Going Through a Phase

Transcription

1 Going Through a Phase Mrs. Timmons asked her class to share their ideas about what causes the different phases of the Moon. This is what some of her students said: Mona: The Moon lights up in different parts at different times of the month. Jared: The phases of the Moon change according to the season of the year. Sofia: Parts of the Moon reflect light depending on the position of the Earth in relation to the Sun and Moon. Drew: The Earth casts a shadow that causes a monthly pattern in how much of the Moon we can see from Earth. Trey: Different planets cast a shadow on the Moon as they revolve around the Sun. Oofra: The shadow of the Sun blocks part of the Moon each night causing a pattern of different Moon phases. Natasha: The clouds cover the parts of the Moon that we can t see. Raj: The Moon grows a little bit bigger each day until it is full and then it gets smaller again. It repeats this cycle every month. Which student do you agree with and why? Explain your thinking. Uncovering Student Ideas in Science 183

2 Going Through a Phase Teacher Notes Purpose The purpose of this assessment probe is to elicit students ideas as to what accounts for the phases of the Moon. It is designed to find out if students recognize the role of light reflection and the positional relationship between the Earth, Moon, and Sun in understanding why we see different phases of the Moon. Related Concept phases of the Moon Explanation Sofia has the best answer. The Moon does not emit light. The moonlight we see from Earth is actually light coming from the Sun that is reflected off the Moon s surface. Approximately half of the Moon is usually illuminated by the Sun; the portion (face) of the Moon that is illuminated depends on the positions of the Sun and Moon. The portion of the lit face of the Moon that can be seen from Earth depends on relative positions of the Earth, Moon, and Sun. As the Moon moves in its orbit around the Earth, different portions of it appear to be lit up, as we look at it from our relative position on Earth. This is why we see the different phases of the Moon. There is no shadow involved except for the unlit side of the Moon that is in its own shadow. The important idea is that the Moon itself doesn t change, nor does the amount of the Moon that is lit by the Sun change. The only thing that changes is the position of the Moon relative to the Earth and the Sun, which gives us our view. This change in relative position results in the different phases of the Moon. These phases repeat in a predictable pattern as the Moon revolves around the Earth. Curricular and Instructional Considerations Elementary Students Phases of the Moon are addressed in most el- 184 National Science Teachers Association

3 ementary curricula. Young children are naturally curious about the moon. However, it is important to recognize that elementary students experiences with Moon phases should be observational, not explanatory. At this level an understanding of Moon-related phenomena involves observing, drawing, and recording the phases of the Moon at regular intervals and noting that there is a repeating pattern but not explaining what causes these changes. This is a grade-level expectation in the national standards. Although the ideas in this probe exceed what elementary students are expected to know, it can be used to determine intuitive ideas students have that begin to form well before they encounter formal explanations for Moon phases in middle school. Middle School Students Students at this age begin to develop ideas about the Earth, Moon, and Sun system. However, the reason why we see different Moon phases is still a difficult idea to grasp (even for adults), even though it is a grade-level expectation in the national standards. This probe is useful in determining the various conceptual models students use to explain the phases of the Moon for the purpose of designing instruction that challenges these ideas. High School Students By high school, students are expected to know what causes the phases of the Moon. However, even after formal instruction in middle school, students may revert to their preconceived notions. This probe is useful in determining if students have a correct conception of the phases of the Moon and whether further instruction is needed before proceeding to more sophisticated astronomy concepts. Administering the Probe You may choose to begin this probe by having students observe, draw, and describe the phases of the Moon they see over a given period of time. Their experience can be used to justify which of the distracters are the correct explanations for their observations. Related Ideas in National Science Education Standards (NRC 1996) K 4 Changes in the Earth and Sky Objects in the sky have patterns of movement. The observable shape of the Moon changes from day to day in a cycle that lasts about a month. 5 8 Earth in the Solar System Most objects in the solar system are in regular and predictable motion. Those motions explain such phenomena as the day, the year, phases of the Moon, and eclipses. Related Ideas in Benchmarks for Science Literacy (AAAS 1993) K 2 The Universe The Moon looks a little different every day, but looks the same again about every four weeks. Indicates a strong match between the ideas elicited by the probe and a national standard s learning goal. Uncovering Student Ideas in Science 185

