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Lesson

Phases of the Moon

The Moon appears to change shape every night; not because it is actually changing, but because of where it sits in its orbit and how much of its lit half we can see from Earth.

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Driving Question
Every human who has ever lived has seen the exact same face of the Moon; so why does it seem to change shape every night, and why can no one on Earth ever see the other side?
🔬 Learning Science Focus 🔍 Phenomenon-First 🧠 Chunked Content 🖼️ Dual Coding ✅ Retrieval Practice 🔀 Interleaved Comparison
Learning Goals

What You'll Be Able to Do

By the end of this lesson, you will be able to:

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I can use a model of the Sun, Earth, and Moon to explain why the Moon appears to change shape.
6.MS-ESS1-1a
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I can name the eight phases of the Moon in order through one lunar cycle.
6.MS-ESS1-1a
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I can explain the difference between waxing and waning phases.
6.MS-ESS1-1a
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I can explain why we always see the same side of the Moon from Earth.
6.MS-ESS1-1a
📚 Instructional Design
Why this section exists
  • Set clear targets for modeling why the Moon appears to change shape.
  • Frame phases as something to explain, not a list to memorize.
Cognitive science
  • Goal setting
  • Advance organizers
Bloom's / DOK
  • Understand to Analyze
  • DOK 1 to 3
Accessibility considerations
  • Student-facing "I can" language
  • One goal per card, short lines
  • Standard badge kept separate from the goal text
Key Vocabulary

Vocabulary to Know

Choose a card to see what each word means.

📚 Instructional Design
Why this section exists
  • Pre-teach phase terms before the model that uses them.
  • Give a reference students can jump back to while learning.
Cognitive science
  • Pre-teaching vocabulary
  • Reduced extraneous load
Bloom's / DOK
  • Remember to Understand
  • DOK 1
Accessibility considerations
  • One card open at a time
  • Click to reveal, no hover
  • Plain, short definitions
Engage · Investigation

What's Happening to the Moon?

Before we explain anything, just observe. These four images show the same Moon, photographed on different nights over two weeks. Nothing was done to it. It's the same Moon each time.

NIGHT 1
NIGHT 8
NIGHT 15
NIGHT 22

What do you think is happening to the Moon itself?

📚 Instructional Design
Why this section exists
  • Anchor the lesson in real photos of the changing Moon.
  • Draw out the common "Earth's shadow" idea before correcting it.
Cognitive science
  • Curiosity gap
  • Phenomenon-based learning
  • Misconception checking
Bloom's / DOK
  • Understand
  • DOK 2
Accessibility considerations
  • Every choice receives feedback
  • No penalty for a wrong prediction
  • Real images paired with the prompt
Explore · Lesson Notes

Phases of the Moon

The Moon doesn't glow on its own; it reflects sunlight. As it orbits Earth, we see different amounts of the lit half. Here's what's really going on.

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Surprising Science Fact

The Face You've Never Seen

Every astronomer in ancient Egypt, Greece, China, and Babylon looked up at the Moon, and saw the exact same face looking back at them. Every human being who has ever lived has seen the same craters, the same dark patches (called maria), the same features. No person on Earth has ever directly observed the Moon's far side with their own eyes. Not because the Moon doesn't rotate; it does. It's because the Moon rotates once in the exact same time it takes to orbit Earth. This perfect synchrony is called tidal locking, and understanding it is the first key to understanding why the Moon appears to change shape every night.

The Earth and the Moon

The Moon is Earth's only natural satellite. It doesn't produce its own light; it reflects sunlight. Earth's gravity keeps it moving in a continuous curved path around our planet called an orbit.

🌐 Orbit
The curved path one object takes around another due to gravity. The Moon orbits Earth once every 27.3 days, roughly one month (that's where the word "month" originally comes from).
🔄 Rotation
Spinning on an internal axis. The Moon takes 27.3 days to rotate once, the exact same time as its orbit. This synchronized motion is why we always see the same side of the Moon from Earth.
🔒 Tidal Locking
When a moon's rotation period and orbital period are exactly equal, so the same face always points toward its planet. Even though the same side always faces Earth, all parts of the Moon still receive sunlight at different times, there is no permanently dark side.
⬇️ Gravity
An attractive force between objects with mass. The greater the mass, the stronger the pull. Earth is far more massive than the Moon, so Earth's gravity keeps the Moon in orbit around it; without gravity, the Moon would fly off into space in a straight line.
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Now that you know the Moon reflects sunlight and stays in a fixed orbit, one question naturally follows: if the lit half is always there, why doesn't it always look the same from Earth?
What Causes Moon Phases?

