1. “Geocentric vs. Heliocentric Theory”
Claims and Evidence from the Ancient Astronomers
Cornell Notes pg. 53

2. Astronomy
The study of celestial objects such as moons, planets, stars, and galaxies.
One of the oldest branches of science.

3. Moving Back in time…
Think back to over 2000 years ago. No high tech equipment for studying the far reaches of space… no idea of the law of gravity… scientists were stilling trying to develop basic explanations for the world.

4. Geocentric Theory CLAIM: Earth is the center of the universe
Sun, planets, and stars all rotate around the earth.
EVIDENCE: things appear to move around Earth; Earth doesn’t feel like its moving…

5. Basic Assumptions about the Universe…
Everything outside of Earth is perfect
Planet shapes are perfect spheres
Orbits of Planets are perfect spheres
Stars are fixed to an outer sphere

6. Aristotle BC
CLAIM: 5 elements each have a natural place in the universe- Earth at center (geocentric).
EVIDENCE: earth sinks in water…gas bubbles rise, fire rises above air… etc.
Scientific teachings were taught for over 1000 years, but never tested!

7. A Problem with the Geocentric Model…
Retrograde Motion- planets temporarily appear to move backwards

8. Ptolemy AD
CLAIM: Celestial bodies revolve around the Earth (geocentric).
EVIDENCE: created a model of space that could be used to predict future positions of planets AND explained retrograde motion

9. Heliocentric Theory
CLAIM: The sun is at the center with planets revolving around the sun.
EVIDENCE: failure of Ptolemy’s model over time; retrograde motion can be explained simply if earth is moving, too.

10. Copernicus AD
Given credit as developing the Heliocentric Theory.
Created a simpler, more accurate model that could be used to predict future locations of planets.
Other Claims: earth is spinning on axis and revolving around the sun.

11. Brahe AD
CLAIM: Moon’s orbit isn’t a perfect circle, it’s an ellipse.
EVIDENCE: careful observations with diligent records.
(Brahe actually supported the Geocentric Theory)

12. Brahe’s model of our solar system to explain the phases of Venus

13. Kepler AD
CLAIM: Planets do not orbit in perfect circles but instead orbit in ellipses.
EVIDENCE: used records collected by Brahe
This discovery was the last piece needed to perfect Copernicus’s model. (Still not widely accepted).

14. Galileo AD
CLAIM: our moon has craters and the sun has spots (they’re not perfect!); and Jupiter has it’s own moons- they don’t orbit the earth!
EVIDENCE: observations made through his carefully crafted telescope

15. Sir Isaac Newton AD
Defined the Laws of Motion and the Law of Gravity.
Now our solar system makes sense!