Module 3: Orbits and Systems

February 18 - March 3

This module looks at the patterns found within the systems and orbits of our Solar System.


Introduction & Perspective

 The chapter on universal motion can be a bit of a struggle for some - stay with it, and bring your questions to the discussion. For those who teach physics, pay attention - you have here colleagues who will show you what your students may find confusing and why.

Before we had all the telescopes and spacecraft, images, etc., we mostly saw spots of light in the night sky. We could figure out position - well, angle. We didn't know how far things were. Once we figured out gravity, though, we could figure out locations of new things by how they move. Even better, we can take a knowledge of motions and distances, and use it to find out how massive things are. (It's not as if you could ever get a world onto a two-pan balance, right?) Also, the way we use gravity is pretty typical for astronomy - first, assume something simple, such as "the orbit is a circle" and observe - that gets you some basic information (the mass of the central object, in this case, or the distance). Then look for the next level of detail - how does the orbit differ from a circle? - that gets you other information, and then a third look may get you yet more. You don't have to know everything at once, but it helps to have a sense of orders of magnitude so you can judge relative importance.


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Assigned Readings:

The Cosmic Perspective
Jeffrey Bennett, etal, Second Edition, 2002, Addison Wesley
- Chapter 5

Online Reading - Bode's Rule

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Study Questions and Problems:

Answers due at the end of the first week of the module:

1.. Imagine you're out in space and kick away from you an object having a mass identical to your own. What happens?

(a) The object stays where it is, and you move away from it at a constant speed.

(b) You stay where you are, and the object moves away from you at a constant speed.

(c) You and the object move away from your starting points at constant speeds.

(d) You and the object move away from your starting points at constant accelerations.

2. When you see Mars at opposition to Sun in the sky, Earth and Mars lie closest together on the same line from Sun. How does the force due to gravity of Mars on Earth compare to the force due to gravity of Earth on Mars at that time?

3. Imagine you are in space in orbit around Earth. You have with you two identical spheres (equal mass). You place them in a line, one "below" (closer to Earth) than the other. Because of their placement, they will experience a tidal effect. What will the tidal effect do to the distance between the spheres?

4. A comet is orbiting our Sun. At what point is its kinetic energy (a) the greatest? (b) the least? (c) What is the value of the total energy of the comet at point (a) compared with point (b)?

5. Imagine that the mass of Earth were increased by a factor of 4, but the size remained the same. (a) What would happen to Earth's density? (b) What would happen to Earth's escape speed? (Note: give answers as factors of current values, not actual values.)


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Assigned Individual & Team Activities:

Individual Activities:

Drawing & Dancing Orbits

Earth-Moon Distance - Image & Reality

Kepler's Laws Simulations

Team Activity:

Testing Bode's Rule

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1. Read the assigned readings.

2. Contribute to the main discussion.

3. Contribute to your group's discussion of the activities.

4. Answer the study questions by the end of the first week.

5. Submit homework by the end of the second week, individually. It should contain:

  • Results of Testing Bode's Rule - The results of your group investigation and answer the four questions discussed in your group
  • Kepler's Laws - Answers to the questions
  • Results of Image Processing Activity
  • Results of Drawing & Dancing Orbits Activities
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Discussion Questions

These questions are to be discussed in the Module 3 Conference Folder

Initial Questions


Supplementary Questions

1. Which planetary body has a greater effect on us and why, Sun or the Moon?

2. Question 2 is written at two levels. You may respond to either question, or to both, depending on your comfort zone:

Beginner: Comets (and all worlds orbiting Sun) move fastest when nearest Sun. That means they have more energy-of-motion (kinetic energy) when close than when far from Sun. What happens to that energy?

Pro: Spacecraft are often sent past a planet to get a "gravity assist". Since energy is neither created nor destroyed, where does the additional kinetic energy of the spacecraft come from?

Also: we should be sure to consider NEAR & asteroid Eros!

Other Module 3 - Discussion Questions

1. How might a comet become indistinguishable from an asteroid?

2. What types of materials cause the apparent red colorations of the asteroids?

3. Why does Earth not have rings?

4. Meteorites are sometimes called "the poor man's space probe" because they cost little to obtain. What would be "the rich man's space probe"?

5. The solar system is often divided into "primordial zones" depending onthe composition of bodies smaller than the giant planets. There is the "soot line" and the "frost line". Where could a new "zone" exist and what would you call it? You discover it...you get to name it!

6. How can a comet get it's "tail" cut off?

7. What is the difference between a moon, an asteroid and a comet?

8. Aristotle believed that the Earth was made from the four elements: fire, water, Earth, and air, while the heavens were made from ether. The literal meaning of quintessence is "fifth element", and the literal meaning of "ethereal" is made of ether. Look up these words in the dictionary. How are their modern meanings related to Aristotle's ancient beliefs?

9. Casting aside the idea the orbits must be perfect circles meant going against deeply entrenched beliefs, and Kepler said that it shook his deep religious faith. Given that only two of Tycho's observations disagreed with a perfectly circular orbit -and only by 8 arcminutes- do you think that most other people would have made the choice Kepler made of abandon perfect circles? Have you ever performed an experiment that disagreed with theory Which did you question, the theory or your experiment? Which do you think your students are more likely to do? Why?

10. Which planet has the most pronounced elliptical path? What evidence do we have for this?

11. Considering Bode's Law what there is a missing planet. What is present in the place of a planet in the Titius Bode's sequence? What other bodies are affected by the Titius-Bode Law?


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last updated 2/10/02