1. Early Models of the Universe

2. Pythagoreans (500 B.C.)
Believed the stars, planets, sun, and moon were attached to crystalline spheres which rotated around the Earth.
Geocentric- everything moved around the Earth.

3. Aristotle (350 B.C.)
Believed the Earth is motionless at the center of the universe and all the stars and planets revolved around it.
Stars and planets moved in circular paths.
Geocentric

4. Ptolemy (140 A.D.)
Expanded Aristotle’s theories to try to account for “retrograde motion” of the planets
Planets traveled in smaller circular paths as they traveled around the Earth (epicycles and deferents) Geocentric
Popular model of the universe for 1,500 years.

5. Copernicus (1543)
Sun at the center of universe and the planets orbit the sun. Heliocentric.
Solved the problem of “retrograde motion”
Theory did not immediately replace Ptolemy’s theory.

6. Tycho Brahe (1600)
Favored an Earth-centered universe different from Ptolemy’s theory.
Thought that other planets revolved around the Sun, and that the sun and moon revolved around Earth.
His theory was incorrect, but made many precise observations of planets and stars.
Geocentric

7. Johannes Kepler (1609) Mathematician Tycho’s assistant
Used Tycho’s observational data to develop laws of planetary motion.
Kepler’s 3 Laws of Planetary Motion
Law of Ellipses
Law of Equal Areas
Law of Periods

8. Galileo Galilei (1609)
First person to use telescope for astronomical observations
Discoveries:
Galilean Satellites (Jupiter’s moons)
Craters on the moon
Sunspots on the Sun
Phases of Venus
Favored Copernicus’s theory over Ptolemy’s

9. Isaac Newton
1687
Explained why planets orbit the Sun, and why moons orbit planets.
Newton’s Law of Motion
Newton’s Law of Gravitation
Completed the work of Copernicus, Tycho, Kepler, and Galileo