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TAMU ASTR 101 - Lecture08_2009C

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Copyright © 2009 Pearson Education, Inc.3.3 The Copernican Revolution• How did Copernicus, Tycho, and Keplerchallenge the Earth-centered idea?• What are Kepler’s three laws of planetarymotion?• How did Galileo solidify the Copernicanrevolution?Our goals for learning:Copyright © 2009 Pearson Education, Inc.How did Copernicus, Tycho, and Keplerchallenge the Earth-centered idea?Copernicus (1473–1543)• Copernicus proposed the Sun-centeredmodel (published 1543).• He used the model to determine thelayout of the solar system (planetarydistances in AU).But . . .• The model was no more accurate thanthe Ptolemaic model in predictingplanetary positions, because it stillused perfect circles.Copyright © 2009 Pearson Education, Inc.Tycho Brahe (1546–1601)• Brahe compiled the most accurate (1arcminute) naked eye measurementsever made of planetary positions.• He still could not detect stellarparallax, and thus still thought Earthmust be at the center of the solarsystem (but recognized that otherplanets go around the Sun).• He hired Kepler, who used Tycho’sobservations to discover the truthabout planetary motion.Copyright © 2009 Pearson Education, Inc.Johannes Kepler(1571–1630)• Kepler first tried to match Tycho’sobservations with circular orbits.• But an 8-arcminute discrepancy ledhim eventually to ellipses.“If I had believed that we couldignore these eight minutes [of arc],I would have patched up myhypothesis accordingly. But, sinceit was not permissible to ignore,those eight minutes pointed theroad to a complete reformation inastronomy.”Copyright © 2009 Pearson Education, Inc. An ellipse looks like an elongated circle.What is an ellipse?Copyright © 2009 Pearson Education, Inc.Eccentricity of an EllipseEccentricity and Semimajor Axis of an EllipseCopyright © 2009 Pearson Education, Inc.Kepler’s First Law: The orbit of each planet aroundthe Sun is an ellipse with the Sun at one focus.What are Kepler’s three laws of planetary motion?Copyright © 2009 Pearson Education, Inc.Kepler’s Second Law: As a planet moves around itsorbit, it sweeps out equal areas in equal times.This means that a planet travels faster when it is nearer to theSun and slower when it is farther from the Sun.Copyright © 2009 Pearson Education, Inc.Kepler's 2nd LawCopyright © 2009 Pearson Education, Inc.More distant planets orbit the Sun at sloweraverage speeds, obeying the relationship p2 = a3 p = orbital period in years a = average distance from Sun in AUKepler’s Third LawCopyright © 2009 Pearson Education, Inc.Kepler’s Third LawKepler's 3rd LawCopyright © 2009 Pearson Education, Inc.Graphical version of Kepler’s third lawCopyright © 2009 Pearson Education, Inc.Thought QuestionA. 4 yearsB. 8 yearsC. 16 yearsD. 64 years(Hint: Remember that p2 = a3.)An asteroid orbits the Sun at an average distancea = 4 AU. How long does it take to orbit the Sun?Copyright © 2009 Pearson Education, Inc.An asteroid orbits the Sun at an average distancea = 4 AU. How long does it take to orbit the Sun?A. 4 yearsB. 8 yearsC. 16 yearsD. 64 yearsWe need to find p so that p2 = a3. Since a = 4, a3 = 43 = 64.Therefore, p = 8, p2 = 82 = 64.Thought QuestionCopyright © 2009 Pearson Education, Inc.How did Galileo solidify the Copernican revolution?Galileo (1564–1642) overcame major objections to the Copernican view. Fourkey objections rooted in the Aristotelian view were the following:1. Earth could not be moving becauseobjects 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 varymust be inside the orbit of themoon.Copyright © 2009 Pearson Education, Inc.Galileo’s experiments showed that objects inair would stay with a moving Earth.Overcoming the first objection (nature of motion): • Aristotle thought that all objects naturally cometo rest.• Galileo showed that objects will stay in motionunless a force acts to slow them down (Newton’sfirst law of motion).Copyright © 2009 Pearson Education, Inc.Overcoming the second objection (heavenly perfection): • Tycho’s observations of comet andsupernova already challenged this idea.• Using his telescope, Galileo saw:— Sunspots on the Sun(“imperfections”)— Mountains and valleys on theMoon (proving it is not a perfectsphere)Copyright © 2009 Pearson Education, Inc.• Tycho thought he had measured stellar distances, solack of parallax seemed to rule out an orbiting Earth.• Galileo showed stars must be much farther thanTycho thought—in part by using his telescope to seethat the Milky Way is countless individual stars.• If stars were much farther away, then lack ofdetectable parallax was no longer so troubling.• Tycho discovered a supernova in 1584 and it had noparallaxOvercoming the third objection (parallax):Copyright © 2009 Pearson Education, Inc.Galileo also saw fourmoons orbiting Jupiter,proving that not allobjects orbit Earth.Copyright © 2009 Pearson Education, Inc.Galileo’s observations of phases of Venus proved that itorbits the Sun and not Earth.Copyright © 2009 Pearson Education, Inc.Galileo GalileiIn 1633 the Catholic Churchordered Galileo to recant hisclaim that Earth orbits theSun.His book on the subject wasremoved from the Church’sindex of banned books in1824.Galileo was “formallyvindicated” by the Church in1992.Copyright © 2009 Pearson Education, Inc.What have we learned?• How did Copernicus, Tycho, and Kepler challenge theEarth-centered idea?—Copernicus created a Sun-centered model; Tychoprovided 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 atone focus.2. As a planet moves around its orbit it sweeps out equalareas in equal times.3. More distant planets orbit the Sun at slower averagespeeds: p2 = a3.Copyright © 2009 Pearson Education, Inc.What have we learned?• How did Galileo solidify the Copernicanrevolution?— His experiments and observations overcame theremaining objections to the Sun-centered solar system.Copyright © 2009 Pearson Education, Inc.3.4 The Nature of Science• How can we distinguish science fromnonscience?• What is a scientific theory?Our goals for learning:Copyright © 2009 Pearson Education, Inc.How can we distinguish science from nonscience?• Defining science can be


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TAMU ASTR 101 - Lecture08_2009C

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