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# Why isn't there an eclipse every month?

Year 7 Earth and space · The moon

<!-- Speaker note: Students state the conditions for solar and lunar eclipses, distinguish the umbra and penumbra, and explain why eclipses do not occur every month (AC9S7U03, node 3). About 22 minutes (Engage and Explore 8 min · Explain 8 min · Evaluate 3 min · Typical adds 3 min, Wider about 2 min). -->

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## Today

**We are learning to** a solar eclipse happens at new moon when the moon's shadow falls on Earth; a lunar eclipse at full moon when the moon passes through Earth's shadow. The moon's orbit is tilted about 5°, so most months the shadows miss: eclipses come in seasons about six months apart.

- I can state the conditions for a solar and a lunar eclipse.
- I can describe the umbra and penumbra.
- I can explain why eclipses don't happen every month.

<!-- Speaker note: Before class: A concept tutorial for introducing or revising the idea; it pairs well with a lamp, a ball and a hoop tilted to show the moon's orbit. Set the width with `?width=minimum`, `typical` (default) or `wider`. The node isn't a hinge, but it depends on the moon-phase model (node 2): a student who thinks phases are Earth's shadow will confuse a lunar eclipse with a phase. The 3D step (the tilted orbit) is the hard part; the board shows it in a side view with the tilt and the shadows exaggerated. Board notes: each step is half a month (29.5-day month). The line where the moon's orbit crosses the plane of Earth's orbit turns relative to the sun once every 346.6 days, so eclipse seasons come a little under six months apart; the board starts so that the first eclipses come in about month 4. Eclipse types use real angle limits (for example, a lunar eclipse is total when the moon is within about 0.4° of the plane). Solar eclipses are seen only from a narrow path or region of Earth. The 2028 total solar eclipse crosses Sydney on 22 July 2028. -->

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## Why it matters

> There's a new moon every month, with the moon between us and the sun. Yet the next total solar eclipse seen from Sydney, on 22 July 2028, will be the first there in over 150 years.

<!-- Speaker note: You know where the moon is at each phase. This lesson: when the sun, Earth and moon line up exactly. -->

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## Engage: predict

**At every new moon the moon is between the Earth and the sun. How often do you think there's a solar eclipse somewhere on Earth?**

- Every month
- About twice a year
- Never: they are too rare to predict
- Once every hundred years

<!-- Speaker note: What happens: About twice a year (between two and five times), and each one is seen only from part of the Earth. Most months the moon passes just above or below the line to the sun. Take two or three predictions before anyone runs it. -->

<!-- Speaker note: Teacher: count how many chose 'every month'. -->

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## Explore it

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fearth-7%2Feclipses%2Fstep-2&code=widget%3A+sky%0Amode%3A+eclipse%0A&fresh=1}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/earth-7/eclipses/?full=step-2#step-2)

<!-- Speaker note: Press **Next full moon** / **Next new moon** again and again. Watch the side view: where is the moon compared with the plane of Earth's orbit? Watch the top view: where is the orange crossing line when an eclipse happens? -->

<!-- Speaker note: Teacher: ask where the orange crossing line points when an eclipse happens. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## What happened?

* About twice a year (between two and five times), and each one is seen only from part of the Earth. Most months the moon passes just above or below the line to the sun.

<!-- Speaker note: Reveal after running. Ask who predicted it, and what made them think so. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Solar or lunar

**Solar eclipse or lunar eclipse?**

Groups: Solar eclipse · Lunar eclipse

Happens at new moon · Happens at full moon · The moon is between the sun and the Earth · The Earth is between the sun and the moon · The moon's shadow falls on part of Earth · The moon turns a dark red

* Answer: Happens at new moon → Solar eclipse; Happens at full moon → Lunar eclipse; The moon is between the sun and the Earth → Solar eclipse; The Earth is between the sun and the moon → Lunar eclipse; The moon's shadow falls on part of Earth → Solar eclipse; The moon turns a dark red → Lunar eclipse

