Module 2: How We Know What We Know

February 4 - February 17

This module examines models, theories, observations and experiments that contribute to the existing understanding of our Solar System.


Introduction & Perspective

Energy is a useful way to approach the structures and processes we'll see later. The text chapter (4) is good in many ways, but has some problems. "Energy being stored for later conversion" (p. 112) is an example of potential energy, but not a definition. "Energy in a stored or inert form" would be more general. Also the text leaves out some important categories: energy stored by position in magnetic or other fields, or gained or lost in deformation of matter (stretching of a spring, for example). Note that energy is stored or released when matter changes phases, even if there's no change in temperature; take a good look at Figure 4.8, too. Energy transfer mechanisms can usually be grouped into conduction, convection, and radiation (light).

Almost all of our information about the Solar System comes through light. We do have a few direct samples (lunar rocks, meteorites) and some physical detection or experiments done by spacecraft probes or landers; these are important, but not plentiful. So...we need to understand light as a tool, especially how light interacts with matter. (Light is also a form of energy; that's important when we study atmospheres.)

"Light" is the whole electromagnetic spectrum, from low to high-energy: radio, microwave, infrared, visible (red through violet), ultraviolet, x-ray, and gamma ray. All these ranges (of energies - or frequencies) are the same basic kind of thing, and they work the same basic ways. They can be emitted, absorbed, reflected (includes being scattered), and transmitted (includes being refracted) by matter. Most of the visible light in our Solar System comes from Sun and is reflected (or not) by various objects - but that's not true of infrared light. Also important, the different energies are not necessarily emitted or absorbed or whatever equally by matter - so if we spread out the light by energy (look at a spectrum), we can learn about the matter by how it has affected the different light energies differently.

Emission comes two main ways: thermal emission by opaque matter, where the spectrum depends only on temperature, and chemical emission by transparent gases, where the spectrum depends on the chemical (element or compound). Chemical absorption spectra take the same patterns as the emission spectra from the same chemical, so the pattern really does tell about the chemical. The chemical spectra tend to have bright (emission) or dark (absorption) lines, though there can be so many that they seem smeared. Thermal emission spectra are smooth - darker at the ends and brighter in the middle.

The way we observe light is with telescopes and instruments - we don't much depend on human eyes anymore. Don't worry about the arrangements of mirrors and/or lenses in telescopes. Read about what they do: concentrate light to make things brighter and clearer. Read about what we do with them: look at images or spectra, timing, etc. using instruments. Read about the different classes of spacecraft and what we do with them.

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Assigned Readings:


  • Chapter 4 - Matter & energy - Note Fig 4.8.
  • Chapter 6 - Light: read 6.1 - 6.4
  • Chapter 7 - 'Scopes & Craft: Read 7.3 - 7.6, also read "misconception" box p. 177

Internet Readings:

Eratosthenes's Shadow

Enhancement (optional):

Multiwavelength Astronomy

Spectral Lines

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Study Questions & Problems

1. James Bond is carrying the wax impression of a key and needs to get across a huge vat of boiling water without having the wax melt. There are two beams: one is hidden in a fog above the boiling water, the other is in clear air above the boiling water. For a change, nobody is shooting at him, so he's only worried about avoiding energy transfer to the wax (he doesn't care about energy transferring to his skin; he feels no pain). Which beam should he use to cross over the vat? Why?

2. We have many kinds of satellites monitoring Earth, especially for weather. Visible-light instruments aren't terribly useful at night, but infrared-instruments can show quite a bit. Why would this be so? (If desired, check current satellite weather data online.)

3. Give two advantages space-based telescopes have over ground-based telescopes.

4. How do we know that sodium exists in our Sun?

5. Why does Moon's spectrum resemble Sun's spectrum (in visible light)?

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Assigned Individual & Team Activities:

Individual Activities:

1 - Image Processing Tutorials - Complete the following tutorials:

  • Contrast/Brightness
  • Histograms
  • Measuring

Turn in the results for:

  • Measuring

2 - How Much Rock is in an Icy Moon?

  • Do individually and discuss in teams
  • Turn in individual results, summary of team discussion

3 - Interpret the Spectrum

  • Examine Figure 6.6 of the text
  • Analyze images (see MSULink post)

Team Activity:

Eratosthenes's Shadow

  • Individually collect data
  • In your teams analyze data

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Study Questions and Problems:

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

1. At what time of year does the setting (and rising) position of the sun change fastest, day-to-day?

2. How do we judge the value of a particular model?

3. Give two advantages that telescopes in space have over ground-based telescopes.

4. (Problem 6, Zeilik p. 22) Use a ruler to measure the size of Moon in Zeilik's Figure 1.18, p. 12. Divide the angular size of Moon (0.5 degree) by the linear size you measure to find the scale of the photo. Use this scale to estimate how far Moon had moved in angle, relative to Venus, between the exposures at the top left and the tenth one down. (For the mathematicians: can you figure out the time between exposures by comparing the angular movement to the angle at which Moon should move, relative to background stars? Assume the motion of Venus is negligible.) Groupwork on this problem is encouraged! You can use the Image program to make these measurments if you'd like!

5. (Ch 1 pr. 5) Sun circuits the zodiac in about a year (from an Earth-centered point of view). What is its average angular speed relative to background stars (a) per month? (b) Per day? (c) Per hour? (d) How does it compare to Moon's average angular speed?

6. You are on an unknown planet. You notice that, at night, the stars circle around, and don't rise or set. You then travel 8000 km in a constant direction and find that, at night, the stars all make half-circles from horizon to horizon (all of them rise and set). What is the circumference of the planet?

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1. Read the assigned readings during the first week of the module.

2. Contribute to the main discussion, which focuses on the discussion questions and module concepts.

3. Contribute to your team discussion. During the first week discussion could include study problems, individual and team activities. During week 2 teams should be focused on completing the activities.

4. Answers to study questions are due at the end of Week 1. Please submit individually and in the body of a message to the Module 2 Homework conference (subconference of Module 2). At this time you will also need to post your measurements for the Eratosthenes's Shadow experiment to your team and the appropriate folder in the Module 2 conference for group discussion and calculations.

5. By the end of the module's second week, put your homework into an e-mail message and send it to "Module 2 Homework" folder.

It should contain:

  • Image Tutorials Answers & Results.
  • Results of the Eratosthenes's Shadow experiment at home, then share your results in your team conference. Individually, turn in your measurements and a summary of how you used this data to find the Earth's size to the homework folder. Note any problems or difficulties and lessons learned. How would you improve the experiment if you were to do it again? As a team, turn in the final calculations and results to the homework folder.
  • Results of the Icy Moons Activity and a short summary of the group discussion.
  • Results of the Interpret the Spectrum Activity - List your three choices and the type of feature each is.
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Discussion Questions

These questions are to be discussed in the Module 2 Conference Folder

Initial Questions

  • What kinds of energy would you add to the text's list?
  • The text (p. 193) suggests that technology drives astronomical discovery. However, aren't the astronomers the ones who ask the questions and come up with the ideas and the specification for the instrumentation they need? What do you think and teach?

Supplementary Questions

  • What arguments (preferably based on observations you can make) can you use to demonstrate the shape of Earth and/or its location in our Solar System? (Note that arguments that Earth is flat or at the center of the system or such are fine, as long as they're supported with reason and/or observation.)
  • How do we know the distances of the sun and planets from Earth?
  • How can we prove that the sun is in the center of the orbits of planets?
  • How do we know the mass of Sun, Earth, planets and moons?
  • How can we prove energy is always conserved?
  • How do we know the Doppler shift is as reliable for light as it is for sound?


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last updated 2/3/02