Phases of the Moon:
Planetarium show for Middle School Children
Pre-Show Activity 1
Students will draw a diagram illustrating the phases of the moon and the positions of the moon when eclipses occur.
3 - 6
Students will be using the materials to create a model of the relative positions of the Earth, Moon, and Sun. The light source will act as the Sun, the white ball will model the Moon, and the student's head will be the Earth. It is assumed that the class has some prior knowledge of moon phases and eclipses. The following paragraphs outline the specific message the model will be simulating.
The phases are due to how the Sun illuminates the Moon and the relative positioning of the Earth, Moon, and Sun. It is important to remember that the Moon is always half illuminated (unless there is a lunar eclipse). What we observe changes because of our perspective. You will observe only a small fraction of the Moon's illuminated side when it is close to the Sun. In fact, the smaller the angular distance between the Moon and the Sun, the less of its illuminated side you see. When the angular distance is less than 90° separation, you will see less than half of the Moon's illuminated side and it will look like a curved sliver of light---the crescent phase. Because the Moon is spherical, the boundary between light and shadow is curved. When the angle is within about 6 degrees you see it in a new phase and is the beginning of the phase cycle. Sometimes that angle = 0 degrees and you have a solar eclipse---the moon is in new phase and it is covering up the Sun. At 90° angular separation you see half of the Moon's illuminated side and the phase is called a quarter phase because you can see a quarter of the Moon's entire surface. The quarter phase a week after the new phase is called first quarter.
The greater the angular distance is between the Moon and the Sun, the more of the Moon's illuminated side you can see. When the angular distance is more than 90° separation, you will more than half of the Moon's illuminated side---the gibbous phase. ``Gibbous'' means a shape that is convex (bulges outward) at both sides. Around 180° angular separation, you see the entire illuminated side of the Moon---the full phase. Around 180 degrees angular separation, you see the Moon in full phase. Sometimes (about twice a year) the Sun-Moon angle is exactly 180 degrees and you see the Earth's shadow covering the Moon---a lunar eclipse. Sometimes a descriptive term is added to the crescent and gibbous phases. If the amount of illuminated side you can see increases with time, it is waxing as in waxing crescent or waxing gibbous. If the illuminated fraction decreases with time, it is waning as in waning crescent or waning gibbous.
- Bright light source; overhead projector or a flood lamp
- White Styrofoam balls
- Drawing paper, pencils
- Explain to the students that they are going to create a model of the Sun, Moon, and Earth to explain the phases of the moon and why eclipses occur.
- Turn on light source and have students face the light. This will be noon. The sun (light source) is straight above the surface of the Earth (head). Explain that the top of their head is like the North Pole, and their chin is the South Pole. Have the students simulate the rotation of the Earth by turning their heads counterclockwise. As their eye face 90' to the left of the light, this is sunset. Continue around until they are facing away from the light; midnight. As they continue around, they will be facing 90' to the right of the light source and this will be sunrise. Practice until students understand the model.
- Introduce the Styrofoam ball. First have students note that the ball is always have illuminated. Let them explore with the ball and the light source for a while. Pose guiding questions like, "What would the model look like at full moon? new moon?" "Can you simulate an eclipse?"
- Depending on the sophistication of the class, you can end the lesson there. If they are ready for more:
- Talk about the ecliptic. Explain how all three objects have to be in line in order for there to be an eclipse. Have the students try it. The Moon's orbit is tilted by 5 degrees with respect to the Earth's orbital plane (the ecliptic). In order for an eclipse to occur, the Moon must be in the ecliptic plane AND exactly at the new or full phase. Usually, the Moon crosses the ecliptic plane at another phase instead of exactly at new or full phase during its approximately month-long orbit around the Earth. During a year the Moon's orbit is oriented in very nearly the same direction in space. The position of the Earth and Moon with respect to the Sun changes while the Moon's orbit direction is approximately fixed. So in one month the Moon will be below the ecliptic at full phase and above the ecliptic at full phase about six months later. Though the Moon crosses the ecliptic twice a month, an eclipse will happen only when it is exactly at full or new phase when it crosses the ecliptic. The tilt of the Moon's orbit explains why eclipses happen only twice a year.
Another source to help explain the phases of the moon and eclipses is
http://www.astro.wisc.edu/~dolan/java/MoonPhase.html. Be sure to choose ``both'' in the point of view pop-up list.
- Have students take drawing paper and draw a model of the Sun, Moon, and Earth to show the different phases and eclipse points. Allow them to share their ideas in small groups or with the whole class.
Use the following questions to check for understanding:
- Why does the Moon have phases?
- If the Moon was full 7 nights ago, what time of day (night) should you look for the Moon to be up high in the sky in the south today? Explain your answer.
- What are the positions of the Earth-Moon-Sun during an eclipse?
- What would the Sun-Moon angular separation be for the New Moon if the Earth's shadow caused the lunar phases? How about Gibbous phase?
- What are the real angular separations for New and Gibbous phase?
- About how much difference in time is there between moonset and sunset at first quarter phase? Does the Moon set before or after the Sun at that phase?
- About how much difference in time is there between moonset and sunset at new phase?
- If the Earth's shadow caused the lunar phases, what would be the difference in time between moonrise and sunrise at new and first quarter phases?
- About when will the Waxing Crescent Moon be on the meridian? Explain your answer.
- The Moon is low in the western sky at sunrise, what is its phase? Explain!
- Why do we not have eclipses every month?
Some information on this page was taken from Nick Strobel's Astronomy Notes. Go to his site at www.astronomynotes.com for the updated and corrected version.
Source: Matt Merritt