The Birth of Modern Science
The stars move in the sky but not with respect to each other
The planets (or “wanderers”) move differently from stars
They move with respect to the stars
They exhibit strange retrograde motion
What does all this mean?
How can we explain these movements?
What does the universe look like?
The accepted model for 1400 years
The earth is at the center
The Sun, stars, and planets on their spheres revolve around the earth: explains daily movement
To account for unusual planetary motion epicycles were introduced
Fit the Greek model of heavenly perfection – spheres are the perfect shape, circular the perfect motion
Sun at center (heliocentric)
Uniform, circular motion
No epicycles (almost)
Moon orbited the earth, the earth orbited the sun as another planet
Planets and stars still on fixed spheres, stars don’t move
The daily motion of the stars results from the Earth’s spin
The annual motion of the stars results from the Earth’s orbit
In the heliocentric model, apparent retrograde motion of the planets is a direct consequence of the Earth’s motion
How do we decide between two theories?
Use the Scientific method:
These are both explanations based on the observation of retrograde motion
What predictions do the models make?
How can these predictions be tested?
Heliocentric predicts that Venus should show a full phase, geocentric does not
Unfortunately, the phases of Venus cannot be observed with the naked eye
It predicted planetary motions and events no better than the Geocentric system
The earth does not move (things do not fly off)