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How
to make a "Moon Stick"
Some of you
asked to make a "Moon Stick" (as several of us call it). For those at
the CLCs you might want to have one available in case questions arise
about distance, size, and scale. And for the teachers it is always good
to have one available in the classroom. The key is to remember that the
distance to the Moon from Earth is about 29 1/2 Earth diameters. Here's
the easiest way to build the Moon Stick:
-
Obtain
a wooden dowel 1/4" or 3/16" in diameter. Cut the dowel to 30 inches.
-
Obtain
two spheres (cork, wood, Styrofoam, etc.). The sphere for the model
Earth should have a diameter as close as possible to 1"; the sphere
for the model Moon should have a diameter as close as possible to
1/4".
-
Obviously
the next step is to put the model Earth on one end of the dowel and
the model Moon on the other end. You might try to sharpen each end
of the dowel (not too sharp, though) to make it easier to place the
spheres on the dowel. Be certain to use some glue to affix the spheres
(some glues don't work very well with Styrofoam).
-
That
should do it. If you use a model Earth that is not 1" in diameter,
just be certain to make the distance to the model Moon about 30 times
that diameter.
How
to make a model to demonstrate the barycenter (center of mass) of the
Earth-Moon system
The Earth
is roughly 80 times more massive than the Moon. That means the barycenter
for the system is about 1/80 from the center of the Earth (i.e., 1/80
of the distance between the center of the Earth and the center of the
Moon). That puts it about 900 miles below the surface of the Earth. What
I do is as follow:
-
Start
as with the Moon stick - i.e., dowel, spheres. It is a little harder
to get this to scale, since you have to cut open the sphere (in half)
for the model Earth, remove much of the inner material, and replace
it with some very dense material. (Try lead; you can get lead sinkers
or lead weights which are used at the bottom of drapes.) I think you
need to start with at least a 2" diameter sphere so that you can hollow
out enough room for the lead weights. Since that would require a 60"
dowel (to properly represent the scaled distance to the Moon), you
probably have to settle for an improper scaled distance for this model.
Fasten the model Moon to the dowel.
-
Tie a
2-foot piece of string onto the dowel and position it close to the
end of the dowel where the model Earth will be located.
-
By trial
and error, figure out how many pieces of lead you need to insert into
the model Earth to have the whole set up balance with the string positioned
right next to the model Earth (i.e., way off center of the dowel).
-
Glue
the model Earth back together (with the lead inside) and fasten to
the free end of the dowel. You can fine tune the balance of the system
by taping small pieces of masking tape to one end or the other.
-
If you
now hold the model in front of you (away from your body), you can
slowly nudge the Moon so that it and the Earth will each orbit the
other, with the center of the motion located at the barycenter.
How
to find the N/S line with an analog watch
(You can
cheat and put a magnetic compass on your wrist - looks like a watch! -
but I know that won't do.) Here's the story:
-
Outside
on a sunny day point the hour hand of your watch directly at the Sun
(keep the face of the watch level with the ground). Don't look directly
at the Sun! You can do this most easily by finding a small twig -
like a toothpick - and holding the twig upright over the center of
the watch face (the center post for the hands). Move the watch until
the shadow of the twig falls directly away from the hour hand - that
will mean the hour hand is directly facing the Sun.
-
Without
moving the watch, locate the hour numeral on your watch face that
approximately corresponds to your "local noon." (That's another way
of saying, find the 1 o'clock point during daylight savings time,
and 12 noon during standard time.)
-
Now take
the angle between the hour hand (which should still be directly facing
the Sun) and the local noon point and bisect that angle. An imaginary
line that runs from the center point of the watch over that point
of bisection is the N/S line. Here's an example: Suppose you go outside
about 10 am standard time. Align your watch so that the hour hand
faces the Sun. (The hour hand would be pointing to both the numeral
10 and the Sun.) Now find noon on the dial face. Bisect the angle
between 10 and noon. Hence, an imaginary line running from that point
over the center of your watch will be the N/S line for you. Got it?
Let us know.
A
fine time to observe the Moon!
-
The best
times to observe the Earth's Moon are during the first and last (a.k.a.
third) quarters. At those times the Sun is also visible. First quarter
moons are visible in the afternoons; the last quarter moons are visible
in the mornings.
-
Go outside
and locate both the Moon and Sun in the sky. Stand in the sunshine.
Take a sphere (a tennis ball is a convenient size) and hold it up
in the sunlight at arm's length. Position yourself so the sphere is
held just under the Moon. Now the sphere models the Moon in space:
the portion of the tennis ball illuminated by the Sun replicates the
portion of the Moon illuminated by the Sun.
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