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# Why are there two high tides a day?

Year 7 Earth and space · The moon

<!-- Speaker note: Students explain tides as caused by the moon's gravity, account for two high tides a day, and link spring and neap tides to the moon's position and phase (AC9S7U03, node 5). About 20 minutes (Engage and Explore 7 min · Explain 7 min · Evaluate 3 min · Typical adds 1 min, Wider about 2 min). -->

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## Today

**We are learning to** the moon's gravity pulls the oceans into two bulges, one facing the moon and one on the far side; the Earth spins through both, so most coasts get two high tides a day. Spring tides (biggest range) come at new and full moon, when the sun and moon pull in line; neap tides at the quarters.

- I can explain that the moon's gravity causes the tides.
- I can explain why most coasts have two high tides a day.
- I can link spring and neap tides to the moon's phases.

<!-- Speaker note: Before class: A concept tutorial for introducing or revising the idea; pair it with a real tide table for a coast near you (Bureau of Meteorology tide predictions). Set the width with `?width=minimum`, `typical` (default) or `wider`. This node integrates the moon's phases (node 2), the line-ups from eclipses (node 3) and gravity (AC9S7U04). Students who can do each part separately often can't put them together; the board joins the bulges, the moon's position and a month of tides in one view. Board notes: the bulges are hugely exaggerated, and the far-side bulge is shown without explaining it fully (at Year 7, "the moon pulls the near ocean more than the Earth, and the Earth more than the far ocean" is enough). The chart is for an idealised coast with a 3.2 m spring range and 1.2 m neap range; real coasts vary a lot, and some have one tide a day. Play a day at the beach ignores the moon's movement during the day (which makes real tides about 50 minutes later each day). -->

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## Why it matters

> At a beach the water creeps up the sand and back down about every 12 and a half hours, and the biggest tides of the month always arrive near a new or full moon.

<!-- Speaker note: You know the moon's phases and positions. This lesson connects them to the oceans. -->

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<span style="background:#dbeafe; color:#1a1a1a; border:1.5px solid #3b82f6; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">💬 Discussion</span>

## Engage: predict

**On most coasts, how many high tides are there in a day?**

- It's random
- Four
- Two: about 12½ hours apart
- One: when the moon is overhead

<!-- Speaker note: What happens: Two, about 12½ hours apart. The moon pulls the oceans into two bulges (one towards it and one on the far side) and the Earth spins through both each day. Take two or three predictions before anyone runs it. -->

<!-- Speaker note: Teacher: count how many chose one high tide a day. -->

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## Explore it

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

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

<!-- Speaker note: Press **Play a day at the beach** and count the high tides. Then press **Moon: next quarter** (or drag the moon) through a whole month. When are the tides biggest on the chart? -->

<!-- Speaker note: Teacher: ask how many high tides the beach passed through. -->

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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?

* Two, about 12½ hours apart. The moon pulls the oceans into two bulges (one towards it and one on the far side) and the Earth spins through both each day.

<!-- 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>

## Cause

**What mainly causes the tides?**

- The Earth's spin alone
- The wind
- The moon's light
- The moon's gravity pulling on the oceans

* Answer: The moon's gravity pulling on the oceans

<!-- Speaker note:  -->

<!-- Speaker note: Teacher: the spring/neap sort 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>

## Spring or neap

**Spring tides or neap tides?**

Groups: Spring tides · Neap tides

New moon · Full moon · First quarter · Last quarter · The sun and moon pull in line · The sun pulls at right angles to the moon · Smallest difference between high and low tide

* Answer: New moon → Spring tides; Full moon → Spring tides; First quarter → Neap tides; Last quarter → Neap tides; The sun and moon pull in line → Spring tides; The sun pulls at right angles to the moon → Neap tides; Smallest difference between high and low tide → Neap tides

<!-- 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 name

**Do spring tides happen only in spring?**

- Only in the Northern Hemisphere's spring
- Only once a year
- Yes, only in September to November
- No: they happen about every two weeks all year, at new and full moon ('spring' as in springing up)

* Answer: No: they happen about every two weeks all year, at new and full moon ('spring' as in springing up)

<!-- 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 new moon, where is the moon?**

- Behind the sun
- At a right angle to the sun
- On the far side of the Earth from the sun
- Between the Earth and the sun, so the sun, moon and Earth are in a line

* Answer: Between the Earth and the sun, so the sun, moon and Earth are in a line

<!-- Speaker note: At new moon the moon is between the Earth and the sun. The sun and moon pull in line, which gives a spring tide. What this step is for: Spring and neap tides depend on where the moon is in its orbit, which the phases lesson laid out. -->

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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: reading a tide chart

**On the board's chart, about how many days are there between one spring tide and the next?**

* Answer: 15

<!-- Speaker note: About 14.8 days: half the moon's 29.5-day cycle, from new moon to full moon. What this step is for: Reading the period of spring tides from a chart connects the model to real data. -->

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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: the best low tide

**Reef walking and gathering shellfish is best at the lowest low tides. When should you plan to go?**

- At a quarter moon
- Near a new or full moon, at low tide: spring tides give the lowest lows
- When the moon is high overhead
- Any day: low tides are always the same

* Answer: Near a new or full moon, at low tide: spring tides give the lowest lows

<!-- Speaker note: Many coastal First Nations communities have long timed fishing and gathering by the moon and tides; ask local knowledge holders about your own coast. What this step is for: Using the moon's phase to predict extreme low tides applies the model, as coastal people have for thousands of years. -->

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## Evaluate: look back

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

**Revision summary**

- The moon's **gravity** pulls the oceans into **two bulges** (towards the moon and on the far side); the Earth spins through both: **two high tides a day** on most coasts.
- **Spring tides** (biggest range): new and full moon, sun and moon **in line**. **Neap tides** (smallest range): quarter moons, sun at **right angles**.
- Spring and neap tides each come about every two weeks, all year.

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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

**When do spring tides happen, and why are they bigger?**

* At new and full moon, when the sun and moon are in line, so their pulls on the oceans add together.

<!-- 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: This completes AC9S7U03. Later: gravity and orbits (AC9S7U04), and First Nations knowledge of the moon and tides from local sources. In class: real tide tables; compare tide heights with moon phases over a month. -->