Summary Table of Solar System Models

This is a comparative, incomplete summary of three key models put forth by the Greek civilization (Ptolemy), the Renaissance (Copernicus), and a modern view based on precise measurements by Brahe (Kepler). These scientists all considered their key problem to be predicting the positions of the planets as viewed from the Earth. 
              Ptolemaic                                 Copernican                                 Keplarian

Fundamental Ideas
The Heavens are perfect.    The Heavens are perfect.    The heavens are not perfect.
The Sun is not a star.                                  There is an underlying,
                                                           unifying pattern.    

Observations
Motions of the planets.     Motions of the planets.     Motions of the planets.
Moon's phases and motions.  Moon's phases and motions.  Moon's phases and motions.
Daily motion of the stars.  Daily motion of the stars.  Daily motion of the stars.

Model
Geocentric, with everything  Heliocentric, with every-  Heliocentric, with elliptical 
  moving in perfect circular   thing moving in perfect    orbits and non-uniform 
  orbits at uniform speed.     circular orbits and        speed.
  This required epicycles,     uniform speed.  More
  or little orbits around      epicycles.
  larger orbits.

Predictions
Positions of the planets    Positions of the planets    Positions of the planets 
  within the accuracy of       within the accuracy of      within the accuracy of  
  observations of the time,    observations of the time,   observations of the time,
  ~2-5 o.                       ~2-5 o.                     ~10'-1 o.
The Earth is still.         The Earth itself moves.     The Earth itself moves.

Tests
Failed to predict planets'  Failed to predict planets'  Predicted planets' positions
   positions to Brahe's        positions to Brahe's        (except Mercury) to modern
   accuracy.                   accuracy.                   accuracy.   
No stellar parallax, so     Venus goes through all      Newtonian laws of motion.
  the Earth doesn't move.      phases, from new to full.Galileo observed the Sun
                            Earth does move in space:      and Moon to be imperfect.
                               stellar parallax measured.

Philosophy
Earth is center of          Sun, not Earth, is center   Planets do not undergo
   Universe.                   of universe.                natural motion.  A
                                                           force is required.
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Beth Hufnagel's home page, email: hufnage4@pilot.msu.edu
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