Why Does the Moon Change Shape?

Your child listens; you read. This page helps you answer the questions that come after the story. The script is available from the script button while the audio plays.

What this episode covers

It starts with a question children really ask: "Why does the moon change shape?" Many adults give a wrong answer to it. So the story does not just give the answer. It separates what people believe from what has been checked. It ends with a way your child can check it at home.

The core of the story is the textbook answer. The moon makes no light of its own. Sunlight falls on it, so the half facing the sun is always bright. As the moon goes around the Earth, we see more or less of that bright half. That is what the phases are. This is a textbook agreement, so we give no citation for it. One physics educator wrote that many people do not see the moon as a ball that is half lit. That picture is the key to the whole answer (DOI: 10.1119/5.0064950).

What people often say, and what is true

"The Earth's shadow covers the moon" — wrong. Wrong ideas about the phases are widespread and hard to change, even in adults. A guide for teachers and curriculum writers, written for the US National Science Education Standards (National Research Council, 1996), says that the most common mistake mixes up phases and eclipses, and claims the phases happen when the moon enters the Earth's shadow (DOI: 10.3847/aer2005002). In the film A Private Universe, Harvard graduates and gifted high school students gave serious wrong answers about the phases. One of the beliefs was that the Earth's shadow is the cause (DOI: 10.1119/1.2344481). This source is a two-page column in The Physics Teacher, and it reports the film second-hand.

You can test the shadow answer by asking where shadows fall. The Earth's shadow points away from the sun. But an evening crescent appears low in the western sky just after sunset, and a morning crescent sits in the eastern sky before sunrise. Both are close to the sun's direction. The Earth's shadow does sometimes cover the moon. A lunar eclipse happens only at full moon, and only now and then. The moon's path is tilted a little against the Earth's path, so a full moon usually passes above or below the Earth's shadow. The phases happen every month (textbook).

"Clouds cover it" — wrong. One study sorted the answers of 30 eighth graders into seven groups. Two groups were scientific: the moon goes around the Earth and its position changes how much lit part we see (the group that also named the Sun–Earth–Moon angle was the correct one). The rest included "clouds cover part of the moon," the Earth's shadow, the Earth blocking the light, changes in sunlight, and the Earth's spin (DOI: 10.1063/1.5094007). Clouds pass in no fixed pattern, but the phases come in order. On a clear night, a half moon is still a half moon.

"The moon really gets smaller and bigger" — wrong. The moon is always the same ball; only the part of its bright half that we see changes. We could not find how common this idea is among children in the papers we collected (we searched in two ways). So the story only offers it to the child as a question.

"Anaxagoras in ancient Greece found it first" — the records disagree. Old reports say Anaxagoras taught that "the moon receives its light from the sun." Most writers read this as "the moon reflects sunlight." But the scholar Couprie argued in 2019 that it more likely meant "the sun sets the moon on fire," since other reports call the moon a fiery stone. He notes that we have no record of how Anaxagoras explained the phases. Reading the moon's light as fire, and taking his earlier paper on eclipses together with this one, Couprie concludes that Anaxagoras was not "the great discoverer of the real cause of lunar eclipses and the moon light" that recent writing makes him. He also writes that Anaxagoras inventively defended a coherent set of ideas that were already out of date, such as a flat Earth and a moon of fiery stone (DOI: 10.36950/hyperboreus.bgzz5935). Another old record, citing Eudemus, names Anaximenes as the first to find that the moon's light comes from the sun (DOI: 10.4324/9781351324489-15). The old records disagree, and scholars read his words in different ways, so the story leaves it that way.

Is the moon out in the daytime? Yes. The same physics educator wrote that adults are often surprised to see the moon in the daytime, while young children often spot it in a bright sky (DOI: 10.1119/5.0064950). No study tells us how common this idea is. It is one educator's observation, so the story presents it as "a physics teacher wrote."

