1. Observing the Solar System

2. Greek Observations
Greek astronomers believed Earth to be the center of the universe. This is known as a geocentric system.

3. The Copernican Revolution
Copernicus thought that the sun is at the center of the system of planets. This system is called a heliocentric system.

4. Galileo's Evidence
Galileo Galilei used the newly invented telescope to make discoveries that supported the heliocentric model.
Galileo saw 4 moons revolving around Jupiter, proving not everything revolves around Earth.
Galileo saw Venus go through a series of phases, which wouldn’t happen if it only circled around Earth.

5. Kepler’s Calculations
Kepler, who was once the assistant to a successful astronomer that died before their research was complete, had concluded from his calculations that the orbit of each planet is an ellipse instead of a perfect circle.

6. Modern Discoveries
Since Galileo’s time, we have discovered Uranus, Neptune, and Pluto (which is no longer a planet).
Today we know that the solar system consists of the sun, the planets and their moons, and several kinds of smaller objects that revolves around the sun.