• the superlunar world was perfect and unchanging and had different laws of physics
• Plato argued that since the circle is the perfect form, everything in the superlunar world must move in circles
• but the changes in position of the planets are surprising: https://www.youtube.com/watch?v=kbynKfNfHk4
• the best working out of a system of circles to describe the actual motions of the planets was provided by Ptolemy in the late Hellenistic period
• he used circles on circles, epicycles, and other devices such as eccentrics--which made less and less physical sense
  
• Arab astronomers struggled to reconcile the model with theories of physics at the time
• but mostly astronomers were concerned with calculations, not physics
  

Copernicus (1473-1543) thought the system was too messy

• he simplified it by putting the sun at the center, explaining retrograde motion by the relative motion of the earth and the planet
  
• but he still believed all heavenly motion must be perfect circles so he still needed epicycles and eccentrics for accuracy
  
• it could give better calculations (though initially it did not), but without a new theory of physics it didn't make much sense, so not many people took it seriously for 50 years
• the counter-evidence was the lack of relative changes in positions of the stars and planets  depending on which side of its orbit the earth is on (parallax)
  
• however at this point many astronomers made their living from astrology, and easier calculations and better tables were a big help
• the church accepted astrology if it was not a prediction of the future but information about a person based on when they were born
• astrology also included weather prediction and predicting the best date to go to war or for other important events
  

Tycho Brahe (1546 – 1601) was shocked to see a new star (the heavens were supposed to be unchanging, comets were understood to be below the moon)

• he measured that comets were beyond the moon--set up a system to take very accurate observations
  
• he proposed a compromise system were the planets revolve around the sun and the sun revolves around the earth

• Kepler had discovered a relationship between the distances of the planets from the sun and the basic three dimensional figures
• after much struggle realized the best way to describe the motion of the planets was as ellipses, not circles
• he needed a pretty strange physics to explain this

Galileo Galilei (1564-1642)

• heard about the invention of the telescope and reinvented it
• turned it to the heavens in 1609
• discovered mountains on the moon, moons of Jupiter, phases of Venus, and sunspots, all contradicting Aristotelian ideas
• experience--what you can observe with your senses--is challenging authority
  
• started down the path of figuring out a new physics in which the copernican theory would work
• this was a sensitive time because of the protestant reformation
  
• also seemed to contradict the bible, particularly the sun standing still in Joshua 10: 12-13
  
• he tried to show how to interpret the Bible to avoid contradictions--this was a bad time for someone who wasn't a priest to be proposing interpretations of the Bible in Catholic Italy
• in 1624 the Copernican theory was condemned by the church

But no one had yet worked out a physics to explain the Copernican theory

• Rene Descartes proposed a system of vortices
• Isaac Newton (1642-1727) worked out the laws of inertia and gravity, though he had to invent calculus to do so
• but the idea of action at a distance was troubling even to Newton, who explained it as the action of God