Moon Phases: Sunlight & Viewing Geometry — Print · Grade 6 Science

Chapter: Earth, Moon, Sun & Space Models

Moon Phases: Sunlight & Viewing Geometry

Learning goals

I can explain phases as a change in viewing geometry rather than a shadow.
I can give evidence that rules out the shadow explanation.
I can explain why the same face of the Moon always points towards Earth.

Readings for this unit

One Sky, Several Mechanisms

Before this unit

Read the chapter opening and the named observation or diagram. Clarify one word or unit that blocks understanding. Recall the relevant earlier measurement, matter, or energy idea.

Study the unit across three 35–45-minute class sessions. Use the worked examples after an attempt and the supplied records when actual equipment is not available.

The Moon produces no light of its own and shines entirely by reflecting sunlight. At any moment the Sun illuminates one half of it, just as it illuminates one half of Earth, and that lit half is always there whatever phase we happen to see.

What changes is our viewing angle. As the Moon travels round Earth over 29.5 days we see the lit half from different directions: almost entirely from behind at new moon, side-on at the quarters, and fully lit at full moon. The phase is a matter of geometry rather than of anything happening to the Moon.

The most common explanation offered, that Earth casts a shadow on it, is wrong and worth taking apart carefully, because the reasoning that disposes of it is more valuable than the fact itself.

Build your understanding

Where this fits

The light and shadow lesson in chapter 4 supplied the geometry this lesson needs, and the next lesson uses the same geometry for eclipses, which genuinely do involve shadows. Telling the two apart is the whole point of the second section here.

Use three flexible 40-45-minute sessions: phases as viewing geometry; why the shadow explanation fails; naming the phases and why the same face always points at us, then the practice set, revision and the exit check. Workshop, review, quiz and exams have separate time.

Part 1 · Always half lit

Hold a ball at arm's length in a darkened room with a single bright lamp across the room, and walk slowly in a circle while keeping the ball between you and the lamp at the start. The lamp always lights exactly half the ball, and what changes as you turn is how much of that lit half you can see from where you stand. With the ball between you and the lamp you see almost entirely the dark side; a quarter of the way round you see half lit and half dark; opposite the lamp you see the whole lit face. That is the entire explanation of phases, and it can be done with a tennis ball and a desk lamp in five minutes.

The geometry also explains when each phase is visible, which is a detail most people never notice. A new moon is in roughly the same direction as the Sun, so it rises and sets with the Sun and is lost in daylight. A full moon is opposite the Sun, so it rises as the Sun sets and is up all night. A first-quarter moon is 90 degrees from the Sun and sits high in the sky at sunset, which is why it is often visible in the afternoon. Anybody claiming to have seen a full moon at midday has made a geometrical impossibility rather than an unusual observation. One further consequence is worth noticing because it is visible on most clear evenings. A thin crescent moon often shows the rest of its disc glowing faintly, a effect called earthshine: sunlight reflecting off Earth, travelling to the Moon and reflecting back. Leonardo da Vinci worked out the explanation around 1510, reasoning that if the Moon lights our night sky then Earth, being far larger, must light the Moon's.

Sunlight comes from the left in the top view; about half each Moon is illuminated while the Earth observer sees different fractions. The lower icons use one schematic orientation, which can vary with viewpoint. Sizes and spacing are exaggerated, and this is not an eclipse or a current sky calendar.

Pause and explain

Why can a full moon never be seen high in the sky at midday?

Check your reasoning after answering

Because a full moon is opposite the Sun as seen from Earth. At midday the Sun is high in the southern or northern sky, so the point opposite it is below the horizon. A full moon therefore rises around sunset and sets around sunrise, and is only ever up during the night.

