1. The Geocentric Universe Aristotle (4 th cent. BC) Aristotle wanted to know “Why?” Used “philosophy” to guide his theory. Crystalline celestial spheres. Hipparchus (2 nd cent. BC) and Ptolemy (2 nd cent. AD) Made or used observations of planetary positions. Epicycles upon equants - complications.

2. The Geocentric Universe Medieval versions Everything is centered on the earth (nearly the lowest, nastiest possible point of the cosmos). Observations are proving problematic - theoretical predictions over the course of 1200 years are becoming unsustainable. The Church needs good astronomy, e.g. to help with calendar-making.

3. The Heliocentric Universe Nicolaus Copernicus (1473 – 1543) Continued to hold celestial spheres to be central. Copernicus needed as many epicycles as Ptolemy; quantitative results were really no better. Qualitatively, this made a bit more sense. De Revolutionibus was not considered a problem by the Church - until Galileo. It was a champion worst-seller.

4. The Heliocentric Universe Galileo Galilei (1564 – 1642) Used the telescope to make new observations. Moons of Jupiter; phases of Venus; mountains on the moon Ran into the Inquisition and Counter-Reformation; bad timing. Galileo was pretty obnoxious! His works were banned, along with Copernicus’s.

5. Galileo His reports of the phases of Venus were very inaccurate. His sketches of the Moon’s surface are unrecognizable. His “proof” of his ideas, the mechanism of the tides, was incorrect. He knew of Kepler’s more accurate theory of elliptical orbits, but did not want to give up on perfect circles.

6. Galileo Sketches of the moon Moons of Jupiter, 1610 1/21/10, midnight 1/22/10, 2:00 AM 1/22/10, 6:00 PM 1/23/10, midnight

7. The Heliocentric Universe Johannes Kepler (1571 – 1630) Used new, careful observations of Tycho Brahe Discarded (after a lot of struggling) the spheres – planetary orbits are elliptical – a new theory. Observations (he could also use a telescope) confirmed that his was a better model. But why did it work?

8. The Gravitational Universe Isaac Newton (1642 – 1727) – Formulates the Law of Universal Gravitation; partially answers the “Why?” question. Neither Earth nor Sun is the center. Albert Einstein (1879 – 1955) – Formulates the Theory of General Relativity; eliminates “spooky action at a distance.” Are we back to Aristotle?