notes: kepler’s laws of planetary...
TRANSCRIPT
Notes: Kepler’s Laws of Planetary Motion Intro Video
Johannes Kepler History (1571-1630)
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
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
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
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)
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
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
Answer the following questions at the end of your
notes. Please write out the question in a different color
or you can highlight or underline the question.
True or False (If false, fix the statement so it is true)
1. Planets move in perfect circles around the sun.
2. The sun is at one focus & the moon is at the other focus.
3. The farther away a planet is, the faster it must go to get
around the sun in the same time as Earth (365 days)
4. Fill in the blank:
A. Earth moves fastest at ______________
B. Earth moves slowest at ______________