How it was checked

Adults get it wrong. In one study, 20 primary school teachers were each interviewed for about an hour. They did not know well what can be seen in the sky. Sometimes they worked backwards from a picture in their head to guess what they should see. Only 4 of the 20 held the scientific model of the universe, and these gaps often led to wrong answers about day and night, the seasons, and the phases (DOI: 10.1080/0267152930080107). In another study, 50 student teachers for young children took part. Before the lessons, none of them understood the cause of the phases. After inquiry lessons with a computer simulation, 82% did (DOI: 10.1002/tea.20227). These two studies stay on this page only, so that the start of the story is not crowded with "adults get it wrong" studies.

Four young children. One study interviewed four young children, twice each. None of the four believed the phases come from the Earth's shadow. But their idea of shadows was also basic: two of the four said a shadow needs no light (DOI: 10.1111/ssm.12084). So this does not mean that children know the answer. With only four children, we also cannot say that "children do not hold this wrong idea." The study's question, when and why the wrong idea appears, is still open. The story stresses that there were only four.

Watching and recording on their own. Thirty-two seventh graders selected as science-gifted were given homework: watch the moon every day for a month, take photos, and record set items on a form. Overall, they did not link the phases to things like the date, their own position, or the sun's position. Some showed the shadow mistake, and many drew the sun in the wrong place. The authors suggest that not only teaching but also research should reach informal settings such as real outdoor observation (DOI: 10.5467/jkess.2011.32.5.514). So their conclusion is to add real sky watching, not to drop it. The story only says that watching and writing on their own did not show them the answer.

Twelve who had lessons on studying their records and modeling with their bodies. A study followed 12 student teachers for several months. Their inquiry lessons included recording and studying moon observations over time, and modeling the phases with their bodies. They were interviewed before, 3 weeks after, and months after the lessons. Shortly after the lessons, they were very likely to hold the scientific view (in line with earlier studies). Most still held it six months or more later, but some went back to their earlier ideas (DOI: 10.1002/tea.20121). There was no comparison group and the study is small, so it cannot show that this way of teaching caused the result. Read "most" and "some" just as they are. What differs between this study and the 32 students is whether there were lessons on studying records and modeling, not whether they kept records. A thesis in our sources found no difference in results whether student teachers collected moon data as a whole class or in pairs with a simulation (DOI: 10.18130/v3pp0d).

Questions your child may ask

"Why is only half of the moon bright?" Because the sun shines from one side. Shine a flashlight on a ball and only that side lights up. Half of the moon is always bright; what changes is how much of that half we see.

"How is that different from an eclipse?" In a lunar eclipse the Earth's shadow really covers the moon, and it happens only at full moon, now and then. Phases happen every month, with no shadow involved. This is exactly where adults most often get mixed up (DOI: 10.3847/aer2005002).

"When I can't see the moon, is it gone?" No. The moon is on almost the same side as the sun. Its bright half faces away from us, and the sun's glare hides it.

"Is the moon out in the daytime?" Yes. Around half moon it is easy to find in the day sky. The growing half moon (first quarter, about day 7–8 of the lunar month) shows in the afternoon; the shrinking half moon (last quarter, about day 22–23) shows in the morning.

"Didn't Anaxagoras find it first?" Some writing says so, but the old records disagree, and scholars read his words in different ways (DOI: 10.36950/hyperboreus.bgzz5935). "That is what people say, but it is hard to check" fits the spirit of the story.

What is known, and what is still open

Known. The cause of the phases (textbook). The shadow answer is the most common mistake, even among adults (teacher guide, DOI: 10.3847/aer2005002). Teachers and student teachers often cannot explain the phases before lessons (several small studies, DOI: 10.1080/0267152930080107, DOI: 10.1002/tea.20227).

Small groups, read with care. The four-child study, the 12 student teachers, the 32 gifted students, and the 30 eighth graders are each one small group. The story keeps the numbers.

Still open. When and why the shadow idea starts (DOI: 10.1111/ssm.12084). Whether moon diaries cause better understanding is also not settled. In a study of sixth graders, students with higher diary scores and more entries also scored higher on a moon test (out of 20 points). Each extra diary point went with 0.18 more points (about 1 percentage point). But this is a link, not proof of cause (DOI: 10.1080/09500693.2015.1052861). The story says this too.

