The Evolution of Our Understanding of the Solar System By Brian Fontaine

A panoramic view of the night sky.

All objects rise and fall because of the Earth’s rotation on its axis.

Aristotle looks at sky and notices that all objects rise and set. He concludes that the Earth must be the center of the universe and thus the center of the solar system. This becomes known as the geocentric model. He explains the backward motion of some planets as epicycles. Aristotle: Greek Philosopher Ancient Greece ~300’s B.C. 1.

Geocentric Model

Aristotle uses a theory called “Epicycles” to explain the strange backward motion of planets. 3. Retrograde motion was the apparent backwards motion of planets as they moved through the sky. Planets seemed to move backward in the night sky Epicycles 2. Retrograde Motion

Claudius Ptolemaeus: Ptolemy: Italian Astronomer 100 A.D. Ptolemy agrees with Aristotle but explains the strange phenomenon of retrograde motion by advancing theory that of “Epicycles”. 2

Nicolas Copernicus: Polish Astronomer Late 1400’s Nicholas Copernicus, was the first astronomer to formulate a scientifically based heliocentric theory that displaced the Earth from the center of the universe. He believed the sun was the center of our solar system. 4

Heliocentric Model

Galileo confirmed Copernicus’ heliocentric theory but broke an agreement not to speak about it to the church. He was placed under house arrest during the Spanish Inquisition. Galileo Galilei- Italian Astronomer Early

6. Geocentric Model

7. Heliocentric Model

“The Earth is in the Middle” Aristotle Ptolemy

“The Sun is in the Middle” Copernicus Galileo

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