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Greek Principles of Understanding Nature • Nature can be understood, not just random events controlled by whims of some gods • Diverse behavior in nature held together by underlying patterns How interesting!! Euclid (600 BC) • Where he was ‘wrong’ – Earth is a flat, rotating disc – Water is basic material; all matter comes from water • Where he was ‘right’ – Predicted solar eclipses – Moon shines because of sun’s reflection – All universe understandable by ordinary reasoning Pythagoras (530 BC) • First to suggest Earth is a sphere • Early advocate for geocentric solar system – Earth at center, surrounded by concentric, rotating, transparent spheres – Bodies attached to spheres in order: Moon, Mercury, Venus, Sun, Jupiter, Saturn, and stars Greek cosmology after Pythagoras How did they know the difference between stars and planets? Planets were the wanderers. Problem with retrograde motion of planets occasional backward motion of seen from Earth Eudoxus (370 BC) • Introduced more than just 7 spheres • Suggested 27, then 55 spheres to explain cycles and epicycles Aristotle (340 BC) • Strong advocate for geocentric view • Developed system of scientific reasoning • Four ‘proofs’ why Earth is a sphere Symmetry - Sphere is perfect shape Pressure - Pieces of Earth fall naturally to the center, pressing it to a spherical shape (circular logic) Shadow - During eclipse Earth’s shadow always circular North Star - North star gets higher in sky the further north one goes LA North Seattle LA North Seattle LA North Aristarchus (240 BC) • Spinning or rotating of Earth on its axis accounts for daily motion of stars • Earth moves or revolves around the sun in yearly orbit • Had it right 200 BC, but ideas did not catch on! Eratosthenes (235 BC) Calculated Earth’s radius to within 2% - maybe. Uncertainty about unit of length (stadia) he used. Ptolemy (AD 120) • More elaborate schemes of epicycles to explain observations • No significant changes for next 1300 years • Ptolemaic synonymous with geocentric view of solar system Copernicus (1500’s) Major Break with Past! • Beginning of end of geocentric view • Advocated heliocentric view, but had no proof – Sun at center of solar system – Planets in order outward from Sun: Mercury, Venus, Earth (moon), Mars, Jupiter, Saturn • Ideas not accepted in his lifetime (died 1543) • Resisted by church • Copernican synonymous with heliocentric view of solar system Sources of web images (1/2): Euclid http://www.storyofmathematics.com/hellenistic_euclid.html Pythagoras http://www.sciencephoto.com/media/227667/enlarge Greek cosmology http://hendrix2.uoregon.edu/~imamura/123/lecture-5/lecture-5.html Discovery of Pluto http://en.wikipedia.org/wiki/Pluto Mars retrograde http://apod.nasa.gov/apod/ap080511.html http://www.sjaa.net/eph/0711/d.html Eudoxus http://www.crystalinks.com/eudoxus.html Aristotle http://www.catholiceducation.org/articles/apologetics/ap0016.html Source of web images (2/2): Aristarchus http://www.nndb.com/people/756/000096468/ Eratosthenes http://www.eg.bucknell.edu/physics/astronomy/astr101/specials/eratosthenes.html Ptolemy http://en.wikipedia.org/wiki/Ptolemy http://abyss.uoregon.edu/~js/ast123/lectures/lec02.html Copernicus http://en.wikipedia.org/wiki/Nicolaus_Copernicus