<!-- Speaker note:  -->

<!-- Speaker note: Teacher: the tilt question is the checkpoint. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Shadows

**Match each shadow to what you'd see from inside it.**

| | |
|---|---|
| `Umbra` | A. The outer part: the light source is partly blocked (a partial eclipse) |
| `Penumbra` | B. The dark centre: the light source is completely blocked (a total eclipse) |

* Answer: Umbra → B. The dark centre: the light source is completely blocked (a total eclipse); Penumbra → A. The outer part: the light source is partly blocked (a partial eclipse)

<!-- Speaker note:  -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## The tilt

**Why isn't there a solar eclipse at every new moon?**

- The moon moves too fast
- The sun is sometimes too bright
- The moon's orbit is tilted about 5° to Earth's, so the moon usually passes above or below the line from the sun to the Earth
- The moon is too small

* Answer: The moon's orbit is tilted about 5° to Earth's, so the moon usually passes above or below the line from the sun to the Earth

<!-- Speaker note:  -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Mix it up

**Looking back at the moon's phases: at which phase is the moon between the Earth and the sun?**

- New moon
- Full moon
- First quarter
- Last quarter

* Answer: New moon

<!-- Speaker note: At new moon the moon is between the Earth and the sun, so we see its dark half. A solar eclipse can only happen at new moon. What this step is for: An eclipse needs a particular line-up of the sun, Earth and moon, so the phase positions come back first. -->

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<span style="background:#fef3c7; color:#1a1a1a; border:1.5px solid #f59e0b; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✍️ Activity</span>

## Elaborate: a flat orbit

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fearth-7%2Feclipses%2Fstep-5&code=widget%3A+sky%0Amode%3A+eclipse%0Atilt%3A+choose%0A&fresh=1}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/earth-7/eclipses/?full=step-5#step-5)

<!-- Speaker note: What this step is for: Removing the tilt in the model and getting an eclipse every time shows the tilt is the reason, not a detail. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## How often

**With a flat orbit, how often were there eclipses?**

- Never
- Only in summer
- About twice a year
- At every new moon and every full moon

* Answer: At every new moon and every full moon

<!-- Speaker note:  -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Elaborate: predicting eclipses

**Many ancient peoples recorded eclipses carefully but found them hard to predict. Babylonian astronomers eventually noticed eclipses repeat in a cycle of about 18 years. Which missing idea makes eclipses predictable from first principles?**

- The speed of light
- The exact date of every new and full moon
- The size of the moon
- The moon's tilted orbit, and where it crosses Earth's orbit plane: eclipses happen only when that crossing line points at the sun

* Answer: The moon's tilted orbit, and where it crosses Earth's orbit plane: eclipses happen only when that crossing line points at the sun

<!-- Speaker note: Long records let people find patterns (like the 18-year saros cycle) even without the model; the 3D model explains why the pattern exists. What this step is for: Seeing why eclipses seemed unpredictable without a 3D model connects the physics to the history of science. -->

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## Evaluate: look back

Write one sentence: "At first I thought… Now I think… because…"

**Revision summary**

- **Solar eclipse:** new moon, moon between sun and Earth, the moon's shadow falls on part of Earth.
- **Lunar eclipse:** full moon, Earth between sun and moon, the moon passes through Earth's shadow (it often looks red).
- **Umbra:** full shadow (total eclipse). **Penumbra:** partial shadow (partial eclipse).
- The moon's orbit is **tilted about 5°**, so most new and full moons miss: eclipses come in **eclipse seasons** about every six months.

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Exit ticket

**Why doesn't every full moon give a lunar eclipse?**

* The moon's orbit is tilted about 5° to Earth's, so at most full moons the moon passes above or below Earth's shadow.

<!-- Speaker note: Students answer on paper or a mini whiteboard before the reveal. Anyone who cannot answer needs the main idea again next lesson. Where this leads: Next: tides (node 5). In class: a lamp, a ball and a tilted hoop; look up the next eclipses visible from your town. -->