SIRC Jan 2013 Motion of Stars—Teacher Notes/Outline
By Rich Hedman hedmanrd@csus.edu
IMPORTANT: This document goes with the PowerPoint. The most important thing about this document
is that it has a list of the expected phenomena, and model statements, for teacher reference. If
students don’t mention some of the phenomena, you need to ask them about it, so you can add it to the
class list. Do the same with important model statements, if they miss any.
1. Develop Model of Motion
Driving Question: How do you know something is moving? What do you have to know to find out if
something is moving?
Pair-share
Share out, synthesize model to match the one in my notes, capture on chart paper “Model of Motion”.
Teacher notes:
I want them to develop our first model, Model 1: Model of Motion:
1. You need an object of interest
2. You need a reference
3. You need a time frame
4. You need to be aware of your point-of-view (1st person or 3rd person)
Explain to teachers: We are going to shift from 1st person perspective (personal, earth-bound view) to
3rd person perspective (view where you see yourself, earth, the whole system, usually from far away in
space). We need to be careful in clarifying the view we are talking about.
2. Sun Motion (Day & Night) Phenomena
Thought Question: So, based on these ideas of motion, in the First Person viewpoint, if you are outside
on a clear day, is the sun moving? Think quietly about that for a moment. We will come back to this
question in a few minutes.
To answer our question, we need to think about the data we have, and look for patterns.
In your groups, answer this question: What are some observations that you have about day and night?
Group work
Share out, compile their observations, “Day & Night Phenomena”.
Teacher Notes: I want the teachers to describe phenomena such as these three things:
1. In the daytime, the sun is visible; at night, the sun is not visible.
Specifically, the sun is visible about half the time, and not visible about half the time.
2. The sun appears to move across the sky with respect to the horizon during the day.
Specifically, it rises in the east and sets in the west.
3. This patterns repeats itself about every 24 hours.
3. Develop Model of Day & Night
What model could explain these patterns? A model is a set of ideas. What set of ideas could explain
these patterns? We need to create a model (in words, diagrams, with 3-D objects) to help us answer the
driving question.
Have the teachers physically write down a list of statements about what causes day and night, AND also
have them use the 3-D objects to be able to explain their model.
Group work.
Share out, synthesize their model statements.
Make sure to call on groups that haven’t been sharing.
Teacher notes: I want them to develop our model, Model 2: Model of Day & Night:
1. The sun produces light.
2. Because earth is a sphere, sunlight can only illuminate about half the planet (the side facing
the sun). The other half is not illuminated and is dark.
3. The earth spins on an axis once every 24 hours.
4. As the earth spins, different parts of the globe are illuminated (daytime)and different parts
are in darkness (nighttime).
4. Apply both Models to answer our Driving Question:
Driving question: Does the sun move during the daytime?
Refer to your earlier prediction, but make sure to use the models to answer the question.
Group dialogue.
Share out/discussion.
Teacher notes: some of the things I would like them to say or consider:
Based on our model, when it is daytime, we are on the side of earth facing the sun. As earth
spins on its axis, the sun’s position in the sky in respect to our local horizon changes; so in this
respect, YES, the sun is moving during the daytime. However, this is misleading, because it is
really the earth’s spin which is causing the sun to appear to move. In respect to the earth (not
our local horizon), the sun is NOT moving (it is not revolving around the earth) during the
daytime.
SO, It is a trick question; it depends. You can’t tell a student they are wrong on this one. Not
without clearly defining all the parameters and understanding (with respect to what).
The sun moves with respect to earth’s horizon; but it could also be the earth’s horizon is moving
relative to the sun. We actually need quite a bit more information to determine it is the earth’s
horizon moving (spinning) relative to the sun. For our purposes, the difference is not that
important actually!
From a truly 1st person perspective, we usually would define this as follows:
Object = sun; Reference = horizon; Time frame = a day; and we would say something like, “The
sun moves across the sky during the day with respect to the horizon”.
As long as we define our parameters and our point of view, no one could argue with you.
Of course, someone else might say, “The horizon moves across the sky, with respect to the sun,”
and you couldn’t argue back. Both are valid, useful statements; and it would take a lot more
data to figure out which one is better to use. But if it usually works, use it!
4. Stars during one night—focus question
Focus Q to ask to begin star motion:
So if you were to go outside on a clear night, and lay out and watch the stars all night (pretend you are
still in college!), what would you predict you would see? Would the stars move? If so, how, exactly?
Quick write
But don’t discuss—we will come back to it later.
5. Introduce Stellarium
6. Stars during one night—Phenomena
Question: What patterns do you notice in the motion of stars in the sky over a night?
(from Sacramento).
Teachers use Stellarium to take detailed observations of what the patterns they notice (group work).
Groups write patterns on white boards.
Share out, compile their observations, “Stars During 1 Night Phenomena”.
Make sure to call on groups that haven’t been sharing.
Teacher Notes: I want the teachers to describe phenomena such as these things:
Stars During 1 Night Phenomena:
Over the course of a night:
 If you look toward the S horizon, it appears that all the stars in the sky are moving
from the left (west) to the right (east).
 If you look E, it looks like stars are coming toward you, or rising.
 If you look W, it looks like stars are setting.
 If you look to the N, it looks like stars are moving in a circle (CCW, around one single
star)
 Also, the stars don’t change their positions with respect to each other—they change
their positions with respect to the horizon.
Teacher notes: Show video clips if there is a problem in using Stellarium—or to emphasize what they
can see:
Show clip 1—identify patterns; make sure teachers identify their reference points, and point of
view. Phenomena to observe:
Stars move from E to W across the sky.
Stars do not move relative to each other.
