ASTR 20 Lecture Notes - Lecture 18: Ellipse

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16 Jul 2020
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Kepler first tried to match tycho"s observations with circular orbits. But an 8-arcminute discrepancy between his predictions and the observations led him eventually to ellipses. An ellipse looks like an elongated circle. Kepler"s first law: the orbit of each planet around the sun is an ellipse with the sun at one focus. Kepler"s third law: more distant planets orbit the sun at slower average speeds, obeying the relationship. An asteroid orbits the sun at an average distance a=4au. Kepler fixed the heliocentric model by introducing ellipses, but even after kepler, objections remained: Overcame major objections to the copernican view. Earth-centered view were: earth could not be moving because objects in air would be left behind, noncircular orbits are not perfect as the heavens should be, if earth were really orbiting sun, we"d detect stellar parallax. Overcoming the first objection (nature of motion): galileo"s experiments showed that objects in air would stay with a moving earth.

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