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Lesson

Eclipses

The Moon passes between Earth and the Sun every single month. So why doesn't the Sun go dark every single month?

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Driving Question
If the Moon orbits Earth every month, why don't eclipses happen every month?
Learning Science Focus Phenomenon-First Conceptual Change Dual Coding Retrieval Practice
Learning Goals

What You'll Be Able to Do

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

🌑
I can use a model of the Sun, Earth, and Moon to explain how solar and lunar eclipses happen.
6.MS-ESS1-1a
🛰️
I can explain why eclipses do not happen every month.
6.MS-ESS1-1a
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I can describe the difference between the umbra and the penumbra of a shadow.
6.MS-ESS1-1a
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I can explain why eclipses only happen when the Moon is near a node.
6.MS-ESS1-1a
📚 Instructional Design
Why this section exists
  • Set targets for modeling how and why eclipses happen.
  • Name the month puzzle up front as a goal to explain.
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 shadow and orbit terms the lesson leans on.
  • Give a reference students can jump back to during the lab.
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
  • Jump link ties each term to the lesson
Engage · Briefing

Two Kinds of Eclipse

Before you investigate, here's the setup. Two types of eclipse exist, and they work in completely opposite ways.

Solar Eclipse
Moon Casts Shadow on Earth
SUN MOON EARTH
PhaseNew Moon
OrderSun → Moon → Earth
EffectMoon's shadow falls on Earth
Lunar Eclipse
Earth Casts Shadow on Moon
SUN EARTH MOON
PhaseFull Moon
OrderSun → Earth → Moon
EffectEarth's shadow falls on the Moon
🤔
Make a prediction: A solar eclipse needs a new moon, and a new moon comes around every 29.5 days. So why don't we get a solar eclipse every single month? Make your best guess, there is no penalty for guessing.
Hold that thought. The Moon really does reach new moon every month, yet a solar eclipse is rare, so something must usually keep the three from lining up exactly. Keep your prediction in mind as you run the Discovery Lab and reach The Tilted Orbit, where you will find out what it is.
📚 Instructional Design
Why this section exists
  • Set up the two eclipse types, then pose the month puzzle.
  • Force a prediction before instruction so the explanation has something to resolve.
Cognitive science
  • Curiosity gap
  • Phenomenon-based learning
Bloom's / DOK
  • Understand
  • DOK 2
Accessibility considerations
  • Prediction locks in one tap, no penalty
  • Reveal appears only after a choice
  • Labeled diagrams paired with the text
Explore · Discovery Lab

Eclipse Discovery Lab

Scientists can predict eclipses centuries in advance. The positions are exact. Can you find them?

Challenge 1
Arrange the Moon to create a solar eclipse.
Moon Phase
Moon Height
✅ Solar Eclipse!

New moon + crossing the orbital plane = solar eclipse. Most months the Moon passes above or below this line, and the shadow misses Earth completely.

Challenge 2
Arrange the Moon to create a lunar eclipse.
Moon Phase
Moon Height
✅ Lunar Eclipse!

Full moon + crossing the orbital plane = lunar eclipse. Both conditions have to be true at exactly the same time.

📚 Instructional Design
Why this section exists
  • Let students find eclipse alignments themselves instead of being told.
  • Show that both a phase and a crossing must line up at once.
Cognitive science
  • Productive struggle
  • Evidence-based reasoning
  • Dual coding
Bloom's / DOK
  • Apply to Analyze
  • DOK 2
Accessibility considerations
  • Interactive with clear success feedback
  • Large drag and click targets
  • No penalty for repeated tries
Explore · What You Discovered

What You Discovered

Here's what you found. The two eclipses look similar from a distance; but the geometry, the phase, and the experience are completely different.

Solar Eclipse Lunar Eclipse
What happens Moon blocks sunlight from reaching Earth Earth's shadow falls on the Moon
Moon phase New Moon, Moon between Sun and Earth Full Moon, Moon on far side of Earth
Alignment Sun → Moon → Earth Sun → Earth → Moon
Seen from Only the narrow path of totality on Earth Anywhere the Moon is visible in the sky
One mystery remains. If the Moon reaches new moon every single month, the alignment you just created should repeat every month; so why don't solar eclipses happen every month? The answer is in the orbit itself.
📚 Instructional Design
Why this section exists
  • Name what the lab revealed about each eclipse type.
  • Set up the orbit question the next section answers.
Cognitive science
  • Comparison and contrast
  • Concrete to abstract
  • Elaboration
Bloom's / DOK
  • Understand to Analyze
  • DOK 2
Accessibility considerations
  • Side-by-side comparison table
  • Short, parallel rows
  • Bridge callout restates the open question
Explore · Why Eclipses Are Rare

The Tilted Orbit

New moon happens every 29.5 days, the exact phase a solar eclipse needs. So why don't eclipses happen every month? A single 5-degree tilt in the Moon's orbit changes everything.

