1. Ancient cultures studied the sky and developed calendars based upon the patterns they saw
Motion of the sun
Motion of the moon
Motion of the stars
Ancient culture based their calendars on repeating cycles they observed in the sky.

2. What is a day? What they saw:
The time it takes the sun to cross the sky and return to the same point
What we know:
Time required for the Earth to rotate once on its axis

3. What is a month? What they saw:
The amount of time required for the moon to move through a complete set of phases
What we know:
The amount of time it takes the moon to revolve around the Earth

4. What is a year? What they saw
The time it takes for a group of stars (constellation) to return to the same part of the sky at a certain time of day
What we know:
Time required for the Earth to revolve around the sun.

5. Early Models of the Universe

6. Pythagoreans (500 B.C.)
Believed the stars, planets, sun, and moon were attached to crystalline spheres which rotated around the earth.
(geocentric)

7. Aristotle (350 B.C.)
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.
(geocentric)

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

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

10. 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 the Earth.
Theory incorrect, but made many precise observations of planets and stars.

11. 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

12. 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

13. 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