In this article, we present a new way of

Transcription

1 A new perspective on teaching the phases of the Moon to upper-elementary students Keywords: Moon Phases at Enter code: SC By Timothy Young and Mark Guy In this article, we present a new way of teaching the phases of the Moon. Through the introduction of a self shadow (an idea of a shadow that is not well-known), we illuminate (pun intended) students understanding of the phases of the Moon and help them understand the distinction between the shadows that cause eclipses and the shadows that relate to the phases of the Moon. Then, we follow with two easy-to-do demonstrations that help students further develop their understanding of the reasons behind the patterns of lightness and darkness in the Moon s phases. 30 Science and Children

2 Shadows First The shadows we think of most commonly are cast shadows. Light from any source can be blocked by an opaque object, reducing the amount of light falling on surfaces in the direct path of the light source and causing a shadow to form on them. For example, a person on the playground on a sunny day in the afternoon will cast a shadow on the cement or anything that enters the light s path. A cast shadow is separate from the object itself. But, there is a second kind of shadow, the self shadow. A self shadow is a shadow on the object blocking the light. It forms on the opposite side of the light source. Unlike a cast shadow, which is separate from the object, a self shadow is physically connected to the object blocking the light. Figure 1 shows the two different kinds of shadows the cast shadow is on the ground, the self shadow is on the body (or object) itself. How do these different kinds of shadows relate to an understanding of the phases of the Moon? Well, most people think the phases of the Moon are caused by cast shadows. The phases of the Moon are not caused by cast shadows as is commonly believed. Instead, the phases of the Moon are seen because we can t see the surface of the Moon that is in self shadow. That part of the Moon is in nighttime or darkness and gets no light. In space, everything is darkness unless it is near a star. This includes all the planets in the solar system and all the Moons orbiting the planets. The Sun s rays, coming from one direction, only light one half of an object at a time, leaving the other half of the object in darkness, or self shadow. On Earth, the half of the Earth that is bathed in sunlight is known as daytime. As the Earth turns on its axis each day, the half-lit side constantly moves across the surface. The unlit side of the Earth is in darkness, what we call nighttime. This unlit side of the Earth is its self shadow, the half of the object that is blocked from light. Figure 1. Cast shadow (on ground), self shadow (on body). Now, apply the idea of self shadow to the Moon. The Moon is an object in the sky and receives light from the Sun as does the Earth. And, because only one half of an object in space faces the Sun at one time, the Sun can only light one half of an object at a time. We know that half of the Moon is lit (daytime), while the other half is in darkness (nighttime). The unlit side of the Moon is its own self shadow. Now factor in that the Moon orbits the Earth each month and that the Earth orbits the Sun each year. The Moon s orbit is tilted at five degrees from the Earth Sun orbit and thus the Moon orbits above the plane of the Earth Sun orbit and below it. This is important because the alignment (in a straight line) of the Sun, Earth, and Moon is a rare event. At those times, cast shadows fall onto another object in space, which result in what we call eclipses. They are rare because the Moon s orbit crosses the plane of the Earth Sun orbit at only two points. If there were no tilt to the Moon s orbit around the Earth, we would have eclipses 24 times a year! This would be great, but not exactly the novelty it is today. Outside and Inside Views Now let s imagine we could view the Earth Moon Sun system from space as if we were outside the system. Imagine looking down at the Earth Moon from the north and the Sun off to one direction (it is not necessary to specify the location but the location must remain fixed). From this perspective we are far enough away that we can see the Earth and Moon in the same view. We would see that half the Earth is in sunlight and half in darkness. The Moon as well would have half of its surface lit up and half in darkness. If we now watched as the Moon orbited the Earth, in one month we would see the same pattern of light and darkness on the Moon, half lit and half unlit. Different specific areas on the surface of the Moon will be lit at different times, but overall, half would be always be lit and half would always be in darkness at any given time. That was looking at the Moon from outside the system, now we will place ourselves on the Earth to see a different perspective. From Earth things look very different. On Earth we are inside the system looking from a unique position. Here we can see the Moon orbiting the Earth and also changing its position with direction toward the Sun. This is important because as the Moon orbits the Earth we see different parts of the sunlit side of the Moon when we look at it. For example, the full Moon occurs when the Moon is on the opposite side of the Earth as the Sun. When viewing from Earth, we are only seeing the surface of the Moon that is lit up by the Sun; we are seeing every bit of daylight on the Moon. The other half of the Moon, September