Part 2 · Why it is not a shadow

The idea that phases are Earth’s shadow falling on the Moon is wrong, and three separate arguments dispose of it. First, if Earth's shadow were the cause, phases would depend on Earth being between the Sun and the Moon, and that is precisely the position at full moon, when the whole disc is lit. The explanation predicts the opposite of what is observed. Second, Earth's shadow is round, so it could only ever produce a curved bite out of the disc; it could not produce the straight edge seen at first quarter.

Third, and most decisively, the Moon is often visible in the daytime sky at the same time as the Sun, with a clear phase showing, and at that moment Earth is not between them at all and could not be casting anything onto it. The shadow explanation is a good example of a hypothesis that sounds reasonable, makes definite predictions and fails all of them, and that failure is why it is worth examining in class rather than simply contradicting. It is worth stating the conclusion in the plainest possible terms, because this is the misconception that survives longest: the phases are not Earth’s shadow. They are the changing fraction of the permanently lit half that happens to face us. Earth's shadow is involved in exactly one lunar phenomenon, and it is not this one. Earth's shadow does fall on the Moon occasionally, and the result is a lunar eclipse, which is the subject of the next lesson.

Pause and explain

Give two observations that rule out Earth's shadow as the cause of the Moon's phases.

Check your reasoning after answering

First, a full moon occurs when Earth is between the Sun and the Moon, which is exactly when the shadow explanation predicts the Moon should be darkest, so it predicts the opposite of what happens. Second, the Moon is often visible in daylight showing a clear phase, at times when Earth is nowhere between it and the Sun and could cast nothing onto it.

Part 3 · Naming what you see

The cycle takes 29.5 days and the names follow a simple logic once two words are understood. Waxing means the lit portion is growing, and waning means it is shrinking. Crescent means less than half is lit, gibbous means more than half. So the sequence runs new moon, waxing crescent, first quarter with half the disc lit, waxing gibbous, full moon, waning gibbous, third quarter, waning crescent, and back to new. Quarter refers to the position in the orbit rather than to how much is lit, which is why a first-quarter moon looks like a half disc.

There is a practical trick for telling waxing from waning in the northern hemisphere. A waxing moon is lit on its right-hand side and a waning one on its left, so a crescent curving like the right side of the letter D is growing and one curving like the left side of C is shrinking. In the southern hemisphere the rule is reversed, because observers there are effectively upside down relative to northern ones, which is a neat reminder that a great deal of astronomical description is written from one particular place. The 29.5 day cycle does not quite match the 27.3 days the Moon takes to complete one orbit, and the difference matters. While the Moon goes round, Earth has moved along its own orbit, so the Moon must travel a little further to return to the same alignment with the Sun. Those two figures, 27.3 and 29.5 days, are the orbit and the phase cycle respectively, and confusing them is a common source of error in calculations.

Pause and explain

A moon is lit on its right-hand side and more than half the disc is bright. Name the phase and say what it will do next.

Check your reasoning after answering

It is a waxing gibbous moon: gibbous because more than half is lit, waxing because the lit area is growing, which in the northern hemisphere shows as light on the right. Over the following days the lit portion will keep growing until the whole disc is lit at full moon, a few days later.

Part 4 · The face we never see

The Moon keeps the same face towards Earth at all times, which led to the phrase the dark side of the Moon. The phrase is wrong twice over: the far side is not dark, receiving just as much sunlight as the near side over a month, and it is better called the far side. Nobody saw it at all until a Soviet spacecraft photographed it in 1959, and the images showed a surface markedly different from the near side, with far fewer of the dark plains.

The reason we see only one face is that the Moon rotates exactly once on its axis for each orbit it makes of Earth, both taking 27.3 days. That is not a coincidence but the result of tidal locking: Earth's gravity pulls slightly harder on the near side of the Moon than the far side, which over billions of years slowed its spin until it matched its orbit. The same process is slowly lengthening Earth's day, by a little under 2 milliseconds per century, and pushing the Moon about 3.8 cm further away each year. That last figure is measured directly rather than inferred. Reflectors left on the lunar surface by the Apollo missions and by Soviet landers return laser pulses fired from Earth, and timing the round trip gives the distance to within a few centimetres. Repeating the measurement over decades shows the Moon receding steadily, which also means that in the distant past it was closer and appeared larger in the sky.

