Download Phases of the Moon - Michigan State University

Anna Cajiga
SME 301
Astronomy Summary Revision
February 17, 2005
Title
Understanding the Phases of the Moon
Benchmark
Area: Earth Science
Category: Solar System, Galaxy and Universe
Benchmark: Describe and explain common observations of the night skies. (Middle School)
Misconception
The phases of the moon are caused by the shadows cast on its surface by other objects in the
solar system, particularly the earth or the sun.
Background
There is a common misconception that the phases of the moon are caused by the shadows
cast on its surface by other objects in the solar system, particularly the earth or the sun. However,
this experiment explores what actually causes the phases of the moon. The moon revolves
around the earth in a counter-clockwise direction. Throughout this orbit, exactly one half of the
moon is always illuminated by the sun – the half that is facing the sun, since this is the half of the
moon that is able to reflect the light of the sun. From earth, we are only able to see the part of the
moon that is facing us and is also reflecting the light of the sun. This is why the appearance of
the moon changes over the course of a four week time span. In addition to revolving around the
earth, the moon also rotates on its own axis. It takes 29.5 days for the moon to complete one full
revolution around the earth and one full rotation on its axis, which is why it also takes 29.5 days
for the moon to complete its cycle.
Materials
One Tennis Ball
Black Permanent Marker
One 6 Inch Wooden Dowel
Hot Glue Gun
Yellow Balloon
Tape
Cutout Paper Circle with a 12 inch Diameter
Directions
1. Pretend the cutout paper circle is a clock. Draw short lines where the 12, 3, 6 and 9 would be.
Now, draw short lines exactly halfway between the 12 and the 3, the 3 and the 6, the 6 and the 9
and the 9 and the 12. Label the lines as shown below:
Cajiga 2
2. Color exactly one half of the tennis ball with the black permanent marker to represent the unlit
half of the sun. Poke a hole in the tennis ball somewhere on the line that separates the black half
from the yellow half. Insert the wooden dowel into this hole and secure it using hot glue. When
done, it should look like this:
Tennis Ball
Wooden Dowel
3. Tape the inflated yellow balloon to a wall so it is at eye level. This balloon represents the sun.
Walk away from the wall in a straight line for about 10 feet and place the paper circle on the
ground so Line A is pointing towards the balloon. The setup should look like this (from a birdseye view):
Balloon
About
10 Feet
Paper Circle
4. Stand on the paper circle so you are facing Line A. You represent the earth. Holding the
wooden dowel, put the tennis ball directly in front of your face at arms length and eye level.
Always make sure that the illuminated half of the moon (the yellow half) is facing the sun. Draw
a picture of what the tennis ball looks like.
5. Rotate your body so you are facing Line B. Continue to hold the tennis ball so it is in front of
your face at arms length and eye level. Once again, make sure that the illuminated half of the
moon is facing the sun. Draw a picture of what the tennis ball looks like.
6. Continue this process for Lines C – H, always making sure that as you rotate your body, the
illuminated half of the moon is always facing the sun.
Results
After completing this experiment, students will have a collection of drawings that
illustrate their observations of the “moon”. These drawings show a progression from the moon
not being visible, to slowly “appearing” from the right side until it is a full circle, and then
slowly “disappearing” from the left side until it is not visible once again.
