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Planetary Motion Intro Video
Johannes Kepler History (1571-1630)
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 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• P = period of orbit (years)
• a = distance from sun (AU)