1. Geocentric Theory and Heliocentric theory

2. Geocentric Theory Who? Greek philosophers and astronomers
What? The earth is stationary and at the center of the universe
Planets orbit on the surface of hollow, transparent spheres
Stars were attached to the outermost sphere
Who? Greek philosophers and astronomers
Hipparchus – ( BC) Greek who developed a mathematical geocentric model and cataloged 850 stars
Ptolemy – (AD ) Egyptian who attempted to improve the geocentric theory

3. Geocentric theory Problems:
1. Planets sometimes appear to be large, bright, and close; other times, smaller, dimmer, and farther away.
2. Could not explain why certain planets were observed to slow down, stop, and then back up before resuming their forward motion.
3. Inaccurate – there were discrepancies between observations and the model

4. Heliocentric theory What? The sun is at the center of the solar system
Who?
Copernicus – stationary sun and moving earth; wrote The Revolutions; Copernican theory = heliocentric theory
Tycho – very accurate astronomical observer
Kepler – laws of planetary motion; modified and improved heliocentric theory; orbits are elliptical; wrote New Astronomy
Galileo – first to see Saturn’s rings; first to see Jupiter’s moons; tried by the Catholic Inquisition for promoting heliocentric theory
Newton – law of gravity; according to tradition, he was inspired by an apple falling from a tree