Planetary Motions

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# Planetary Motions - PowerPoint PPT Presentation

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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## PowerPoint Slideshow about 'Planetary Motions' - beatrice

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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
• 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
• 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
• 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
• 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