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Planetary Motions

Planetary Motions. Kepler’s Laws: Physics not optional!. Planetary Motions. Periodic : Repeat at fixed intervals of time Shape : Follow a regular geometric path--what is it? Layout : Heliocentric, with spacing set by observations and geometry. Kepler’s Laws.

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Planetary Motions

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  1. Planetary Motions Kepler’s Laws: Physics not optional!

  2. Planetary Motions • Periodic: Repeat at fixed intervals of time • Shape: Follow a regular geometric path--what is it? • Layout: Heliocentric, with spacing set by observations and geometry

  3. Kepler’s Laws • Empirical: Based on observations, not theory (geometrical) • Law 1: Orbits elliptical • Law 2: Equal areas in equal times • Law 3: Regular spacing of planets from sun

  4. Law 1 • Planetary orbits are elliptical with the sun at one focus • Distance between sun and planet varies (perihelion-closest; aphelion-farthest) • Average distance = semimajor axis • Orbits are not very elliptical (small eccentricities)

  5. Law 2 • A line between a planet and the sun sweeps out equal areas in equal times • Orbital speeds vary with distance from the sun • aphelion, slowest; perihelion, fastest • Sense of a force from sun acting on planet (magnetic?)

  6. Law 3 • Orbital period squared divided by average distance cubed is a constant • Orbital speeds decrease (a lot!) with increasing distance from the sun • Objects at same distances follow same orbits

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