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. Believed the stars, planets, sun, and moon were attached to crystalline spheres which rotated around the Earth. Geocentric- everything moved 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 rotated around it. Believed the Earth is motionless at the center of the universe and all the stars and planets rotated around it. Stars and planets moved in circular paths. Stars and planets moved in circular paths. Geocentric Geocentric

4. Ptolemy (140 A.D.) Expanded Aristotle’s theories to try to account for “retrograde motion” of the planets 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) Planets traveled in smaller circular paths as they traveled around the Earth (epicycles and deferents) Popular model of the universe for 1,500 years. 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. Sun at the center of universe and the planets orbit the sun. Heliocentric. Solved the problem of “retrograde motion” 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. 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. 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. His theory was incorrect, but made many precise observations of planets and stars.

7. Johannes Kepler (1609) Mathematician Mathematician Tycho’s assistant Tycho’s assistant Used Tycho’s observational data to develop laws of planetary motion. Used Tycho’s observational data to develop laws of planetary motion. Kepler’s 3 Laws of Planetary Motion Kepler’s 3 Laws of Planetary Motion Law of Ellipses Law of Ellipses Law of Equal Areas Law of Equal Areas Law of Periods 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 1687 Explained why planets orbit the Sun, and why moons orbit planets. Explained why planets orbit the Sun, and why moons orbit planets. Newton’s Law of Motion Newton’s Law of Motion Newton’s Law of Gravitation Newton’s Law of Gravitation Completed the work of Copernicus, Tycho, Kepler, and Galileo Completed the work of Copernicus, Tycho, Kepler, and Galileo