The Sun always illuminates exactly half of the Moon. The phase we see from Earth depends entirely on where the Moon is in its orbit, specifically, how much of the lit half is facing us at any given moment.

As the Moon orbits Earth over about 29.5 days, our viewing angle of the lit half continuously changes. When we see the entire lit half, it's a Full Moon. When the lit half faces completely away from us, it's New Moon. Every phase in between is a different fraction of the lit half turned toward Earth.

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Key Misconception: Moon phases are not caused by Earth's shadow falling on the Moon. That's a lunar eclipse, a different and much rarer event. Phases happen simply because of our changing angle of view as the Moon orbits Earth. On most nights, the Moon is not in Earth's shadow at all.
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You now understand the mechanism. Scientists have precise names for each position in the cycle, learning them lets you describe any Moon phase you see with a single, exact term.
Phase Vocabulary

Six key terms describe how the Moon's lit portion changes across the cycle. Learning these gives you the language to name any phase you see.

🌒 Waxing, Growing
When the lit portion of the Moon is increasing each night. The right side of the Moon is lit. The Moon is moving from New toward Full. Think: waxing candle = getting bigger.
🌘 Waning, Shrinking
When the lit portion of the Moon is decreasing each night. The left side of the Moon is lit. The Moon is moving from Full back toward New.
🌖 Gibbous, More than Half
When more than half of the Moon's visible surface is lit. Can be Waxing Gibbous (growing toward Full) or Waning Gibbous (shrinking away from Full). "Gibbous" comes from Latin for "hump-shaped."
🌙 Crescent, Less than Half
When less than half of the Moon's visible surface is lit, a thin sliver or C-shape. Can be Waxing Crescent or Waning Crescent.
🌓 Quarter, Half Lit
When exactly half of the Moon appears lit. "Quarter" refers to the Moon being one-quarter or three-quarters of the way through its cycle, not half the Moon being lit, even though that's what it looks like.
📅 Lunar Cycle
The complete sequence of Moon phases from one New Moon to the next, taking approximately 29.5 days. This is slightly longer than the Moon's orbital period (27.3 days) because Earth is also moving around the Sun, the Moon has to travel a little extra to "catch up" to the same Sun-Earth-Moon alignment.
📚 Instructional Design
Why this section exists
  • Build the core model: we see different amounts of the lit half.
  • Replace the shadow idea with reflected sunlight and orbit position.
Cognitive science
  • Cause-and-effect modeling
  • Dual coding
  • Elaboration
Bloom's / DOK
  • Understand to Analyze
  • DOK 2
Accessibility considerations
  • Labeled diagram paired with text
  • Key terms defined in place
  • Short paragraphs, one idea each
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Now that you understand why phases happen and what each one is called, the next step is knowing how to draw them accurately, and how to remember all eight in order, every time.
Explore · Drawing Rules

Drawing Moon Phases

Two tools to help you draw any phase correctly and remember the sequence every time.

The 4 Rules

1
Find the Sun. Always establish which direction sunlight is coming from; it is always to the right in standard diagrams.
2
New Moon = Moon between Sun and Earth. When the Moon is on the same side as the Sun, its lit half faces away from us, we see nothing.
3
Move counterclockwise. The Moon orbits Earth counterclockwise when viewed from above the North Pole, phases always progress in that direction.
4
The lit side always faces the Sun. Draw the bright part of the Moon as the side closest to the Sun's direction, never the far side.