Show clip 2—Identify patterns; make sure teachers identify their reference points, and point of
view. Phenomena to observe:
Stars move in a CCW circle around a star (Polaris).
Stars do not move relative to each other.
7. Stars during one night—Model
What model could explain these patterns? A model is a set of ideas. What set of ideas could explain
these patterns?
Have the teachers physically write down a list of statements about what cause the patterns in
star motion they recorded, and since saying the model is one thing and showing it is another,
also have teachers use3 dimensional objects to: a) illustrate the model, and b) explain the
patterns we see in the sky.
Group work.
Share out, synthesize their model statements.
Make sure to call on groups that haven’t been sharing.
Teacher notes:
I want them to develop our model, Model 3: Model of Stars over a Night:
1. Stars look like points of light, and they are relatively far from earth.
2. The stars are in fixed positions (not moving) relative to each other.
The stars might be in fixed positions on a celestial sphere, all equally distant from earth.
3. Earth is spinning on an axis.
We see the illusion of the stars moving across the sky, but it is really us spinning on our
axis.
Explanation of patterns we see using the model:
As the earth’s spins, the stars in the sky appear to be moving.
A competing model would be that the earth is not spinning, but that the “sphere of stars” is
moving, not the earth.
More Teacher Notes:
Interestingly, if you already have a model of day & night, you already have a head start on the star
model. The new part here is the details of that motion. In particular, the one new thing that needs to
be explained is why the stars appear to rotate around a star. Also the stars don’t move with respect to
each other—but this is more phenomenon than model.
If teachers have trouble explaining the circular pattern after grappling with it for a few minutes, to a
quick lab activity: (have a star or Polaris taped to the ceiling somewhere).
Optional Quick Lab Activity if needed:
Me: Spin in your chair. Tell me what you see.
Them: They say, “Well, things look like they are moving.”
Me: Which way?
Them: “Well, if I am spinning CCW, things move from E to W.”
Me: Interesting, okay, that explains video clip 1. Now, look up and tell me what you see.
Them: Hey, things are moving in a circle!
Me: Interesting. Does this explain video clip 2? Just leave this question hanging out there, then
move right into the next item.
8. Apply Models to answer our Driving Question:
Group Quickwrite Explanation—Use the model to construct an explanation—an answer to our driving
question:
In a sentence or two, explain what causes the patterns in star motion (with respect to the horizon) over
the course of a night.
Group dialogue and quick write on white board.
Share out/discussion?
9. Stars Over Days, Weeks, Months—Phenomena
Question: What patterns do you notice in the motion of stars in the sky at the same time of night, over
days, weeks, and months?
(from Sacramento).
Teachers use Stellarium to take detailed observations of what the patterns they notice (group work).
Groups write patterns on white boards.
Share out, compile their observations, “Stars Over Days, Weeks, Months Phenomena”.
Make sure to call on groups that haven’t been sharing.
Teacher Notes: I want the teachers to describe phenomena such as these things:
Stars Over Days, Weeks, Months Phenomena:
 Each night, the stars appear to shift slightly.
 As months pass, you begin to see different sets of stars.
 Over 12 months, the stars return to their original positions.
10. Stars Over Days, Weeks, Months —Model
What model could explain these patterns? A model is a set of ideas. What set of ideas could explain
these patterns?
Have the teachers physically write down a list of statements and or a diagram about what cause
the patterns in star motion they recorded, and since saying the model is one thing and showing
it is another, also have teachers use3 dimensional objects to: a) illustrate the model, and b)
explain the patterns we see in the sky.
Group work.
Share out, synthesize their model statements.
Make sure to call on groups that haven’t been sharing.
Teacher Notes:
I may want to give them some time to try to figure it out.
Then I could pause them, and provide them with more tools: the diagrams of a prominent
constellation from each time of the year, along with Polaris.
Then they could go back to work. They will need to be able to show this physically.
We will have a light bulb in the middle of the room—maybe we create a full class model?
Where I have constellations on different walls?
Then we move to the model of stars at different times of the year. This requires adding to the
model that earth orbits the sun, and when it is dark/night we see a different patch of the sky.
I want them to develop our model, Model 4: Model of Stars over Days, Weeks, Months:
These will probably just be added to our earlier model:
1. That the earth orbits the sun
2. It takes one year to go around the sun.
3. The stars we see at night are in different sections of the sky.
11. Make a Star Finder
Make a star finder
12. Use our Models to Explain the Star Finder and how it works.
Questions for groups to answer:
1. Figure out how to use the star finder to predict the patterns of stars you would see in the sky
at 9pm tonight.
[Make sure everyone can do this-- use a scene from stellarium and have them identify
constellation]
2. Why are the East and West compass directions reversed?
3. Why do we need an oval cut-out, what does this represent?
4. Take the “wheel” of stars out and look at it. What does this represent?
5. Explain why there are marks indicating the time of day—what does that represent physically?
6. Explain why there are marks indicating the month and date—what does that represent
physically?
7. Could you use this star finder to predict the patterns of stars you would see in the sky if you
lived in Brazil? Explain.
8. Could you use this star finder to predict the patterns of stars you would see in the sky if you
lived on the earth’s North Pole? Explain.
13. Conclusion. Models are useful but have limits.
For example, the star finder itself is very useful, but only under certain conditions (in this case
the Northern Hemisphere of earth), and they could talk about it.
Then at the end have a brief lecture on stars: 1) that stars are at different distances from us, and
2) they are actually all moving. This calls into question what frame of reference we are using.
You don’t notice the motion on normal time frames; you only notice the motion over thousands
of years (verify).
14. Then have a very short time at the end for questions and answers.