3 the unlit side, is in darkness, or nighttime and its self shadow. In contrast, when the Moon is between the Earth and the Sun, on Earth we are seeing the unlit side of the Moon, so we are looking at its nighttime or self shadow; the new Moon. Because it is so dark we see nothing. Those are the two unique positions at either end of the Moon s cycle. The rest of the positions of the Moon around the Earth reveal varying amounts of sunlight and self shadow. As the Moon moves from new Moon to full Moon (counterclockwise motion when viewed from the North pole), the Moon gradually moves so that more and more of the lit-up side is visible. This is called the waxing part of the cycle and includes in order, the waxing crescent, the first quarter (a half moon), and the waxing gibbous. To complete the Moon s orbit (continuing counterclockwise as viewed from the north), we see the Moon change from full Moon to new Moon. The name for this part of the cycle is waning. Then in order after the full moon, we see the waning gibbous, third quarter Moon (a half moon), and waning crescent. Deepening Understanding The explanations above can be reinforced through the activities and models used below. Glow Moon Model Objective: As the Moon orbits the Earth, we observe different portions of the Moon s lit half from Earth s perspective. Materials: Styrofoam ball Stick Glow-in-the-dark latex paint (available in many paint and hardware stores) Flashlight, overhead projector Moon phase worksheet (Figure 2) Glow Moon demonstration setup diagram (Figure 3) Figure 2. Moon phase worksheet. New Moon Beginning at position A on Figure 3, this is the new Moon position. This is the first phase we see and the image is black. We are seeing all of the self shadow (nighttime) on the Moon. Waxing Crescent As the Moon orbits counterclockwise (B in Figure 3), we are seeing mostly the self shadow (nighttime) on the Moon. First Quarter In this position (C in Figure 3), we are seeing equal amounts of the lit-up part (daytime) and self shadow part (nighttime) on the Moon. Waxing Gibbous Continuing in a counterclockwise revolution (D in Figure 3), we see more of the lit-up half (daylight) than the self shadow half (nighttime) of the Moon. Full Moon Continuing in a counterclockwise revolution (E in Figure 3), we see all of the lit-up half (daylight) and none of the self shadow half (nighttime) of the Moon. Waning Gibbous Continuing in a counterclockwise revolution (F in Figure 3), we see more of the lit-up half (daylight) than the self shadow half (nighttime) of the Moon. This is different than the waxing gibbous because the left side of the Moon is bright. Third Quarter Moon In this position (G in Figure 3), we are seeing equal ammounts of the lit-up part (daytime) and self shadow part (nighttime) on the Moon. This is different from the first quarter Moon because the left side of the moon is bright. Waning Crescent The last phase in the Moon s cycle is the waning crescent. In this position (H in Figure 3), we are seeing mostly the self shadow (nighttime) on the Moon. It is different from the waxing crescent because the left side is bright. A B C D E F G H 32 Science and Children

4 The Moon s Phases and the Self Shadow Figure 3. Demonstration setup diagram. Use this diagram for understanding the positions for the Glow Moon model and the setup for the positions of the golf balls in the Moon Ring model. Waxing Gibbous (D) Full Moon (E) Waning Gibbous (F) Earth First Quarter (C) Third Quarter (G) Waxing Crescent (B) To Sun New Moon (A) Waning Crescent (H) Note: Set golf balls equally spaced in a circle with each golf ball on a separate desk. Turn golf balls so that the half-white side is facing toward one wall. Proceed to do this with all the golf balls. The wall that they are oriented toward is indicating the direction of the Sun. Teacher Preparation: Before instruction, paint half of the ball. Let dry as directed. Apply a second coat. Make sure the edge line (called the terminator) is very sharp. We used a rubber band to set the halfway mark on the ball. When the ball is dry, gently push the dowel into the Styrofoam ball at any boundary point between painted and unpainted parts. Before using the model in class, place the model in a bright light source for minutes to charge the ball. (We found that placing the model directly on a lit overhead projector works well.) Students are given a worksheet with eight Moon phases showing the phase appearance and name (Figure 2). These photos are the view from Earth and represent the view inside the system. Exploration: Have students sit in a circle in the middle of darkened classroom. Explain to students that the Sun is a long distance away and always faces a specific direction (you choose which). Next, demonstrate the Moon s orbit for students, walking around the students in a counterclockwise direction holding the Glow Moon model straight Figure 4. Assessment rubric for the Glow Moon and the Moon Ring models. High Medium Low Phases Identifies all eight phases Identifies four six phases Identifies less than four phases Shadow Types Cause of Moon Phases Sun Earth Moon Positions with a model Perspective (Moon View Ring only) Shows and describes properties of a self shadow and a cast shadow in detail Explains the role of the self shadow in the cause of Moon phases accurately with detail Positions Sun Earth Moon model correctly for all eight phases Demonstrates and clearly describes how perspective determines the occurrence of Moon phases Shows and describes properties of either a self shadow or cast shadow Explains the role of the self shadow in the cause of Moon phases in general terms Positions Sun Earth Moon correctly for four six phases Demonstrates and partially describes how perspective determines the occurrence of Moon phases Shows limited ability to show or describe properties of a self shadow or a cast shadow Limited explanation of the role of the self shadow in the cause of Moon phases Positions Sun Earth Moon for less than four phases Limited demonstration or description of how perspective determines the occurrence of Moon phases September