Mnemonic: Remember the Sequence

"New Cats Quickly Get Fat With Lazy Cats"
LetterWordPhase
NNew🌑 New Moon
CCats🌒 Waxing Crescent
QQuickly🌓 First Quarter
GGet🌔 Waxing Gibbous
FFat🌕 Full Moon
WWith🌖 Waning Gibbous
LLazy🌗 Third Quarter
CCats🌘 Waning Crescent
📚 Instructional Design
Why this section exists
  • Give students a rule for drawing any phase correctly.
  • Offer a memory hook for the order of the eight phases.
Cognitive science
  • Dual coding
  • Elaboration
  • Pattern recognition
Bloom's / DOK
  • Understand to Apply
  • DOK 2
Accessibility considerations
  • Step-by-step rule with an example
  • Mnemonic laid out in a simple table
  • Short, parallel rows
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With those rules and the mnemonic in hand, you're ready to meet each phase by name, and see exactly where it sits in the Moon's orbit around Earth.
Explore · The 8 Phases

Eight Phases of the Lunar Cycle

The lunar cycle contains eight distinct phases, each with a name that tells you whether the Moon is growing or shrinking, and how much of its surface is lit. Click any card to expand it.

👆 Click each phase card to read what's happening at that moment in the Moon's orbit, then explore the interactive diagram below.
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New Moon
Start of Cycle
The Moon sits between Earth and the Sun. Its lit half faces the Sun, away from Earth. We see no illuminated surface. This marks day 0 of the lunar cycle.
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Waxing Crescent
Waxing
Moving counterclockwise, the Moon's right edge begins to catch light. A thin sliver (less than half) is visible from Earth. The lit area grows a little each night.
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First Quarter
Waxing
The Moon is 90° from the Sun. Exactly half (the right half) is lit as seen from Earth. One-quarter of the cycle is complete. Often visible in the afternoon sky.
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Waxing Gibbous
Waxing
More than half of the Moon is now lit on the right side. The lit area is still growing toward Full Moon. "Gibbous" describes the hump-shaped, rounded lit region.
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Full Moon
Midpoint
Earth is between the Sun and Moon. The Moon's entire lit half faces Earth, we see a complete bright circle. The Moon rises at sunset and sets at sunrise.
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Waning Gibbous
Waning
Past Full, more than half is still lit, but the lit area is now shrinking. The left side is lit. The Moon rises after sunset and remains visible into the morning hours.
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Third Quarter
Waning
The Moon is 270° through its orbit. The left half is now lit. Three-quarters of the cycle is complete. Rises around midnight and is visible in the morning sky.
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Waning Crescent
Waning
A thin sliver on the left side, shrinking toward New Moon. Less than half is lit. Almost back to the start, the cycle will restart at New Moon in just a few days.
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Click a Moon Position to See Its Phase
The Sun is always to the right. Earth is in the center. Click any Moon position in the orbit.
← Click a Moon position
Select any of the 8 Moon positions in the diagram to learn about that phase.
📚 Instructional Design
Why this section exists
  • Let students click each phase and connect its name to a position.
  • Show the cycle as a loop rather than eight isolated pictures.
Cognitive science
  • Dual coding
  • Pattern recognition
  • Elaboration
Bloom's / DOK
  • Understand to Analyze
  • DOK 2
Accessibility considerations
  • Click to reveal, no hover
  • Diagram positions paired with labels
  • Large targets, one phase at a time
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You've now seen all eight phases in action. Before the quiz, make sure the vocabulary is solid, these are the exact terms you'll need to use with precision.
Quick Recall

Brain Check

Three quick questions before you reason it through. These are not graded. Pulling answers from memory now will help them stick.

Quick Recall · 1 of 3
Just a quick brain check. Not graded.
What actually causes the Moon to appear to change shape through the month?
Quick Recall · 2 of 3
Just a quick brain check. Not graded.
A Moon is described as waxing. What is the lit part doing?
Quick Recall · 3 of 3
Just a quick brain check. Not graded.
About how long does one full lunar cycle take?
📚 Instructional Design
Why this section exists
  • Pull key phase facts from memory before the reasoning wrap-up.
  • Surface gaps early, while there is still time to reread.
Cognitive science
  • Retrieval practice
  • Generation effect
Bloom's / DOK
  • Understand to Apply
  • DOK 1 to 2
Accessibility considerations
  • Ungraded and low stakes
  • Immediate feedback
  • Try Again resets each item
Explain · Wrap-Up

Reason It Through

Three questions to put the pieces together before the quiz. Select an answer, feedback appears right away.