Pause and explain

Why is 'the dark side of the Moon' a misleading phrase?

Check your reasoning after answering

Because the far side is not permanently dark. It receives as much sunlight as the near side over the course of a month, including full sunlight at new moon when the near side is unlit. What is true is that we never see it from Earth, because the Moon rotates once per orbit, so the correct term is the far side.

Apply the whole unit · Work out the view

At a given moment the Moon is positioned so that the angle between the Sun, Earth and the Moon is 90 degrees, with the Moon having passed new moon four days earlier. An observer in the northern hemisphere looks at it at sunset.

Your task

Name the phase, describe how it looks and where in the sky it will be, and explain each from the geometry.

What your response should show

Identifies first quarter from the 90 degree geometry
States that the right-hand side is lit and links this to waxing
Places the Moon high in the sky at sunset using the 90 degree separation
Notices and comments on the inconsistency in the stated timing

Response guidance — after your attempt

Name the phase and say what fraction of the disc appears lit.
Describe which side is lit and how you know.
Say where it will appear at sunset and explain why.

At 90 degrees between Sun, Earth and Moon, we are looking at the lit half edge-on, so exactly half the visible disc appears lit: this is first quarter. The name refers to the Moon having completed a quarter of its 29.5 day orbit, which fits with it having passed new moon roughly seven days earlier rather than four, so either the timing or the angle in the question is approximate and the angle is the more reliable guide. The lit side is the right-hand side for a northern observer, because the Moon is moving away from the Sun's direction in the sky and the lit portion is growing, which makes it waxing. At sunset it will be high in the sky, roughly overhead towards the south, because it sits 90 degrees from the Sun: as the Sun touches the western horizon, a point 90 degrees away from it along the Moon's path is close to the highest part of the sky. It will then set around midnight, six hours after the Sun.

Scientific references for this unit

Parent / teacher teaching notes

Begin with the literal observation, quantity, or diagram relationship. Ask the learner to explain why the evidence supports the claim, check a relevant unit or boundary, then discuss an alternative. The supplied numbers are illustrative, not real observations. A parent can read aloud and allow a spoken rehearsal before writing. Judge science and evidence rather than political, faith, or family agreement. Use the complete data option whenever an actual procedure is unavailable or unsuitable.

The key idea

The Sun always lights exactly half the Moon. Phases are the changing fraction of that lit half we can see from Earth as the Moon moves round us, and they are not Earth’s shadow falling on it, which is a different and far rarer event.

Worked examples

Example 1 · Explain a phase record

Explain illumination, waxing/waning intervals, and why the record does not require monthly eclipses.

Keep sunlight direction and observer view distinct.
Identify increasing and decreasing visible fractions.
Include tilted-orbit alignment limits.

Answer: The visible illuminated fraction grows from new toward full and then decreases toward last quarter. About half the Moon remains sunlit throughout; the viewing fraction changes. Eclipses need additional alignment with the tilted orbit, so the full-phase entry does not guarantee Earth’s shadow crosses the Moon.

Example 2 · Transfer to a different case

The same lunar face is broadly visible from Earth. Does that mean the Moon never rotates?

Identify the distinction and conditions in this new case.
Use the relevant mechanism or quantity; preserve units and the chosen boundary.
Explain why the answer fits and what further evidence would be needed.

Answer: No. Its rotation is coordinated with its orbital motion. A model must include both to explain the familiar facing side.

Example 3 · Trace a sequence through an orbit · a focused example

What distinguishes waxing from waning?

State the literal condition or observation in the example.
Apply the distinction explained in this teaching part.
Check whether the claim stays within the given evidence.

Answer: Whether the observed illuminated fraction is increasing or decreasing, not a universal left/right rule for every observer.

