1. Kepler’s Laws: Physics not optional!
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