The Misconception

What actually causes the Moon to appear to change shape from night to night?

The Night Sky

You look up and see a crescent Moon with the lit sliver on the right side, growing slightly larger each night. Which phase are you seeing?

The Big Idea

Every person who has ever lived has seen the exact same face of the Moon. What explains this?

📚 Instructional Design
Why this section exists
  • Have students reason across phases, the cycle, and tidal locking.
  • Turn separate facts into one connected explanation before the quiz.
Cognitive science
  • Schema building
  • Elaboration
  • Coherent narrative
Bloom's / DOK
  • Understand to Analyze
  • DOK 3
Accessibility considerations
  • Ungraded, no time pressure
  • Feedback appears right away
  • One question at a time
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Your reflections don't need to be perfect; they just need to be honest. When you're ready, the quiz is next.
Evaluate · Quiz

Moon Phases Quiz

10 questions on Moon phases, tidal locking, and the lunar cycle. Fill in your info below, your score will be sent to your teacher when you submit.

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🧠 Show Your Thinking

Scientists don't just know the answer. They explain their thinking.

Write your own explanation first. Then submit your work to compare your thinking with a model answer.

Tonight the Moon is a thin crescent. Two weeks from now it will be a full circle. The Moon itself never changes shape, and the Sun always lights exactly half of it. So explain why the phase we see keeps changing. Build the whole chain: what stays the same about the lit Moon, what the Moon is doing as it moves, and why our view of the lit part changes from night to night. Use the word half.

One strong way to say it The Sun always lights exactly half of the Moon, so the lit part never actually changes size. What changes is our view. As the Moon orbits Earth, it moves to a new position each night, so we look at that lit half from a different angle. When the lit half faces fully toward us we see a Full Moon; when it faces away we see a New Moon; and in between we see part of the lit half, a crescent, a quarter, or a gibbous. The Moon is not growing or shrinking, and it is not in Earth's shadow. We are simply seeing different amounts of the same lit half as the Moon travels around Earth.

🔍 The Question You Came In With You started this lesson asking: "Why does the Moon seem to change shape every night, and why can no one ever see the other side?" The Sun always lights half the Moon, so the phase you see is just how much of that lit half faces Earth as the Moon orbits. And because the Moon rotates once for every orbit, the same face always points toward us, so no one on Earth ever sees the far side.
📚 Instructional Design
Why this section exists
  • Assess phases, the lunar cycle, and tidal locking together.
  • Send results to the teacher for a quick check of understanding.
Cognitive science
  • Retrieval practice
  • Feedback loops
Bloom's / DOK
  • Understand to Apply
  • DOK 1 to 2
Accessibility considerations
  • Answer explanations provided
  • Plausible, evenly placed options
  • Try Again to review missed items
Go Further

More Learning

The lesson is just the beginning, go deeper, explore the real data, or connect it to the bigger picture.

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The Moon Tonight
Use what you learned to read the real Moon in tonight's sky, live phase, the 29.5-day mystery, special moon names, and an observation challenge.
Explore
📚 Instructional Design
Why this section exists
  • Offer optional depth for students who want to keep going.
  • Connect the model to the real Moon students can watch tonight.
Cognitive science
  • Interest-driven extension
  • Transfer
Bloom's / DOK
  • Apply to Analyze
  • DOK 2 to 3
Accessibility considerations
  • Optional and self-paced
  • No penalty for skipping
  • Clear label on the card
Connections

Connections

Phases are one piece of a bigger picture. Here are the ideas that help explain them.

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Sun-Earth-Moon System
Phases come from the way these three bodies move together. Step back to see the whole system that produces them.
Explore
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Eclipses
Eclipses only happen at certain phases, a new Moon or a full Moon. See why the same orbit that makes phases also makes eclipses.
Explore
🌊
Nature of Waves
A phase is really just sunlight reflecting off the Moon. Explore how that light travels to your eyes as waves.
Explore