4 6 8 The Earth The Moon s orbit around the Earth once in about 28 days changes what part of the Moon is lighted by the Sun and how much of that part can be seen from the Earth the phases of the Moon. Related Research Explanations of the phases of the Moon are very challenging to students as well as adults. Students may not be able to understand explanations of these phenomena until they can reasonably grasp ideas about the Earth, Moon, and Sun system, including relative size, motion, and distance (AAAS 1993). Baxter (1989) investigated students ideas about phases of the Moon and identified five predominant ideas, including the correct one (which is that a portion of the Moon reflects light depending on our position in relation to the Sun and Moon). All four of the incorrect ideas involved a shadow: (1) clouds cover part of the Moon, (2) planets cast a shadow on the Moon, (3) the shadow of the Sun falls on the Moon, and (4) the shadow of the Earth falls on the Moon. In a study by Philip Sadler (1987), 37% of the students sampled thought the phases of the Moon were caused by the Earth s shadow. Suggestions for Instruction and Assessment Elementary students experiences should be focused on opportunities to observe, draw, and record the phases of the Moon in order to develop an understanding of the repeating pattern and predictability of the phenomenon. It is too early to ask students to explain what causes the phases of the Moon. Combine experiences learning about light reflection and light sources with understanding how we see parts of the Moon at night. Students need to understand that light is reflected by the Sun and that we see part of the reflection. It also dispels the notion some students have that the Moon lights up from the inside. This is an example of a phenomenon that is best explained and understood using physical models. However, for students, merely observing a model the teacher presents is less effective than constructing, using, and making sense of one s own model. Comparing the lunar eclipse phenomenon using a model to explain the phases of the Moon may help students see why the shadow notion works for the eclipse but not phases of the Moon. Comparing and contrasting the two different phenomena with middle school students may help students better understand why the shadow model does not support the phases of the Moon. Related NSTA Science Store Publications and NSTA Journal Articles American Association for the Advancement of Science (AAAS) Atlas of science literacy (See Indicates a strong match between the ideas elicited by the probe and a national standard s learning goal. 186 National Science Teachers Association

5 Solar System, pp ) New York: Oxford University Press. Driver, R., A. Squires, P. Rushworth, and V. Wood- Robinson Making sense of secondary science: Research into children s ideas. London and New York: RoutledgeFalmer. Gilbert, S. W., and S. W. Ireton Understanding models in earth and space science. Arlington, VA: NSTA Press. Hermann, R., and B. Lewis Moon misconceptions: Bringing pedagogical research of lunar phases into the classroom. The Science Teacher (Nov.): Keeley, P Science curriculum topic study: Bridging the gap between standards and practice. Thousand Oaks, CA: Corwin Press. Lindgren, J Why we have seasons and other misconceptions. Science Scope (Jan.): Rider, S Perceptions about moon phases. Science Scope (Nov./Dec.): Smith, S Project earth science: Astronomy. Arlington, VA: NSTA Press. Taylor, I Lunar phases: Students construct fundamental knowledge of moon phases. The Science Teacher (Nov.): Volkmann, M., and S. Abell Seamless assessment. Science and Children (May): Related Curriculum Topic Study Guide (Keeley 2005) Earth, Moon, and Sun System References American Association for the Advancement of Science (AAAS) Benchmarks for science literacy. New York: Oxford University Press. Baxter, J Children s understanding of familiar astronomical events. International Journal of Science Education 11(Special Issue): Keeley, P Science curriculum topic study: Bridging the gap between standards and practice. Thousand Oaks, CA: Corwin Press. National Research Council (NRC) National science education standards. Washington, DC: National Academy Press. Sadler, P Misconceptions in astronomy. Paper presented at the Second International Seminar: Misconceptions and Educational Strategies in Science and Mathematics, July 26-29, Cornell University. Uncovering Student Ideas in Science 187

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