5 up by the dowel that was inserted. This is to eliminate any hands blocking the light. Keep the dowel straight up and as steady as possible when walking. Alternatively a stand and a cart can be used to put the model on and wheel it around. It is important to keep the glow-painted side of the ball facing toward the direction chosen for the Sun (for example, the chalkboard) at all positions in the orbit around the children. You may have to turn the dowel in your hand to achieve this. This mimics the fact that the Sun always shines on the half of the Moon facing it. A fully darkened room produces the best results, so be sure to have a flashlight to aid you in walking the Moon. As the Moon orbits the students the students will continuously see the glowing part on the Moon first increasing and then decreasing. During demonstrations of the model, the students will call out the phases of the Moon as the teacher walks around, full Moon, third quarter, and so on. Figure 5. Moon Ring views. Space View: Outside the Moon Ring Outside the orbit of the Moon, we see all eight moons and they are all the same phase. Note: Students must be some distance away (more than feet) from the ring to see this effect. Earth View: Inside the Moon Ring Photographed inside the orbit towards the waxing crescent (right) and first quarter or half moons (left). Continuing around the orbit we see all the phases of the Moon. Discussion and Assessment: What we have seen is that a 3 D sphere that is half lit can produce all the shapes of the phases of the Moon. It is an example of a simple model that works. The glowing part of the sphere is the reflected sunlight, and the part that is dark is the self shadow. Interestingly, for astronomical bodies, the self shadow is as dark as space and thus you cannot see that surface of the object. In the classroom, the darker the room the less light will fall on the sphere and will represent the self shadow ex- 34 Science and Children

6 The Moon s Phases and the Self Shadow ceptionally well. One drawback of the model is that the light is being generated at the surface of the paint. But we feel that this is in essence a delayed reflection! To assess student understanding, have students place other 2 D or 3 D shapes in a light source, and then identify the self shadow and the cast shadow for each object. Figure 4, p. 33, is a rubric to evaluate students understandings of these concepts. Moon Ring Objective: Students will understand that the Moon phases occur only from the Earth s perspective. Materials: Eight hollow plastic golf balls or any set of similarsized spheres (to represent Moon phases) Eight golf tees or any type of stand to hold the golf balls up Black or white paint Demonstration Setup Diagram (Figure 3, p. 33) Teacher Preparation: Paint half of each ball black. If the balls are colored, paint the other half white. The teacher should puncture each ball with a sharp implement and then students can insert the sharper edge of the golf tee into the golf ball and glue it into place. Exploration: Place eight Moons on desks around a central area where children can sit. Be sure to orient all of the balls with the white side facing one direction. It can be any direction, but they have to face the same way. Explain that is the direction of the Sun. Follow the setup shown in Figure 3. From inside the ring (Earth view), one sees all the phases. From outside the ring, one can only see one phase on all Moons (space view). Note: Students must be some distance away (more than feet) from the ring to see this effect. Figure 5 shows the views from both the inside and outside perspectives. Discussion and Assessment: Students are usually amazed when they see this model in action. Even though they may have understood the concept on paper, once the demonstration is set up and students see it for themselves, almost every one excitedly says, I can see all the Moons! [phases] [inside the ring]; The Moon stays in the same phase! [outside the ring]. By the end of the demonstration, students are really comfortable with the understanding that the Moon s phases only occur from Earth s perspective. Using this model, it is easy to relate the idea that Connecting to the Standards This article relates to the following National Science Education Standards (NRC 1996). Content Standards Grades 5 8 Standard D: Earth and space science Earth in the solar system National Research Council (NRC) National science education standards. Washington, DC: National Academy Press. the Moon itself is blocking the light from the Sun and causing the self shadow. The black paint is where no sunlight can hit and be reflected. This is the nighttime we are seeing on the Moon. An Earth model can be put in the center and also have half of its surface black, thus identifying its self shadow or nighttime. As before, use the rubric (Figure 4, p. 33) to assess students understandings of these concepts. Light From Shadows Teaching students about phases of the Moon using the idea of a self shadow is a new and by our way of thinking easier way of introducing these concepts to students. The extrapolation of night and day, which are well-known concepts on Earth, to objects in space helps make these intangible objects less mysterious. Every planet, Moon, and asteroid has a night and day. By using a specific name for a different part of the shadow self shadow a distinction from the cast shadow can be made. These self shadows can be routinely identified in the classroom by turning on a bright light source and making the distinction between the cast shadow that is responsible for causing the eclipses and identifying the shadow on the surface of the object as the self shadow and responsible for causing the phases. Thus, the concept of the phases can be separated more easily from the idea of eclipses. This clear, simple distinction will help children move on to deeper scientific thinking and make comparisons with daily objects and their two shadows. When they think of objects in the sky, they will no longer be in the dark! n Timothy Young (tim.young@und.nodak.edu) is an associate professor in the Department of Physics, and Mark Guy (mark.guy@und.nodak.edu) is an associate professor in the Department of Teaching and Learning, both at the University of North Dakota in Grand Forks, North Dakota. September