Lecture 3n - Lecture # 3 Motions on the Sky 1 Physics 20...

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Physics 20 Lecture 2 1 Lecture # 3 Motions on the Sky
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Physics 20 Lecture 2 2 In this lecture we discuss: Patterns on the sky, Constellations and the Milky Way Celestial sphere, ecliptic, longitude, latitude… Locating objects on the sky Why do stars rise and set ? Reason for Seasons Why summer days are longer ? Precession of the Earth’s orbit
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Physics 20 Lecture 2 3 Brief Revision of the last lecture
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Physics 20 Lecture 2 4 Copernican Revolution Nicholas Copernicus (1473-1543) overturned the concept of an Earth- centered cosmos which was proposed by Ptolemic 1700 yrs before him. Measured planetary orbits around the Sun and proposed that the Sun is in the center of the cosmos and not the Earth.
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Physics 20 Lecture 2 5 Nicolas Copernicus (1473-1543) 1543-”De Revolutionibus Orbium Coelestrom • Heliocentric Model • Sun at Center • All planets revolve about Sun • Moon revolves about Earth • Earth rotates daily, tilted 23.5 0 • Sphere of Stars much further away-no parallax • Astronomical Unit (AU) Model was no more accurate than Ptolemaic model in predicting planetary positions, because it still used perfect circles.
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Physics 20 Lecture 2 6 Tycho Brahe (1546-1601) • Compiled the most accurate (one arcminute) naked eye measurements ever made of planetary positions. • Still could not detect stellar parallax, and thus still thought Earth must be at center of solar system (but recognized that other planets go around Sun) • Hired Kepler, who used Tycho’s observations to discover the truth about planetary motion. • Like Copernicus, Tycho considered the planetary orbits to be circular and could never come up with a convincing explanation for planetary motion.
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Physics 20 Lecture 2 7 Johannes Kepler (1571-1630) • Kepler first tried to match Tycho’s observations with circular orbits • But an 8-arcminute discrepancy led him eventually to ellipses… “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.” Kepler’s main accomplishment was that planetary motions do not follow circular orbits but elliptical.
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Physics 20 Lecture 2 8 An ellipse looks like an elongated circle What is an ellipse?
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Physics 20 Lecture 2 9 Kepler’s First Law: The orbit of each planet around the Sun is an ellipse with the Sun at one focus. What are Kepler’s three laws of planetary motion?
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Physics 20 Lecture 2 10 Kepler’s Second Law: As a planet moves around its orbit, it sweeps out equal areas in equal times. means that a planet travels faster when it is nearer to the Sun and slower when it is farther from the Sun.
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Physics 20 Lecture 2 11 More distant planets orbit the Sun at slower average speeds, obeying the relationship p 2 = a 3 p = orbital period in years a = avg. distance from Sun in AU Kepler’s Third Law
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Lecture 3n - Lecture # 3 Motions on the Sky 1 Physics 20...

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