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•Kepler used Tycho’s Mars observations to discover the laws governing planetary motion. •He first tried to match Tycho’s observations with circular orbits, but found that elliptical orbits better matched the data. Johannes Kepler (1571-1630) © 2010 Pearson Education, Inc. Clicker Question Who actually made a device to replicate the planet’s motions in the sky? A. B. C. D. E. Aristotle Plato Ptolemy Tycho Kepler © 2010 Pearson Education, Inc. Clicker Question Who actually made a device to replicate the planet’s motions in the sky? A. B. C. D. E. Aristotle Plato Ptolemy Tycho Kepler © 2010 Pearson Education, Inc. Clicker Question Who was a proponent of the heliocentric model? A. B. C. D. E. Copernicus Plato Ptolemy Tycho Kepler © 2010 Pearson Education, Inc. Clicker Question Who was a proponent of the heliocentric model? A. B. C. D. E. Copernicus Plato Ptolemy Tycho Kepler © 2010 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. © 2010 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. © 2010 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 = avg. distance from Sun in AU © 2010 Pearson Education, Inc. Kepler’s Third Law © 2010 Pearson Education, Inc. Clicker 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 © 2010 Pearson Education, Inc. Clicker 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. © 2010 Pearson Education, Inc.
