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Class 3Laws of planetary motion

Galileo confirms the heliocentric model!

Kepler’s three laws of planetary motion

I : Definitive evidence for theheliocentric model

400 years ago, Galileo looked at the skywith a telescope for the first time

Dramatic discoveries followed Phases of Venus

Venus changes size significantly and also hasphases (like the Moon);

Varies from fully illuminated to a thin crescent In geocentric model, you would only ever expect

to see thin crescent phases of Venus. Observations clearly disagree with geocentric

model

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Expectations from thegeocentric model…

Expectations from the heliocentric model

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Real observations of Venus

Galileo’s discoveries (cont) Moons of Jupiter

Galileo found four bright moons close to Jupiter Could track their orbits… they were definitely

orbiting Jupiter If things can orbit Jupiter, why not the Sun?

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II : Kepler’s laws of planetarymotion

Assuming that planets orbit the Sun incircular orbits still did not preciselyexplain the positions of the planets

Tycho Brahe (1546-1601) Collected best pre-telescope data on

position of stars, planets (+ a supernovaand a comet)

One of first scientists to carefully thinkabout errors and error bars!

Data finally explained by the mathematicianJohannes Kepler

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Kepler’s first law : The orbit of aplanet about the Sun is an ellipsewith the Sun at one focus

Notes

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If ellipse hassemimajor axis a andsemiminor axis b, theeccentricity is definedas

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Kepler’s second law : a line joininga planet and the Sun sweeps outequal areas in equal intervals oftimes

Notes

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Kepler’s third law : The square ofthe period “P” of a planet is directlyproportional to the cube of thesemimajor axis “a” of the orbit

Notes

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