1. History of our Knowledge of the Solar System
Early Ideas
Geocentric Universe
Early Astronomers
Separated Stars from Planets
Planets movements can be tracked because stars do not move.
Ptolmey: Geocentric Universe
Earth is the center of the universe.
All the planets and the Sun revolve around the Earth

2. Geocentric Model of the Universe
Problems
1). Retrograde Motion
A sudden change in planetary motion
Planets switch from moving East to moving West
Very hard problem to solve
Scientists began looking for a better model of the universe/solar system.

3. Retrograde Motion Solved
Copernicus
Retrograde Motion Explained
In 1543, he suggested a heliocentric model of the universe
Sun centered
Earth and all other planets orbit the Sun
Why do we see planets moving backwards?
Inner planets move faster then outer planets around the Sun.
Earth will “pass” a slower moving planet
This planet appears to move backward temporarily.

4. Kepler’s Laws 1st Law Astronomical Unit
Most planets orbit the Sun in an elliptical shape
More oval like
Earth being the exception
Earth believed to move between an elliptical orbit and a circular orbit every 100,000 yrs or so.
Planets orbit while staying centered around 2 points.
Sun is one point
How we measure the average distance between the Sun and planets.
Sun to Earth = 1 astronomical unit (AU)

5. What is Eccentricity Eccentricity Details
Ratio of distance between the foci to the semi-major axis.
Change in distance from the focus points; such as distance of a planet from the Sun.
Planets are not always the same distance away from the Sun.
The point in orbit when the planet comes nearest to the Sun = Perihelion
The point in orbit when the planet is farthest from the Sun = Aphelion

6. Gravity
In the late 16th century early 17th century Galileo was working with gravity.
Performed experiments dropping objects off the Tower of Pisa and rolling balls down inclines
Galileo found
Gravity accelerates the fall of all objects at the same rate.
Air resistance causes lighter objects to fall more slowly.

7. Gravity Sir Isaac Newton Basics Two objects attract each other
1687 Newton published his theory of universal gravitation.
Also called the inverse square law
This theory helped discover Neptune.
Watched Uranus’s movements
Something large was affecting the movements of the planet.
Two objects attract each other
Depends upon their mass AND the distance between them.

8. Law
This knowledge of gravity produced the law of universal gravitation.
The larger the objects (m) the stronger the force of gravity between them.
The farther apart the objects (d) the weaker the force of gravity.
Distance squared weaker
G = Constant
X 10-11