If the Orbit Were Flat
If the Moon's orbit were flat, the shadows would line up every month.
The Real Orbit Is Tilted
Because the Moon's orbit is tilted, the shadow usually misses Earth.
🎯
Nodes: The Eclipse Windows
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The Moon's orbit crosses Earth's plane at only two points. Eclipses can only happen at those exact crossings.

The Moon's tilted orbit crosses Earth's orbital plane at exactly two points, called nodes, the only places where an eclipse is geometrically possible. Both conditions must be true at the same time:

Condition 1
Correct Moon Phase
New moon (for solar) or full moon (for lunar), the Sun, Moon, and Earth must be roughly in a line.
Condition 2
Near a Node
The Moon must be near one of the two crossing points called nodes, otherwise the shadow passes above or below Earth.

A new moon happens every ~29.5 days, but the Moon is near a node for only a few days each orbit. That rare overlap is exactly why eclipses feel special.

Node
One of the two places where the Moon's orbit crosses Earth's orbital plane.
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That's the answer. New moon happens every month; but the 5-degree orbital tilt means the shadow usually misses Earth entirely. Only at the nodes does everything line up. That's why eclipses are rare, and why, when they happen, they feel extraordinary.
When an eclipse does happen, not everyone sees the same thing. Whether you witness totality or just a crescent Sun depends on exactly where you're standing.
📚 Instructional Design
Why this section exists
  • Resolve the month puzzle with the 5-degree orbital tilt.
  • Trace the chain from tilt to missed shadow to node alignment.
Cognitive science
  • Cause-and-effect modeling
  • Misconception checking
  • Dual coding
Bloom's / DOK
  • Understand to Analyze
  • DOK 3
Accessibility considerations
  • Flat and tilted orbits shown side by side
  • Key terms defined in place
  • Short paragraphs, one idea each
Explore · Shadow Models

The Anatomy of a Shadow

The Moon casts two shadow zones during a solar eclipse, and they produce completely different experiences. Which one you're in determines everything.

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Umbra: Total Darkness
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The innermost, completely dark cone. Stand inside it and you see a total eclipse.

The umbra is the innermost, darkest cone of the Moon's shadow. Stand inside it and the Moon completely covers the Sun, the sky goes dark, stars appear, and the Sun's corona glows around the Moon's edge. The umbra can be as narrow as a few kilometers and rarely exceeds about 270 km wide on Earth's surface.

What You Would Experience
  • Sun completely covered by the Moon
  • Sky noticeably darkens, stars and planets may appear
  • Solar corona glows around the Moon's edge
  • Only a narrow path on Earth experiences totality
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Penumbra: Partial Shadow
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The outer, lighter shadow zone: from here, the Moon only partly covers the Sun.

The penumbra is the outer, lighter shadow zone. Here, the Moon covers only part of the Sun, you see a partial solar eclipse, a crescent Sun still bright enough to need eye protection. The penumbra spreads wider than the umbra across Earth's surface. Most people observing a solar eclipse are in the penumbra and see only a partial eclipse.

What You Would Experience
  • Part of the Sun remains visible
  • Daylight stays relatively bright
  • Sun appears crescent-shaped
  • Eclipse glasses are still required
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A remarkable coincidence. The Sun is 400 times wider than the Moon; but also 400 times farther away. From Earth, they appear almost exactly the same size. Without this coincidence, the umbra would never reach Earth's surface, and total solar eclipses wouldn't exist.
📚 Instructional Design
Why this section exists
  • Separate the umbra from the penumbra as two shadow zones.
  • Tie which zone you stand in to what you actually see.
Cognitive science
  • Comparison and contrast
  • Dual coding
  • Elaboration
Bloom's / DOK
  • Understand to Analyze
  • DOK 2
Accessibility considerations
  • Labeled shadow diagram paired with text
  • Umbra and penumbra named where shown
  • Short, parallel descriptions
Quick Recall

Brain Check

Three quick questions before we put it all together. 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.
The Moon orbits Earth every month, so why don't we get an eclipse every month?
Quick Recall · 2 of 3
Just a quick brain check. Not graded.
In a solar eclipse, what moves into whose shadow?
Quick Recall · 3 of 3
Just a quick brain check. Not graded.
What is the umbra of a shadow?
📚 Instructional Design
Why this section exists
  • Pull key eclipse facts from memory before the 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

Why Not Every Month?

