1. Planetary Motion
Intro Video

2. Johannes Kepler Came up with 3 laws of Planetary Motion
History ( )
Liked the idea that numbers and math
underlie the mysteries of the cosmos
While trying to make sense of data collected
by Tycho Brahe, he realized that the old ideas about astronomy couldn’t be correct
Came up with 3 laws of Planetary Motion

3. What is an Ellipse? Ellipse A flattened circle (oval)
Eccentricity: How close the ellipse is to a straight line
E = 1 (straight line)
E = 0 (circle)
A circle is also an ellipse but both foci
are in the center

4. Kepler’s 1st Law of Planetary Motion
Planets move in elliptical orbits with the Sun at one focus of the ellipse
**The sun lies at one of two focal points
Aphelion: The point in orbit furthest from the star
Perihelion: The point in orbit closest to the star

5. Kepler’s 2nd Law of Planetary Motion
A line from the Sun to the planet sweeps out equal areas in equal times
Therefore…the closer a planet is to the sun the faster it travels (they don’t move at a constant speed)
When closer to the sun, the gravitational force is stronger so the planet covers more distance in the same time (speeds up)

6. Kepler’s 3rd Law of Planetary Motion
(Period of orbit)² is proportional to (semi-major axis of orbit)³
What? …
What this means is that the amount of time it takes a planet to go around the sun (period of orbit) is mathematically related to how far away it is (semi-major axis of orbit).
P² = a³ with P in years and a in A.U.
P = period of orbit
a = distance from sun

7. Why do planets move this way?
Kepler thought some magnetic force
No theory to describe why this motion occurred.
Became a major question in the 17th century and beyond