Avoid these mistakes

Misleading claim: Every ordinary Moon phase is caused by Earth’s shadow.
Misleading claim: A cloudy missing observation proves the Moon has disappeared.
Omitting units, measurement timing, or a model boundary makes the evidence hard to interpret.
Treating illustrative classroom data as real field measurements or copying a model is not an independent explanation.

Check your understanding

Does a half-lit disk mean only half as much Moon exists?

Discuss the answer

No. The body remains whole; the observer sees a different fraction of the illuminated portion.

What distinguishes waxing from waning?

Discuss the answer

Whether the observed illuminated fraction is increasing or decreasing, not a universal left/right rule for every observer.

Why is “Earth’s shadow makes every crescent” incorrect?

Discuss the answer

Most phase positions do not place the Moon in Earth’s shadow. Illumination and viewing direction explain the ordinary sequence.

What does the cloudy entry establish?

Discuss the answer

Only that the observation was unavailable or obscured under the method; it does not show the Moon vanished.

Sustained scientific explanation and revision

These activities are scheduled in the parent plan. Save a first response, a correction, and the reason for the change in the science notebook.

Activity 1 · Moon Phases: Sunlight & Viewing Geometry · Model and explain

Assigned reading and preparation

Read One Sky, Several Mechanisms and the linked unit teaching; use its complete supplied case and diagrams.

Your task

For Moon Phases: Sunlight & Viewing Geometry, study Parts 3–4 and attempt the worked case: Explain illumination, waxing/waning intervals, and why the record does not require monthly eclipses. Explain the steps before opening the model.

Success criteria

An accurate scientific claim answering the actual question.
Relevant evidence or a labeled model with quantities and units when applicable.
Reasoning that connects evidence with the mechanism rather than only repeats it.
A specific method, boundary, scale, or alternative-explanation limit.

Hint

Begin with the actual observation, quantity, or diagram relationship. Explain how it bears on the question before making a broad claim.

Response guidance — discuss after your attempt

The visible illuminated fraction grows from new toward full and then decreases toward last quarter. About half the Moon remains sunlit throughout; the viewing fraction changes. Eclipses need additional alignment with the tilted orbit, so the full-phase entry does not guarantee Earth’s shadow crosses the Moon.

Activity 2 · Moon Phases: Sunlight & Viewing Geometry · Practice, revise and exit

Assigned reading and preparation

Read One Sky, Several Mechanisms and the linked unit teaching; use its complete supplied case and diagrams.

Your task

For Moon Phases: Sunlight & Viewing Geometry, complete practice questions 1–6 and both choice checks, then answer independently: The same lunar face is broadly visible from Earth. Does that mean the Moon never rotates? Save a correction explaining one changed claim or method.

Success criteria

An accurate scientific claim answering the actual question.
Relevant evidence or a labeled model with quantities and units when applicable.
Reasoning that connects evidence with the mechanism rather than only repeats it.
A specific method, boundary, scale, or alternative-explanation limit.

Hint

Begin with the actual observation, quantity, or diagram relationship. Explain how it bears on the question before making a broad claim.

Response guidance — discuss after your attempt

No. Its rotation is coordinated with its orbital motion. A model must include both to explain the familiar facing side.

Words to know

waxing: An increase in the Moon’s observed illuminated fraction over the phase sequence.
waning: A decrease in the Moon’s observed illuminated fraction over the phase sequence.
evidence: A relevant observation, measurement, or record used to evaluate a claim.
system boundary: The chosen limit specifying what is included and which exchanges cross it.
uncertainty: A stated limit in measurement or knowledge, distinct from a careless mistake.

In the real world

Exit check

The same lunar face is broadly visible from Earth. Does that mean the Moon never rotates?

Check your explanation after answering

No. Its rotation is coordinated with its orbital motion. A model must include both to explain the familiar facing side.

Explain your reasoning without notes. If a key idea is missing, revisit the lesson and try again.