Discussion
As this activity illustrates, rather than being caused by the shadow of the earth or the sun,
the phases of the moon are actually caused by the moon’s location relative to the sun and the
earth. The moon both rotates on its own axis, and revolves around the earth. Both the revolution
and rotation of the sun take 29.5 days, which is why on earth, we always see the same side of the
moon. Only half of the moon is ever lit, because the light can only hit the half of the moon that is
facing the sun. However, we cannot always see the entire lit half of the moon – we can only see
the part that is facing us and is also lit by the sun. In this activity, the tennis ball represents the
moon, the yellow balloon represents the sun and the person holding the tennis ball represents the
earth. As is shown in this activity when we face Line A, sometimes, we cannot see any of the
moon because it is between the earth and the sun. When this happens, the side that is lit is facing
Cajiga 3
away from us. This is called a new moon. After about a week, the moon continues to revolve
counterclockwise around the earth and we can now see the right half of the moon, because only
half of the lit half is facing us on earth. This is called the first quarter moon. In this activity, we
can see the first quarter moon at Line C. The transition from the new moon to the first quarter
moon is shown when the participant is facing Line B. This transition is called a waxing crescent
moon. After the first quarter moon, the moon continues to revolve for about another week until
we have a full moon. A full moon is when the whole lit half of the moon is facing us on earth, so
we can see it entirely. The full moon in this activity occurs at Line E. Between the first quarter
and the full moon, the moon is called a waxing gibbous moon. We can see what a waxing
gibbous moon looks like at Line D. After the full moon, the moon is called a waning gibbous
moon (shown at Line F) until about a week later, when would be facing Line G in this activity,
and we can see the left half. This is called the third quarter moon, and we can just see the left
half because only half of the lit half is facing us. The moon continues to revolve around the earth
for about another week until it is back in the new moon position. Between the third quarter moon
and the new moon, the moon is called a waning crescent, and we can see what this looks like at
Line H. In all it takes 29.5 days, the time it takes for the moon to travel all the way around the
earth, for the moon to complete all of these phases. As the phases of the moon change, the time
that it rises and sets each night also changes. This is because the moon is in a different spot of its
orbit around the earth each night, and we only have the ability to see the moon when the earth is
at a point in its rotation that we are facing the direction of the universe that the moon is located
in. During a new moon, we can’t see the moon at all, so there is no moonrise or moonset. During
the first quarter moon, the moon rises at noon and set at midnight. When there is a full moon, it
rises at sunset and sets and sunrise, and during the third quarter moon, the moon rises at midnight
and sets at noon.
Just like the revolution of the moon around the earth is responsible for the phases of the
moon, the earth’s revolution around the sun combined with the tilt of the earth is the reason the
seasons change. The earth’s axis is tilted 23 degrees from being vertical. As the earth revolves
around the sun, the northern tip of its axis is always pointed towards the North Star. The
changing of seasons is based on the changing number of hours of sunlight each day. In its
revolution around the sun, there are times when the northern hemisphere is tilted towards the
sun. At this time, it is summer in the northern hemisphere because as the earth rotates to
complete one 24 hour day, the northern hemisphere is receiving more hours of light. However, it
is winter in the southern hemisphere because this hemisphere is receiving less hours of light. On
the other hand, when the southern hemisphere is tilted towards the sun, it is summer in the
southern hemisphere and winter in the northern hemisphere, because the southern hemisphere
has more hours of light. Spring and Fall are the intermediate seasons when the axis of the earth is
not tilted towards or away from the sun. During these seasons, both hemispheres receive about
the same amount of light. Having more hours of light is one reason that it is warmer in the
summer months, but warmer temperatures also occur because the intensity of the sun increases in
the summer. During the summer, the sun is at a higher angle in the sky than it is in the winter
because during the summer we are tilted toward the sun, so we have to look higher in the sky to
see it. In Michigan, the sun is about 70 degrees high in the summer and about 24 degrees high in
the winter. This means that when it is summer, the energy from the sun is concentrated in a
smaller area, which makes it much more intense than winter when the sun’s energy is spread out
more. The increased intensity of sunlight in the summer is a major factor in warmer
temperatures.
Cajiga 4
The sun and the moon are not the only things we can see in the sky – we can also see
stars, constellations and planets. Constellations are groups of stars that resemble objects, people
or animals. The stars that make up the constellations are huge balls of gas, like our sun, that are
located very, very far away from the earth so they appear to be just tiny dots in our night sky.
While stars are always made of gasses, planets can be solid masses or gaseous masses. Mercury,
Venus, Earth, Mars and Pluto are solid, and the rest of the planets are made of gasses. Another
difference between planets and stars is that stars glow from their own light, but planets glow
because they reflect the light of our star, the sun. The planets in our solar system are, in order
from the sun, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. The
only planets that are large enough or close enough to the Earth to be seen with the naked eye are
Mercury, Venus, Mars, Jupiter, and Saturn. Some people say that they can also see Uranus. Like
the moon and the sun, the planets and stars both appear to rise in the east and set in the west.
