1. Early Ideas

2. Early Astronomers
Ancient astronomers could recognize the difference between stars and planets
These astronomers believe that the Sun, planets, and stars orbited a stationary Earth
This is called the geocentric model (Earth centered)

3. Geocentric Model

4. Early Astronomers (cont.)
Normally planets, as observed from Earth, move to the east
Sometimes a planet will look like it moves the opposite direction (west)
This is retrograde motion
In 1543, Nicolas Copernicus suggested the sun was the center of the solar system
This is the heliocentric model (Sun centered)

5. Heliocentric Model

6. Kepler’s First Law
The ideas of Copernicus were not very accepted unit further evidence was discovered
Tycho Brahe made very accurate observations of planetary positions to help him out
Johannes Kepler discovered that the planets travel in ellipses, rather then a circle
Each planets elliptical orbit is different in size and shape
The distance between the Sun and Earth is 1 AU (astronomical unit)

8. Eccentricity
Because planets have elliptical orbits they have points that they are closer and farther away from the sun
Perihelion is when the planet is closest
Aphelion is when the planets are furthest away

11. Kepler’s Second Law
Kepler found an imaginary line between the Sun and planets sweeps out equal amounts of area in equal amounts of time

12. Kepler’s Third Law
Kepler also figured out a relationship between the size of it’s orbit and the time it takes the planet to orbit the Sun (P2 = a3)
This was his 3rd law
While Kepler was figuring the solar system out mathematically Galileo Galilei developed the telescope
His discoveries proved the heliocentric model

13. Gravity and Orbits
Isaac Newton started figuring out gravity by looking at the Moon’s motions
He figured out that any two bodies attract with a force that depends on their masses and distance apart
This is known as the law of universal gravitation