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Lecture 19

PHIL 160D3 Lecture 19: Tycho, Kepler, Galileo

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Philosophy Main
PHIL 160D3
Marc Johansen

Tycho, Kepler, Galileo Tycho Brahe Rejects the Copernican model Makes his own geocentric version Copernican model strengths: accurate predictions, no equant points for perfect circle fact, simple explanation for retrograde motion Copernican model weakness: available evidence supports geocentrism, not heliocentrism Tychonic model: Sun and moon orbit the Earth, all other planets orbit the sun Tychonic model is mathematically equivalent to Copernican model Modern geocentrism: Tychonic system is best available geocentric model Johannes Kepler Believed in heliocentric model Keplers first law Law of ellipses The path of planets around the sun is elliptical The center of the sun is located at one focus Keplers second law Law of equal areas Line drawn from center of sun to center of planet will sweep out equal areas in equal intervals of time Planets do not orbit at a constant speed Speed is faster when close to the Sun Keplers third law Law of harmonies The square of the orbital period of a planet is proportional to the cube of the semimajor axis of its orbit Orbital period: time it takes for a planet to complete an orbit Semimajor axis: distance from the center of the ellipse to its longest edge A planets distance from the sun is related to the time it takes to complete an orbit Motivation: Reading the Mind of God Kepler: Why did God create a universe with exactly 6 planets? 5 perfect solids A perfect solid is a threedimensional figure such that (a) each face is an identical two dimensional figure that (b) is made of lines of equal length When nested in the right order, the relative distances between their outer edges is very close to the relative distances between the six planets on a heliocentric model of the solar system. Galileo Evidence for heliocentrism: Mountains on the moon, Sunspots, Ears of Saturn, Moons of Jupiter, New stars, Phases of Venus
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