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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

Planetary Motions

Kepler’s Laws:

Physics not optional!


Planetary motions1
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
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


Law 1
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)


Law 2
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?)


Law 3
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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