--- title: "Why isn't there an eclipse every month?" subtitle: "Year 7 Earth and space · The moon · Lesson 4" --- # Why isn't there an eclipse every month? **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** 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. :::teacher-note **In one sentence:** 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). **Students should be able to say:** 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. **Width:** the unmarked parts are the floor every student does (Engage, Explore, Explain, Evaluate). Typical and Wider sections add contexts, not difficulty. Students work in the online tutorial alongside this sheet. ::: ## Engage :::keep-together **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 My prediction, and why: {{lines-3}} ::: :::teacher-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. ::: ## Explore Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Explain: conditions for an eclipse :::keep-together ### Solar or lunar **Solar eclipse or lunar eclipse?** Groups: Solar eclipse, Lunar eclipse - Happens at new moon: {{field-15}} - Happens at full moon: {{field-15}} - The moon is between the sun and the Earth: {{field-15}} - The Earth is between the sun and the moon: {{field-15}} - The moon's shadow falls on part of Earth: {{field-15}} - The moon turns a dark red: {{field-15}} ::: :::teacher-note **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 ::: :::keep-together ### Shadows **Match each shadow to what you'd see from inside it.** A. The outer part: the light source is partly blocked (a partial eclipse) B. The dark centre: the light source is completely blocked (a total eclipse) {{columns 60% 40%}} {{rows 1}} | Part | Letter | |---|---| | `Umbra` | | | `Penumbra` | | ::: :::teacher-note **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) ::: :::keep-together ### 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 ::: :::teacher-note **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 ::: ## Mix it up :::keep-together **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 ::: :::teacher-note **Answer:** New moon 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. ::: :::typical ## Elaborate: a flat orbit :::keep-together ### 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 ::: :::teacher-note **Answer:** At every new moon and every full moon ::: ::: :::wider ## Elaborate: predicting eclipses :::keep-together **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 ::: :::teacher-note **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 Long records let people find patterns (like the 18-year saros cycle) even without the model; the 3D model explains why the pattern exists. ::: ::: ## 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. ## Exit ticket :::keep-together **Why doesn't every full moon give a lunar eclipse?** {{lines-3}} ::: :::teacher-note **Answer:** 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. :::