Planetary Motions

1
Planetary Motions

• Keplers 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
Keplers 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