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 ______________