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Lecture 10

AST 101 Lecture Notes - Lecture 10: Deferent And Epicycle, Natural Philosophy, Celestial Sphere

Course Code
AST 101
Walter Freeman

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From Ptolemy to Kepler
Branch of “natural philosophy”
oSaw behavior of nature as something we can understand
oProposed natural, not supernatural causes for things when possible
oProposed models for things in nature
Used mathematics in these models
Any model made had to agree with observation
oSaw astronomy as a separate discipline from natural philosophy (split between
groups of people)
Learned Earth was round and Moon is lit by the Sun, not from within in solar eclipse model
Nat Philosophy:
Concerned with fundamental truth of things
Very concerned with logic
Saw heavens as mostly outside their purview
Concerned with PREDICTING the motions of stars and planets
Not all that concerned with the “transcendent truth” of models
Wrote Almagest – a treatise on astronomy
Celestial sphere doesn’t account for ellipses orbits ad retrograde motion
Circle being orbited by loop to loop motion
oIt worked – not far off from actuality
Alexandria in Egypt – center of scholarly work – burned down
Muslims accumulated knowledge about motions of the sky, named stars, refined
Ptolemy model, and made strides in mathematics
Europe – Ptolemaic model was known, mostly just applied (didn’t have scientific tradition)
Copernicus (early 1500s)
Proposed that everything orbits the Sun in perfect circles
oPtolemy – geocentric (earth is center)
oCopernicus – heliocentric (sun is center)
This allowed him to explain retrograde motion – without epicycles
Less precise than Ptolemy’s at predicting celestial motion
Perfected the telescope
There are things orbiting Jupiter
o4 moons, “Galilean moons”
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