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Unformatted text preview: 1 Astro 102/104 1 Lecture #3: Orbits and Gravity Laws of Planetary Motion: Kepler's Laws. Newton's Laws. Gravity. Planetary Orbits. Spacecraft Orbits. Astro 102/104 2 The Main Point Motions of planets, moons, and asteroids can be very accurately predicted because of the underlying laws of planetary motion and gravity discovered by Kepler and Newton. Astro 102/104 3 Laws of Planetary Motion Ptolemaic (geocentric) and Copernican (heliocentric) systems made specific predictions about where the planets would be over time. What was needed was high quality data to test these competing models. Tycho Brahe (1546-1601): 20+ years of careful measurements. Best data available, and pre-telescopic! Astro 102/104 4 The Scientific Method 2 Astro 102/104 5 Laws of Planetary Motion Tycho provided the data , but his apprentice provided the key new model . Johannes Kepler (1571-1630) Mathematician, theorist. Tycho's data weren't fit well by models of Ptolemy or Copernicus. Worst case: Mars. WHY? Kepler's 20 years of studies led him to discover three Laws of Planetary Motion. Astro 102/104 6 Kepler's First Law Each planet moves about the Sun in an orbit that is an ellipse , with the Sun at one focus of the ellipse. Fundamentals of an Ellipse Astro 102/104 7 Kepler's Second Law The straight line joining a planet and the Sun sweeps out equal areas in space in equal time....
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