Planetary Motion Interactive

Kepler’s Second Law of Planetary Motion

Go to the following website:

Click on “Interactive”. You can then click on the name of each of the six objects in the bottom left to watch them complete their orbital path around the Sun.

1. Which of the six objects (4 planets, a dwarf planet, and a comet) has the leasteccentric orbit?

2. Which has the most eccentric orbit?

3. How does distance from the sun appear to relate to eccentricity?

4. Now click on Earth and watch the bar graph to the right. What area is covered during each time interval? Does it ever change?

5. Click on the comet. Since Earth’s orbit very close to circular all of its triangles look very similar. Describe the differences you notice in the comet.

6. What area does the comet cover each time? Does the area ever change?

Kepler’s Third Law of Planetary Motion

Go to the following website:

*Click on “Interactive”

1. Set the eccentricity slider to zero and click go. Does the speed of the planet appear to change or remain constant throughout the orbital period? If it changes, when does it appear to move the fastest?

2. Set the eccentricity slider to 0.7 and click go. Does the speed of the planet appear to change or remain constant throughout the orbital period? If it changes, when does it appear to move the fastest?

3. Does the orbital period (P) (time it takes to orbit) change when eccentricity changes?

*Use the semi major axis (a) slider now to test how the size of an orbit affects its period. Alter the Semi-Major Axis value several times to see the effect.

4. Complete the following statement: The smaller the distance from the sun (a), the ______the orbital period (P).

5. Using the same slider and keeping the star’s mass at 1 solar mass, use guess and check to figure out what distance from the sun would give an orbital period of 4 years.

6. What distance gives an orbital period of 8 years?

*Reload the interactive so that it starts over.

7. Click each of the planets listed in the center box at the bottom, and allow them to make one complete revolution. Sketch the pattern that they make on the graph in the bottom left corner of the interactive.

8. What is the graph comparing? (What does each axis represent)?

9. Based on the graph, what can you conclude about how the value of P² and a³ are related to each other? (hint: 3rd law)

Review Question

  1. Summarize each of Kepler’s 3 laws of planetary motion.
  2. What is an ellipse?
  3. What does eccentricity means in terms of an ellipse?
  4. What is the aphelion?
  5. What is the perihelion?
  6. Kepler was able to describe and predict the motions of the planets using math. However there was still much to learn. What major question did his work inspire?