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)