Chabot College Astronomy

Scott Hildreth

Geocentric vs. Heliocentric Theory Comparison Study Sheet


This worksheet is intended to help you understand and review the observed motions of the sky, and how the two different models of our solar system accounted for or explained those motions. A second benefit of completing this worksheet may also be to help you see how any science progresses, by developing theories and models to explain phenomena, and revising those theories as new observations are made. Answers will be posted (and discussed in weekly sections).


For each of the observations listed below, note how each model explained why the observation occurred. The first has been completed for you as an example. You might want to answer these on a separate piece of paper.

 Geocentric Explanations  Heliocentric Explanations  Comments
 1. Stars rise in the East, and set in the west, over 24 hours. The sky is a fixed celestial sphere The Earth rotates on its axis once per day circling the earth  Geocentric observers felt the sky moved around the Earth. Since it does not appear the Earth is spinning, the only other option is that the sky must move. The Heliocentric theory accounted for this observation by having the Earth spin on its axis once per day.
 2. The sky seems to revolve around Polaris, the North Star.      
 3. The Sun moves east among a select group of stars (the zodiac constellations).      
 4. The Sun rises at different points along the eastern horizon, reaches different max. heights, and sets at different points in the west, during the year. (Seasons)      
 5. The Moon goes through phases.      
 6. The planets show retrograde motion, appearing sometimes to move backwards in the sky.      
 7. The stars do not seem to shift positions ("Parallax").      
 8. Venus shows different phases and sizes (in a telescope).      
 9. Jupiter shows four moons that stay with the planet as it moves in the sky (with a telescope).      
 10. Polaris was not always the north star in our sky. ("Precession")      

Sample answers are noted here.

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