This is because the earth’s rotation is counterclockwise so we see different parts of the night sky
as the night progresses. Over the course of a year, the planets and constellations that we see in
the night sky change. This is because as the earth revolves around the sun, the part of the
universe that can be seen above the horizon at nighttime changes because we are facing different
parts of the universe. The stars that we see on one night of a year will be the exact same stars that
we see on that same night another year because the stars are fixed objects – they do not revolve.
However, the planets that we see will change from day to day and year to year. This is because in
order for us to see it, a planet has to be in the direction of the universe that we are facing at
nighttime. If a planet is not within our horizon between sunset and sunrise, we cannot see it.
Each planet has a different sized orbit because each planet is a different distance from the sun.
The planets that are closer to the sun travel once around the sun much faster than the planets that
are further from the sun. This means that some planets make one trip around the sun faster than
the earth, and some take longer than the earth to travel once around the sun. Because of this, the
planets that are in the direction of the universe that we are facing are constantly changing.
Understanding more about the solar system helps us to understand many phenomena that
we observe in our everyday lives. For example, the importance of wearing sunscreen during the
summer is based on astronomy. Since the sun is at a higher angle in the sky during the summer,
the energy coming from it is much more intense and concentrated than the energy coming from
the sun in the winter when it is at a lower angle. This means that during the summer there is
more potential for the sun to cause damage to our skin. Also, understanding these concepts also
explains why when the moon is full, it seems like we can see it more often. This is not only
because the full moon is when the moon is at its largest and brightest, but because the moon is in
the sky for more hours of the night at a full moon. During a full moon, the moon rises at sunset
and sets and sunrise, so it is always in the sky while it is dark out. During the other phases of the
moon, rather than being in the sky at night when we can see it, the moon is often in the sky
during the daytime when the sun is too bright for us to see the moon. Learning more about the
moon also helps explain why we can always see the “man in the moon.” Since one rotation and
one revolution of the moon both take 29.5 days, the same side of the moon is always facing the
earth. This is why we can always see the same craters on the surface of the moon that make up
the face of the “man in the moon.” Lastly, the fact that we will see the same stars on a particular
day every year explains the origination of Zodiac signs. Since the stars are fixed objects in the
sky and we see them change in the same cycle year after year, the Zodiac signs designed to
represent what stars are visible in the month that we were born.
Cajiga 5
Classroom Resources
1. “Stars” by Meish Goldish (Sung to the tune of “Twinkle Twinkle, Little Star”)
Twinkle, twinkle, little star,
I know what you really are:
Giant ball of glowing gas,
One of billions in a mass!
Twinkle, twinkle, little star,
Oh, how big you really are!
Twinkle, twinkle, giant star,
Larger than the Earth by far!
Since your distance is a lot,
You look like a tiny dot.
Twinkle, twinkle, giant star,
Very bright, yet very far!
Stars are twinkling, every one,
Some are bigger than the sun!
Just a twinkle in the sky,
Just because you're oh, so high!
Twinkle, twinkle, little star,
Oh, how big you really are!
This song would be very helpful when attempting to illustrate the true qualities and
characteristics of stars – that they are huge balls of gas that are extremely far away from the
earth. (Lyrics found at <http://www.canteach.ca/elementary/songspoems34.html>.)
2. Kids Discover – Solar System issue
This magazine includes in-depth discussions of many aspects of the solar system including stars,
comets, meteors, asteroids, and all nine planets, and it also describes how theories about
astronomy have changed and progressed throughout history.
3. Take a Walk Through Our Solar System Activity
(Found at < http://rip.physics.unk.edu/astronomy/Walk(3-12).html>.)
In this activity, our entire solar system is scaled down to what it would look like if our sun were
about the size of a dodge ball, which illustrates not only the sizes of the planets in relation to the
sun, but also the vast distances between each.
Credits
This activity is based on the activity we did in class where we used two-toned bouncy
balls to demonstrate how the appearance of the moon from earth changes over a four week
period of time. This activity really helped me solidify and clarify my knowledge of the moon
phases, which led me to believe that it would also be very helpful to my future students. I
adapted the materials used for this activity, because I felt that the bouncy ball was very small, so
perhaps a tennis ball would be easier to see. I also added more “check-points” for where students
should make observations of what they see to help them understand what the moon looks like at
stages other than just new, first quarter, full and third quarter.