Lecture2 - Planetary Motions Planets are observed to move...

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Planetary Motions Planets are observed to move relative to the background stars ¾ Motion is generally regular, but sometimes shows retrograde motion that was very difficult to explain in geocentric theories ± Led to use of epicycles ¾ In a heliocentric theory retrograde motion is a natural consequence of the inner planets orbiting more quickly than the outer planets Distances to planets can be determined using simple geometric calculations ¾ Interior: measure angle between planet and the Sun at greatest elongation (when they are farthest apart) ¾ Exterior: measure same angle, separated in time by planet’s orbital period. ¾ Copernicus had excellent values for all of the known planets, but they were less precise for the outermost ones (Jupiter and Saturn) because their periods are long enough that fewer oppositions could be observed
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Kepler’s Laws Based on very accurate orbital data obtained by Tycho Brahe, Kepler derived three laws of orbital motion: 1. A planet orbits the Sun in an ellipse, with
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Lecture2 - Planetary Motions Planets are observed to move...

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