NATS 1745 Lecture Notes - Lecture 5: Circular Motion, Firmament, Scientific Revolution

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12 Feb 2016
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Lecture 5
Copernicus:
oPlanets moved (revolved) around the sun with uniform circular motion (constant speed)
oDidn’t want to publish his ideas, as it went against the church techniques
oRemoved earth from the center of the universe --- undignified
oStudied Cannon Law, Medicine, Financial administration
oMotivation:
1. Asked by church to improve Ptolemy’s model --- predictions of the moon, sun, planets and stars
were becoming inaccurate
2. Personal: Aesthetics---Ptolemy’s model was ugly; “postulate” of Copernicus’s Heliocentric Theory
oMotions of the ‘firmament’ (stars, planets) arises due to motion of the earth
oEarth rotates around its own axis and it revolves around the sun
oUniverse is an enormous dimension with stars far away
oHeliocentric model recovers an Aesthetic model
oExplains retrograde motion as a perspective effect
oIn 1543, his book was released the Revolution of the Heavenly Spheres
oHowever, his model was not any more accurate than Ptolemy’s, as he imagined orbital motion in perfect
circles
oPublisher --- Osiander --- gave an introduction but wasn’t very happy with it
oBenefit of the model: simpler, aesthetic and retrograde motion explained
Tycho Brahe:
oHe created tables of data
oDidn’t believe in Copernicus’ model----devised his own hybrid model --- geo-heliocentric model
oBuilt two observatories (Hven, Prague)
oMade exceedingly accurate positional measurements of the stars and planets
oObserved --- supernova ----- published 1573
oObserved comet --- 1577 --- led to question Aristotle’s idea that the heavens where eternal, perfect and
unchanging
othus his own model
ohired Johannes Kepler
What was Tycho Brahe's main contribution to our understanding of the Universe?
He observed a supernova and comet. This is significant because it disproves Aristotle's philosophy that the
universe is unchanging and perfect.
Johannes Kepler:
oThe secret of the universe, defended Copernicus’ model but no prove
o3 laws of planetary motions:
1. Planets’ orbits are elliptical with sun at one focus
2. Planets move faster when they are near the sun, they sweep out “equal area in equal time”
3. A square = p cube
oAfter Brahe died --- he finally got all the extensive and accurate data of the planets. Esp. Mars
oHence improved Copernicus’ model
oPlanets moved in ellipses
oBlue e=0.7; Red e=0.3, Green e=0
oPredictions of where planets are supposed to be at a certain time were more accurate.
Review
How did Copernicus' model improve on Ptolemy's?
What is retrograde motion, and how is it explained in the context of the heliocentric Universe?
What was Tycho Brahe's main contribution to our understanding of the Universe?
How did Kepler's insights and techniques contribute to the Scientific Revolution?
Describe Kepler's 3 Laws of planetary motion.
Define semi major axis, and eccentricity, for an ellipse.
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