Things to try together

• Moon diary. For a few weeks, at about the same time and place each day, draw the moon's shape and where it is, and write the date. On some evenings you will not see it. "No moon" is a record too.
• Talk the diary over together. About once a week, open the diary and talk about "Is it getting bigger or smaller?" and "Is it moving away from the sun's side, or closer?" Studying the records was part of the student teachers' lessons (DOI: 10.1002/tea.20121). Given the gifted students who only recorded on their own (DOI: 10.5467/jkess.2011.32.5.514), this step is the heart of what the story suggests.
• Ball and lamp. Darken the room and turn on one lamp. Your child holds a ball at arm's length and turns slowly in a circle. The head is the Earth, the ball is the moon, the lamp is the sun. When the ball is behind the head, away from the lamp, the head's shadow covers it — that is an eclipse. In this model an eclipse comes almost every turn, so also show that the real moon's path is tilted: hold the ball a little higher and it passes above the shadow. A thesis with student teachers reported that this foam-ball activity was a significant part of their learning (DOI: 10.18130/v3pp0d).
• Find the day moon. Around first quarter, look at the afternoon sky; around last quarter, the morning sky.

Sources

• "Learning about Phases of the Moon and Eclipses: A Guide for Teachers and Curriculum Developers" (2005) — DOI: 10.3847/aer2005002 — Wrong ideas about phases and eclipses are common and hard to change, even in adults; the most common is that phases come from the Earth's shadow.
• "Bringing the moon into the classroom" (1996) — DOI: 10.1119/1.2344481 — In A Private Universe, Harvard graduates and gifted high schoolers held wrong ideas about phases, including the Earth's shadow (two-page teacher column).
• "Young Children Do Not Hold the Classic Earth's Shadow Misconception to Explain Lunar Phases" (2014) — DOI: 10.1111/ssm.12084 — None of four young children held the shadow idea, but two said a shadow needs no light.
• "Development of 8 students' scientific concept in cause of moon phase by using metacognitive strategy" (2019) — DOI: 10.1063/1.5094007 — Thirty eighth graders' explanations fell into seven groups, including the correct one and wrong ones such as clouds and the Earth's shadow.
• "Some primary-school teachers' understanding of the Earth's place in the universe" (1993) — DOI: 10.1080/0267152930080107 — Twenty primary teachers lacked sky observation knowledge; four held the scientific model.
• "The use of a computer simulation to promote scientific conceptions of moon phases" (2007) — DOI: 10.1002/tea.20227 — None of 50 student teachers understood the phases before lessons; 82% did after.
• "Investigation of the 7th Grade Science-gifted Students' Understanding about the Lunar Phase through Their Own Observation and Interpretation" (2011) — DOI: 10.5467/jkess.2011.32.5.514 — Thirty-two gifted students who watched and recorded the moon daily for a month as homework still did not link phases to the key factors; the authors suggest widening teaching and research to real outdoor observation.
• "A longitudinal study of conceptual change: Preservice elementary teachers' conceptions of moon phases" (2006) — DOI: 10.1002/tea.20121 — After lessons with observation records and body modeling, most of 12 kept their understanding for six months or more; some went back.
• "Student Moon Observations and Spatial-Scientific Reasoning" (2015) — DOI: 10.1080/09500693.2015.1052861 — Sixth graders with better moon journals and more entries scored higher on a phases test (a link).
• "Simulating Science: Using a Computer to Facilitate Conceptual Change for Moon Phases" (2014) — DOI: 10.18130/v3pp0d — Thesis with student teachers; the foam-ball activity was a significant part of learning.
• "A Room-Size Model of Lunar Phases" (2023) — DOI: 10.1119/5.0064950 — Many people do not see the moon as a half-lit ball; adults are surprised by the day moon, young children often spot it (educator's article).
• "Anaxagoras on the Light and Phases of the Moon" (2019) — DOI: 10.36950/hyperboreus.bgzz5935 — Argues Anaxagoras more likely meant the sun ignites the moon, and that he was not the discoverer of the real cause of eclipses and moonlight, though he inventively and coherently defended older ideas.
• "Eudemus Fr. 145 Wehrli and the Ancient Theories of Lunar Light" (2018) — DOI: 10.4324/9781351324489-15 — A record citing Eudemus names Anaximenes as first to find that moonlight comes from the sun.