You started with a puzzle: the Moon lines up with the Sun every month, so why don't eclipses happen every month? Here is the whole answer, step by step.

The Setup
Every month the Moon passes between Earth and the Sun.
Once per orbit the Moon sits roughly on the line between Earth and the Sun (New Moon). If orbits were perfectly flat, that would line up an eclipse every single month.
The Twist
The Moon's orbit is tilted about five degrees.
Because of that tilt, the Moon usually passes a little above or below the Sun-Earth line. Its shadow misses Earth, so most months bring no eclipse at all.
The Payoff
An eclipse needs alignment AND the Moon near a node.
Only when New or Full Moon happens near a node do the three bodies truly line up. Then the umbra and penumbra fall in the right place to make a solar or lunar eclipse.
The full chain:
Moon lines up every month Tilted orbit usually misses Alignment must happen near a node Shadow falls in the right place Solar or lunar eclipse
Eclipses are rare not because the Sun and Moon rarely line up, but because they rarely line up in the same plane. The tilt is the whole answer.
📚 Instructional Design
Why this section exists
  • Answer the opening puzzle in one connected chain.
  • Return to the prediction students made so the loop closes.
Cognitive science
  • Schema building
  • Coherent narrative
  • Elaboration
Bloom's / DOK
  • Understand to Analyze
  • DOK 3
Accessibility considerations
  • Steps shown as ordered chips
  • One idea per step
  • Plain summary sentence at the end
Evaluate · Quiz

Eclipses Quiz

10 questions covering solar and lunar eclipses, shadows, orbital tilt, and nodes. Answer every question, then 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.

The Moon reaches new moon every single month, the exact phase a solar eclipse needs. Yet a solar eclipse is rare. Explain why we do not get an eclipse every month. Build the whole chain: what lines up every month, why the Moon's tilted orbit usually makes the shadow miss Earth, and what has to be true about the Moon's position for an eclipse to actually happen. Use the word node.

One strong way to say it Every month the Moon reaches new moon and passes roughly between Earth and the Sun, so it seems like the shadow should line up every time. But the Moon's orbit is tilted about five degrees, so most months the Moon passes a little above or below the exact Sun-Earth line and its shadow misses Earth completely. An eclipse only happens when a new moon (or full moon) lands near a node, one of the two points where the tilted orbit crosses Earth's orbital plane. Only there do the Sun, Moon, and Earth line up closely enough for the shadow to connect. The Moon lines up every month, but it rarely lines up in the same plane, and that is why eclipses are rare.

🔍 The Question You Came In With You started this lesson asking: "If the Moon orbits Earth every month, why don't eclipses happen every month?" The Moon lines up with the Sun every month, but its orbit is tilted about five degrees, so the shadow usually passes above or below Earth. Only when a new or full moon lands near a node, where the tilted orbit crosses Earth's plane, do the three bodies line up closely enough for an eclipse.
📚 Instructional Design
Why this section exists
  • Assess solar and lunar eclipses, shadows, tilt, and nodes.
  • Give a quick check of understanding after the wrap-up.
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

Ready to push your eclipse knowledge deeper? These optional extensions are waiting for you.

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Eclipse Alignment
Manipulate moon phase, orbital tilt, and node alignment to discover why solar and lunar eclipses only happen sometimes.
Simulation
📚 Instructional Design
Why this section exists
  • Offer optional depth for students who want to keep going.
  • Let learners test tilt and node ideas in a simulation.
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

An eclipse is all about alignment. Here are the ideas that help explain how it happens.

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Sun-Earth-Moon System
An eclipse depends on alignment. See how the motion of these three bodies sets up the rare moments when they line up.
Explore
🌙
Phases of the Moon
Solar eclipses happen at a new Moon and lunar eclipses at a full Moon. See how phases and eclipses share the same orbit.
Explore
🌊
Nature of Waves
Eclipses are made of light and shadow. Explore how light travels in straight lines, which is why the Moon can block the Sun.
Explore