1. Our Planet and Solar System

2. Ancient and Pre-Modern Theories of the Universe/Solar System
Aristotle’s theory of four elements
Astronomy and Astrology exist
But Heavens are considered to be a separate realm from earthly objects

5. Geocentric vs. Heliocentric Models
Geocentric Model
Places Earth at the center of the universe
Objects in Heavens (planets, Sun, stars) are fixed crystalline “spheres,” nested around Earth
Big achievement: it explained the motion of the Sun, moon, planets, and stars.
Established c. 500 B.C.

7. Based on: Ptolemy c. A.D. 100-170 Observations
Belief that the heavens must be geometrically “perfect.”
Ptolemy c. A.D
Combined others’ work to create a quantitative model
Used circles within circles
Was fairly accurate for the time

8. Problems with Geocentrism
Circles within circles inelegant (or imperfect)
Didn’t explain phases of the moon
These problems not considered big enough to push alternative theory

9. Nicholas Copernicus (1473-1543)
Used a heliocentric model of the solar system
Surprisingly, not new. Proposed by a Greek astronomer Aristarchus in 260 B.C.
Copernicus applied math to the data (which wasn’t all that good)
Even though more correct, still assumed perfect circles
predictions weren’t much better than the geocentric model.

10. Tycho Brahe (1546-1601) Took really good data
Was funded by the king of Denmark and then the German emperor.
Died without knowing whether he had made any breakthroughs.

11. Johannes Kepler (1571-1630) Worked for Tycho
Approached the data mathematically, assuming circular orbits.
After 8 years, found a mathematical model for a circular orbit of Mars that almost matched Tycho’s observations. There was a difference of 8 minutes of arc.

12. His “aha” moment:
“If I had believed that we could ignore these eight minutes [of arc], I would have patched up my hypothesis accordingly. But, since it was not permissible to ignore, those eight minutes pointed the road to a complete reformation in astronomy.”
This decision to trust the data over his preconceived beliefs marked an important transition point in the history of science.

13. Kepler discovered that the planets did not orbit in circles, but rather ellipses.
Even through Kepler’s model made much better predictions, many scientists still believed in the geocentric model.
Earth couldn’t be moving, otherwise birds and thrown rocks would be left behind.
Non-circular orbits meant heavens weren’t perfect

14. Galileo Galilei ( )
Demonstrated Newton’s 1st law before Newton was even born (in 1642…)
Saw sunspots on the sun
so now the Sun isn’t perfect
Saw topography (mountains) on the moon
So the Moon isn’t perfect either
Discovered that Jupiter has moons that orbit it
Clearly don’t revolve around the Earth
Aristotle’s doctrines considered part of Catholic Church doctrine.
Galileo persecuted by church for publishing his results late in life

16. The imperfections just pile up. . .

17. Phases of the Moon

18. Retrograde Motion – Planets switch directions in their paths in the sky

19. Seasons

20. Our Solar System What was evidence for Geocentrism?
What was evidence for Heliocentrism?

21. Comets and Minor Planets

22. Extra-solar Planets