellations
(365 day period) • The orbits of the planets
(both complicated arcs and periods) Model 1 fails simplest test: it does not explain all
the basic observations!
Improving Model 1
What: Add more to Model 1 to improve it
How?? Creative guessing! This is the essence of science
Guidelines (cultural bias + experience) Simplicity: “Everything should be made as simple as
possible, but no simpler” - Einstein
Symmetry: Nature seems to like it, humans certainly do
Improving Model 1: Example
Consider orbit of Mars • Generally follows arc, but sometimes
moves backward • Epicycle! Circle on a circle:
Improving Model 1: Example (cont.)
Why is epicycle a good solution? It can explain the observation of planets “going
backwards”
It retains the circle as the primary element, and circles are “perfect”
E
Greek Model Improvement
Pythagoras (600 BC): Musical circles/spheres (like model 1)
Eudoxas (400 BC): 27 spheres, different periods and axes
Aristotle (350 BC): 54 spheres; outermost: “seat of
unchangeable order”; inner: “unsettled and changeable”
Hipparchus (100 BC): offset centers and epicycles
Ptolemy (100 AD): “wrote the book” on ancient cosmology, a full treatise which would become THE book for a millenium+
Society and Politics I
Greeks at Athens (600-300 BC) • Learning encouraged (Pythagoras-Aristotle) • Flourishing society could afford schools
Greeks/Egyptians at Alexandria (300 BC - 400 AD) • Learning encouraged (Hipparchus-Ptolemy)
• Built the “Museum”: school and huge library; like an international research institute
Romans (200 BC - 400 AD): • Learning tolerated; some scholars move East
Society and Politics II
Christians in Europe (200 AD - middle ages) • Alexandria Library destroyed in 389 • Learning discouraged: e.g. St. Augustine (400 AD): “It is not necessary to probe into the nature of things as was done by (…)
the Greeks. (…) It is enough for the Christian to believe that the only cause of all created things (…) is the goodness of the Creator.”
Arabs in Middle East (100 BC - middle ages) • Applied learning encouraged: Haroum al-Rashid’s
“House of Wisdom” in Bagdad • Preserved Greek writings rescued from Alexandria • Deveolped new number system and algebra
The Cosmology Saga: continued
Review Aristotle/Ptolemy Cosmology: Earth-centered model “Perfect” circular orbits, spherical shells Multiple epicycles; offset centers Fit data fairly well
Book “lost” after destruction of Museum, preserved by Islamic scholars
A New Idea?
Dissenting Greek: Aristarchus (100 BC) Sun-centered (heliocentric) model Still assumed perfect circular orbits Didn’t fit some observations as well as
geocentric epicycle model Explained some observations more simply Aristotle’s reputation and insistance that Earth
does not move held sway Heliocentric model forgotten for more than a
millenium...
Politics and Society: middle ages
Moslems take southern Spain: Cordoba becomes new center of learning New library collects Arabic translations of
remaining Greek manuscripts and new Arab works
Cordoban astronomers (Gabir Ben Aflah & Mur Ed Din al Betugi) question Ptolemaic system No new model forthcoming
Politics and Society: middle ages
Christians Ease up Charlemagne (784): monasteries should have
schools Thomas Aquinas: Natural laws created by God,
can be learned with rational methods Christians retook Spain (1100’s), discovered
libraries Church embraces Aristotelian/Ptolemaic
cosmology (1200’s): Earth centered is now Church doctrine!
Politics and Society: middle ages
Toward Renaissance Scholars begin to speak out Roger Bacon: Reliance on ancients and Church
dogma not good enough Erasmus: argues against Church practices
Politics Fall of Constantinople: scholars flee to Europe Holy Roman Empire disintegrates; National
monarchies rise.
Ptolemaic System Supreme (?)
Ptolemaic system used to compute Alfonsine astronomical tables and calendar Very difficult, complicated calculations Alphonse X: “If God had asked my advice, I
would have suggested a simpler design for the Universe”
Inaccuracies develop in computation of celestial events: not even that accurate...
Renaissance (1300’s - 1600’s)
Across-the-board emergence of new ideas, creativity Art: explosion of nature and human art Invention: printing press, e.g. Exploration: age of transcontinental voyages Science: Cosmology, motion, medicine...
Cosmology: Copernicus
Copernicus
Observation and good idea: Sun-centered cosmological model Circular orbits with 34 epicycles Afraid to publish it: Inquisition Urged by follower, it was published in 1543
when Copernicus is near death
Preface: “These hypotheses need not be true or even probable; if
they provide a calculus consistent with the observations, that alone is sufficient”
Copernicus and the Church
Catholic Curch banned his book Bruno, 1590’s, is burned at stake for
“worshipping the sun and other heresies” However: Pope Gregory used his model to
compute new Gregorain calendar in 1582!
Protestant countries more open: England, Germany, Denmark
Tycho Brahe
Protestant country, Denmark, gives him land for an observatory (1560’s) Remember: no telescopes! Lifetime of observations, the most detailed ever
of planetary motion
Designed his own Earth-centered cosmology
Johannes Kepler
Born in Germany, moves to Prague to escape Cathollic/Protestant fighting (1560’s) Inherited Tycho’s data upon his death (1601) Tried to fit it with mystical “perfect solids” and
“perfect harmonies” models
Pretty good fit, but off by 8 minutes of arc!
Johannes Kepler (continued)
Kepler: “Since divine goodness has granted us a most
diligent observer, Tycho Brahe, from whose observations the error in this calculation of 8 minutes is Mars revealed, it is fitting that we recognize and make use of this good gift of God with a graceful mind.”
He threw out his own mystical beliefs based on the 8 minute error! A true scientist...
Johannes Kepler (1571-1630)
New Model based on Tycho’s data: Heliocentric cosmology with planets
orbiting Sun in Elliptical orbits Planet-Sun line sweeps out equal areas in
equal times (Period)2 (Avg. radius)3 ∝
Kepler’s “Laws”
Period - Radius relation Means planets closer to Sun have shorter
period All planets have same relation
P2
r3 = same number for all planets