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History of Astronomy Day Four Greeks were really smart… Figured out… Size of the Earth Size of the Moon Distance to the Moon Distance to the Sun Size of the Sun Early Greek Theory Science needs both observations and theories, but the observations must match the theories! Greeks had some holes in their early theory. What to do??? Greek Problems Their theories couldn’t predict the motion of the planets or explain retrograde motion. The planets looked like they would start moving backward! https://youtu.be/0bkxcf0vwQ4 Retrograde Motion All planets orbit the sun in the same direction (eastward). The planets appear to move backward because the planet closer to the sun (the inner planet) moves faster. When the inner planet passes the outer planet, each sees the other as moving backwards (relative to its usual motion). Scientists in hundreds of cultures all over the world tried to predict and explain planetary motion. Greek Problems The Greeks also could not explain why summer was longer than winter. Greeks were really smart… but they had issues. What are your options when your theory is not quite correct 1. Throw out Observations 2. Throw out the Theory 3. Modify the Theory The Greeks chose to modify their geocentric theory. Problem #1: Ptolemy solved their issues predicting the motion of the planets with epicycles . . . sort of. Greek Idealism Recall…theory called for… Perfect Unchanging Heavens Objects in the heavens must move “perfectly” . . . so what about retrograde motion??? . . . they never said much more about that . . . Late Geocentric Theory Problem 2: They had no explanation why summer is longer than winter. So, instead of developing a new theory, they just shifted the orbits off center to explain how summer was longer than winter. Is their solution getting simpler or more complex? The simple explanation tends to be the right explanation. Not to mention the fact they were trying to prove the Earth was at the center . . . and then they moved it out of the center, to prove that it was at the center?! However . . . .