1. Models of the Solar System
Chapter 27
Section 2

2. Two Basic Models of the Universe
Geocentric –
The Earth is located in the center and all other celestial bodies orbit around the Earth.
Heliocentric -
The Sun is located in the center and all other celestial bodies orbit around the Sun.

3. Aristotle (384 – 322 BC)
Greek philosopher
Developed the first geocentric universe (Earth-centered)
Believed the Sun, Moon, five visible planets (Mercury, Venus, Mars, Jupiter, and Saturn), and the stars orbited the Earth.
By 1500s this theory was believed as fact.

4. Challenges to a Geocentric Universe
The 5 visible planets seem to wander in the sky.
Each evening, they move east, briefly slow down, then reverse direction (retrograde motion)

5. Claudius Ptolemy Greek astronomer Wrote the Almagest
expanded Aristotle’s theory by explained how the circular pattern of the planet’s motion creates retrograde motion depending on the size of the circle.
Was able to predict the locations of the planets in the night sky.

6. Nicholaus Copernicus (1473-1543)
Proposed a heliocentric model of the universe (Sun centered)
he theorized that the retrograde motion of planets could be explained in a heliocentric system as a result of differences between the time it takes each planet to orbit the Sun.
His theories were dismissed

7. Johannes Kepler (1571 – 1630)
Polish mathematician and astronomer used the measurements of Tycho Brache
Theorized that the planets revolve around the sun in elliptical orbits and that the sun is not in the exact center of the orbits.
Suggested a force within the Sun that has great effect on nearby objects

8. Galileo Galilei ( )
1609 constructed a small telescope with which he could observe mountains on the Moon, four Moons orbiting Jupiter, and Venus goes through phases like our Moon.
His observations confirmed the incorrect statements of a geocentric universe.

9. First to explain inertia – motion is due to a force and a moving body will stop or change direction only in response to a force.
Concluded a force pulls all falling bodies to the earth – identified the force as magnetism.

10. Isaac Newton ( )
Reasoned that the force identified by Galileo acts on an apple, so it must also act on the moon.
Developed the law of universal gravitation – every body in the universe attracts every other body and that attraction is affected by size and distance.