OUTLINE OF LECTURE 11

THEORIES OF PLANETARY MOTION FROM THE GREEKS TO NEWTON (Part I) 

Problem: How to explain the motions of Stars, Sun, Moon, and Planets?
  • The Geocentric Universe 
    • Aristotle's Dicta: 
      • i)Earth is center of the universe. 
      • ii)All bodies in the heavens (above the sphere of the moon) must move in perfect circles at uniform speed. 
    • Aristotle's explanation: Nested crystalline spheres. 
    • Click here for a more detailed summary of Aristotle's physics and his concept of the universe. 
    • Ptolemy's solution: Planets moving on epicycles, whose centers move along deferents, which, in turn, move around the earth. 

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  • The Heliocentric Universe Major Players: Copernicus, Brahe, Kepler, Galileo, Newton 

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    1) Copernicus: Promulgated the idea. De Revolutionibus (1543). 

    • Daily motion of celestial sphere due to rotation of earth. 
    • Sun's annual motion due to revolution of earth around sun. 
    • Aspects of the planets have simple interpretation. 

    •  

       
       
       
       
       
      Aspects of 
      an 
      Inferior 
      Planet
      Aspects of
      a
      Superior 
      Planet

    • Simple explanation of retrograde motions of the planets. 
    • But there were good reasons to be skeptical 
      • i) System wasn't much simpler. 
      • ii) No one had observed annual stellar parallax. 
      • iii) Seemed to violate Biblical statements (Joshua commands sun to stand still.) 
      • iv)Most important: violated Aristotle's laws of physics. If earth wasn't center of the universe, then what made things go around? (Only answered 125 years later by Newton) 
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Copyright 2007, Laurence A. marschall, Gettysburg College