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Recent History of Astronomy AST 111
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The Geocentric Model IT IS WRONG!
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Fine. So what is it? The Geocentric Model places Earth at the center of the Universe. Everything (Sun, Moon, stars, etc.) revolves around the Earth.
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The Geocentric Model But it’s wrong. So why did it survive for 2000 years?
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Parallax and the Geocentric Model Things at different distances line up differently when you move around This change in angular separation is called parallax – Hold your two index fingers at different distances and move your head around. Lack of observable parallax kept geocentric model alive.
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Opposition to the Geocentric Model Sun-centered model proposed by Aristarchus – Assertions: If Earth orbits sun, angular separations of stars change If they don’t, stars must be unrealistically far away – Observation: The angular separations of stars don’t seem to change
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A “Conspiracy” in the Geocentric Model Venus maintains about the same brightness – Suggests it orbits Earth! – Should get dimmer as it gets farther away? This is a truly wretched coincidence. – As it gets farther, it gets dimmer. – BUT its phase increases (just like the Moon)
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The Geocentric Model Greeks placed “Wandering Stars” at different distances Stars all on outer sphere Wandering stars “do their own thing in their own sphere”
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Retrograde Motion “Wandering Stars” exhibited a strange feature Move forward, reverse briefly, resume going forward Notice that the size and brightness change
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Geocentric Model “Wandering Stars” drove ancient astronomers up the wall, especially retrograde motion – Ultimately broke the geocentric model Was extremely difficult to fit to geocentric model – Became more and more complicated until simply unreasonable
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The Ptolemaic Model Greek knowledge of astronomy represented by the Ptolemaic Model “Wandering Stars” move around on small circles that rotate around a large circle
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The Ptolemaic Model Little circle: Epicycle Big circle: Deferent
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The Ptolemaic Model Still did not predict “Wandering Star” motion accurately enough – Smaller circles added to the small circles – Positioned some of the larger circles off-center Accurate to “within a hand at arms length”
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The Copernican Revolution This is where they started to get it right. Nicholas Copernicus born February 19, 1473 in Poland By then, tables of planetary motion from Ptolemaic model inaccurate
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The Copernican Revolution Copernicus knew about Aristarchus’s sun- centered system Went farther with mathematical details Went from philosophical arguments to predictive geometry
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It didn’t work very well.
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The Copernican Revolution Was not very accurate Was made as complicated as Ptolemaic model to make reasonable predictions – He added epicycles
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The Copernican Revolution It didn’t work because: Copernicus held onto ancient belief that heavenly motion must occur in perfect circles.
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Tycho Brahe Good data was hard to come by – No telescopes! Built a naked-eye observatory – Measured to within an arcminute – 30 years of measurements
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Tycho Brahe Took large amounts of high quality data at his observatory
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Tycho Brahe Aristotle : patterns of stars are “immutable” (unchanging) One blew up in 1572 Tycho Brahe made parallax measurements – It was determined to be as far away as the stars
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Johannes Kepler Tycho Brahe’s apprentice Worked to match circular motions to Tycho’s data – Didn’t work – Trusted Tycho’s data Assumed sun-centered system
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Johannes Kepler Kepler deduced that planetary orbits are in the shape of ellipses, not circles.
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Johannes Kepler Rather than brushing aside small errors to validate a preconceived notion, Kepler trusted the carefully taken data and revolutionized astronomy.
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Ellipses and Circles
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Kepler’s Laws Newton was later able to derive Kepler’s Laws from more fundamental principles
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Galileo Galilei Opponents of sun- centered system with elliptical orbits argued: – Non-circular orbits mean celestial realm imperfect – Stars don’t shift position as Earth orbits the Sun Galileo used the telescope to counter these arguments
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Galileo Galilei Moons of Jupiter
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Galileo Galilei The surface of the Sun:
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Galileo Galilei Resolved individual stars in band of Milky Way:
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Galileo Galilei Phases of Venus
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Galileo Galilei Phases of Venus
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Stellar Parallax Stellar parallax was finally observed in 1838 Measured in arcseconds – 1/60 of the thickness of a fingernail at arm’s length Can be used to find the distance to nearby stars This marked the definitive end of the Earth- centered model.
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“Wandering Stars” The “Wandering Stars” are PLANETS. – (And planets are not stars!) They “wander” because they orbit the Sun and are much closer to Earth than stars
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