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.
- The Heliocentric Universe Major Players: Copernicus, Brahe, Kepler, Galileo, Newton
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.
- 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)
- Models of Planetary motion: A nice page with animations describing the Ptolemaic and Copernican approaches.
| Go back | Astronomy Home Page |
Copyright 2007, Laurence A. marschall, Gettysburg College