1. Historic Models of Astronomy

2. Greek Ideas: Earth at the Center
Thought they were inside a rotating dome with stars pasted on it
Called this dome the “celestial sphere”
Believed Earth was stationed at the center of the universe

3. Greek Ideas: Earth at the Center
Geocentric system- Earth is at the center of the revolving planets

4. Ptolemy (140 AD) Believed the geocentric model
Thought the moon and sun also revolved around earth

5. Ptolemy (140 AD)
Said planets moved in little circles that moved in bigger circles
This explained why planets seemed to wander
Theory believed for years

7. Copernicus’ Idea: Sun at the Center
Early 1500s
Developed another explanation for the motion of the planets
Thought the sun was at the center of the system of revolving planets
Called this the “heliocentric system”
Did not publish his ideas until after his death

9. Galileo Thought heliocentric explanation was correct
Used the telescope and moon phases to prove
Saw four moons revolving around Jupiter
Saw Venus go through phases
Persecuted for his ideas

10. Kepler
Found that the orbit of each planet is an ellipse (ovalish)

11. Newton- Inertia
Newton explained that inertia and gravity combine to keep the planets in orbit
The tendency of a moving object to continue in a straight line or a stationary object to remain in place is called inertia

12. Newton- Gravity
Gravity is the force that attracts all objects to one another
Strength of gravity depends on
Masses of the objects
Distance between them
Newton figured out that the planets stay in orbit around the sun because the sun’s gravity pulls on them while their inertia keeps them moving ahead