1. Observing the Solar System
Notes

2. Earth at the Center
Most early Greek astronomers believed that Earth was the center of the universe.
They also noticed that a few “stars” in the sky seemed to follow paths separate from the others. The Greeks called them planets from their word for “wanderers.”
The Romans would later label these planets Mercury, Venus, Mars, Jupiter, and Saturn after their own gods.

3. Earth at the Center
A model of the universe in which Earth is at the center is called a geocentric system. In a geocentric system, all planets and stars revolve around Earth.

5. Sun at the Center
A system in which the sun is at the center is called a heliocentric system. Earth and other planets revolve around the sun in a heliocentric system.
In 1543, a Polish astronomer named Nicolaus Copernicus developed a good heliocentric model of the universe, but still contained a few flaws.
Despite the fact that he was mostly correct, his theories would still be rejected for over 50 years.

7. Sun at the Center
In the 1600s, the Italian scientist Galileo Galilei made discovers that supported the heliocentric model.
For example, using his telescope, Galileo discovered the four largest moons of Jupiter (known now as the Galilean Moons), proving that not everything orbited Earth.
His observations of a full set of phases for Venus also supported a heliocentric view of the universe.
Galileo too was ridiculed for his theories. He was even put under house arrest by Pope Urban the VIII for “vehement suspicion of heresy,” where he would eventually die.

9. Sun at the Center
Johannes Kepler worked for an astronomer named Tico Brahe. Brahe was worried that Kepler was smarter than him so he only allowed him to work on minor tasks, such as figuring out Mars’ orbit.
However, this would actually allow Kepler to make the greatest discovery of his life.
Kepler discovered that not only Mars, but all of the planets travel in a path in the shape of a flattened circle, or an ellipse.

11. The Scientists