3 3 The Copernican Revolution Our goals for learning How did Copernicus Tycho and Kepler challenge the Earth centered idea What are Kepler s three laws of planetary motion How did Galileo solidify the Copernican revolution Copyright 2009 Pearson Education Inc How did Copernicus Tycho and Kepler challenge the Earth centered idea Copernicus 1473 1543 Copernicus proposed the Sun centered model published 1543 He used the model to determine the layout of the solar system planetary distances in AU But The model was no more accurate than the Ptolemaic model in predicting planetary positions because it still used perfect circles Copyright 2009 Pearson Education Inc Tycho Brahe 1546 1601 Brahe compiled the most accurate 1 arcminute naked eye measurements ever made of planetary positions He still could not detect stellar parallax and thus still thought Earth must be at the center of the solar system but recognized that other planets go around the Sun He hired Kepler who used Tycho s observations to discover the truth about planetary motion Copyright 2009 Pearson Education Inc Kepler first tried to match Tycho s observations with circular orbits But an 8 arcminute discrepancy led him eventually to ellipses Johannes Kepler 1571 1630 Copyright 2009 Pearson Education Inc If I had believed that we could ignore these eight minutes of arc I would have patched up my hypothesis accordingly But since it was not permissible to ignore those eight minutes pointed the road to a complete reformation in astronomy What is an ellipse An ellipse looks like an elongated circle Copyright 2009 Pearson Education Inc Eccentricity of an Ellipse Eccentricity and Semimajor Axis of an Ellipse Copyright 2009 Pearson Education Inc What are Kepler s three laws of planetary motion Kepler s First Law The orbit of each planet around the Sun is an ellipse with the Sun at one focus Copyright 2009 Pearson Education Inc Kepler s Second Law As a planet moves around its orbit it sweeps out equal areas in equal times This means that a planet travels faster when it is nearer to the Sun and slower when it is farther from the Sun Copyright 2009 Pearson Education Inc Kepler s 2nd Law Copyright 2009 Pearson Education Inc Kepler s Third Law More distant planets orbit the Sun at slower average speeds obeying the relationship p2 a3 p orbital period in years a average distance from Sun in AU Copyright 2009 Pearson Education Inc Kepler s Third Law Kepler s 3rd Law Copyright 2009 Pearson Education Inc Graphical version of Kepler s third law Copyright 2009 Pearson Education Inc Thought Question An asteroid orbits the Sun at an average distance a 4 AU How long does it take to orbit the Sun A B C D 4 years 8 years 16 years 64 years Hint Remember that p2 a3 Copyright 2009 Pearson Education Inc Thought Question An asteroid orbits the Sun at an average distance a 4 AU How long does it take to orbit the Sun A B C D 4 years 8 years 16 years 64 years We need to find p so that p2 a3 Since a 4 a3 43 64 Therefore p 8 p2 82 64 Copyright 2009 Pearson Education Inc How did Galileo solidify the Copernican revolution Galileo 1564 1642 overcame major objections to the Copernican view Four key objections rooted in the Aristotelian view were the following 1 Earth could not be moving because objects in air would be left behind 2 Noncircular orbits are not perfect as heavens should be 3 If Earth were really orbiting Sun we d detect stellar parallax 4 All astronomical objects that vary must be inside the orbit of the moon Copyright 2009 Pearson Education Inc Overcoming the first objection nature of motion Galileo s experiments showed that objects in air would stay with a moving Earth Aristotle thought that all objects naturally come to rest Galileo showed that objects will stay in motion unless a force acts to slow them down Newton s first law of motion Copyright 2009 Pearson Education Inc Overcoming the second objection heavenly perfection Tycho s observations of comet and supernova already challenged this idea Using his telescope Galileo saw Sunspots on the Sun imperfections Mountains and valleys on the Moon proving it is not a perfect sphere Copyright 2009 Pearson Education Inc Overcoming the third objection parallax Tycho thought he had measured stellar distances so lack of parallax seemed to rule out an orbiting Earth Galileo showed stars must be much farther than Tycho thought in part by using his telescope to see that the Milky Way is countless individual stars If stars were much farther away then lack of detectable parallax was no longer so troubling Tycho discovered a supernova in 1584 and it had no parallax Copyright 2009 Pearson Education Inc Galileo also saw four moons orbiting Jupiter proving that not all objects orbit Earth Copyright 2009 Pearson Education Inc Galileo s observations of phases of Venus proved that it orbits the Sun and not Earth Copyright 2009 Pearson Education Inc In 1633 the Catholic Church ordered Galileo to recant his claim that Earth orbits the Sun His book on the subject was removed from the Church s index of banned books in 1824 Galileo Galilei Copyright 2009 Pearson Education Inc Galileo was formally vindicated by the Church in 1992 What have we learned How did Copernicus Tycho and Kepler challenge the Earth centered idea Copernicus created a Sun centered model Tycho provided the data needed to improve this model Kepler found a model that fit Tycho s data What are Kepler s three laws of planetary motion 1 The orbit of each planet is an ellipse with the Sun at one focus 2 As a planet moves around its orbit it sweeps out equal areas in equal times 3 More distant planets orbit the Sun at slower average speeds p2 a3 Copyright 2009 Pearson Education Inc What have we learned How did Galileo solidify the Copernican revolution His experiments and observations overcame the remaining objections to the Sun centered solar system Copyright 2009 Pearson Education Inc 3 4 The Nature of Science Our goals for learning How can we distinguish science from nonscience What is a scientific theory Copyright 2009 Pearson Education Inc How can we distinguish science from nonscience Defining science can be surprisingly difficult Science comes from the Latin scientia meaning knowledge But not all knowledge comes from science Copyright 2009 Pearson Education Inc The idealized scientific method Copyright 2009 Pearson Education Inc Based on proposing and testing hypotheses Hypothesis educated guess But science rarely proceeds in this idealized way For
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