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Re: phases of moon, earth-sun system



Hi-
You can take this demo one step further by thoughtful positioning
of your light source, ball, and observer. The Greeks measured the
distance to the sun by noting when the moon was at exactly 1/2 lit, and
measuring the angle between the lines of sight to the sun and moon. They
knew the lunar distance from parallax (I wouldn't call that
"triangulation" I don't think), in units of the earth's radius.
Their number for the solar distance was not too good, as I recall,
because the relevant angle is so close to pi/2.
Regards,
Jack



On Fri, 14 Sep 2001, Dean Livelybrooks wrote:

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I have attempted to teach these things in a 100-level, non-science student class.
Moon Phases:
For the moon I use a white volleyball, a globe and an old projector that gives me collimated white light.
Have students cluster near "Earth" and observe.
Hold the projector in front of class, lighting the Earth. Ask the students which side of the "Earth" is the dark side. Have them move there for "lunar observations."
Have a student hold the volleyball between "Earth" and "moon," and at the various positions in an orbit around "Earth."
One can see the phases of the moon on the volleyball. (You may need to swing the projector/sun out to each side of the front of the classroom as the moon goes through its orbit, so that the "sun" is shining normally on the moon.
Sun-Earth:
I stole ideas from Project Star. Purchase and distribute clear, acrylic hemispherical domes that one can buy at art supply shops cheaply. Also distribute a piece of paper with the outline of the dome on it, a center point, and a North arrow.
Have students take both of these things, a pen and a compass outside several times during a day when the sun is clearly observable. They should hold the pen so that the (sun's) shadow of the tip falls on the center of the hemispherical dome, and mark where the tip is on the half-dome. Date and time stamp each observation. Take several on the same day. Take observations for a day twice or more, spaced several weeks
or months apart. Use different colored pens for different days.
This represents a set of observations. I have students bring these to class/lab on a specified day. We start using the Ptolemaiic model (spelling?). Tape an unmarked dome over where you are on a globe (centered over your city or region). Have a student hold the globe with dome. Start with the sun (a laser pointer) in the center and have the Earth orbit the sun.
Make model-derived data by marking where the laser pointer light hits the unmarked dome on the globe. This is made easier as the laser light will reflect directly back at the "sun" (laser pen holder) when the light is aimed directly at the (local) normal to the dome. Have a student mark where the laser pointer hits the dome for several positions of the sun in its orbit around the Earth.
Compare the real observations to the synthetic ones. Are they consistent with each other (yes, for one day only).
Now ask how one can generate seasonal variations in the Sun's elevation vs. azimuth using the Earth-centric model (they have observations on their domes which demand this change).
Finally, I put the sun the center, hold the globe in one position (hit seasons while you're there since the Earth is tilted) and make synthetic data for one day. It matches observations for one day. Try another day. See if students can figure out where the Earth needs to be relative to the sun for the days of their observations.

That's it. I have a write-up if anyone is interested. The main point here is that many students have difficulty seeing balls representing the Earth, moon and sun in their minds. "Geometric thinking" (for lack of a better phrase) is not well-developed in many students. These exercises are meant as a concrete model to encourage this type of thinking. I hope to get an article in TPT soon about this.

Cheers,
--
------------------------------------------------------------------------
Dr. Dean Livelybrooks Department of Physics
Rm. 221 Willamette Hall 1274 University of Oregon
541.346.5855 Eugene, OR 97403-1274 USA

541.346.5861 FAX "God is subtle, but he is not malicious."
Albert Einstein

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I have attempted to teach these things in a 100-level, non-science student
class.
<br>Moon Phases:
<br>For the moon I use a white volleyball, a globe and an old projector
that gives me collimated white light.
<br>Have students cluster near "Earth" and observe.
<br>Hold the projector in front of class, lighting the Earth.&nbsp; Ask
the students which side of the "Earth" is the dark side.&nbsp; Have them
move there for "lunar observations."
<br>Have a student hold the volleyball between "Earth" and "moon," and
at the various positions in an orbit around "Earth."
<br>One can see the phases of the moon on the volleyball.&nbsp; (You may
need to swing the projector/sun out to each side of the front of the classroom
as the moon goes through its orbit, so that the "sun" is shining normally
on the moon.
<br>Sun-Earth:
<br>I stole ideas from Project Star.&nbsp; Purchase and distribute clear,
acrylic hemispherical domes that one can buy at art supply shops cheaply.&nbsp;
Also distribute a piece of paper with the outline of the dome on it, a
center point, and a North arrow.
<br>Have students take both of these things, a pen and a compass outside
several times during a day when the sun is clearly observable.&nbsp; They
should hold the pen so that the (sun's) shadow of the tip falls on the
center of the hemispherical dome, and mark where the tip is on the half-dome.&nbsp;
Date and time stamp each observation.&nbsp; Take several on the same day.&nbsp;
Take observations for a day twice or more, spaced several weeks or months
apart.&nbsp; Use different colored pens for different days.
<br>This represents a set of observations.&nbsp;&nbsp; I have students
bring these to class/lab on a specified day.&nbsp; We start using the Ptolemaiic
model (spelling?).&nbsp; Tape an unmarked dome over where you are on a
globe (centered over your city or region).&nbsp; Have a student hold the
globe with dome.&nbsp; Start with the sun (a laser pointer) in the center
and have the Earth orbit the sun.
<br>Make model-derived data by marking where the laser pointer light hits
the unmarked dome on the globe.&nbsp; This is made easier as the laser
light will reflect&nbsp; directly back&nbsp; at the "sun" (laser pen holder)
when the light is aimed directly at the (local) normal to the dome.&nbsp;
Have a student mark where&nbsp; the laser pointer hits the dome for several
positions of the sun in its orbit&nbsp; around&nbsp; the Earth.
<br>Compare the real observations to the synthetic ones.&nbsp; Are they
consistent with each other (yes, for one day only).
<br>Now ask how one can generate seasonal variations in the Sun's elevation
vs. azimuth using the Earth-centric model (they have observations on their
domes which demand this change).
<br>Finally, I put the sun the center, hold the globe in one position (hit
seasons while you're there since the Earth is tilted) and make synthetic
data for one day.&nbsp; It matches observations for one day.&nbsp; Try
another day.&nbsp; See if students can figure out where the Earth needs
to be relative to the sun for the days of their observations.
<p>That's it.&nbsp; I have a write-up if anyone is interested.&nbsp; The
main point here is that many students have difficulty seeing balls representing
the Earth, moon and sun in their minds.&nbsp; "Geometric thinking" (for
lack of a better phrase) is not well-developed in many students.&nbsp;
These exercises are meant as a concrete model to encourage this type of
thinking.&nbsp; I hope to get an article in TPT soon about this.
<p>Cheers,
<br>--&nbsp;
<hr>
<table BORDER=0 CELLSPACING=3 CELLPADDING=0 WIDTH="550" >
<tr>
<td><a href="http://hendrix.uoregon.edu/~dlivelyb/index.html";>Dr. Dean
Livelybrooks</a></td>

<td><a href="http://physics.uoregon.edu";>Department of Physics</a></td>
</tr>

<tr>
<td>Rm. 221 Willamette Hall</td>

<td>1274 University of Oregon</td>
</tr>

<tr>
<td>541.346.5855</td>

<td>Eugene, OR 97403-1274 USA</td>
</tr>

<tr>
<td>541.346.5861 FAX</td>

<td>"God is subtle, but he is not malicious." Albert Einstein</td>
</tr>
</table>
</html>

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