--- title: "Why are there two high tides a day?" subtitle: "Year 7 Earth and space · The moon · Lesson 5" --- # Why are there two high tides a day? **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** 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. :::teacher-note **In one sentence:** 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). **Students should be able to say:** 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. **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 **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 My prediction, and why: {{lines-3}} ::: :::teacher-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. ::: ## Explore Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Explain: the moon, the sun and the tides :::keep-together ### 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 ::: :::teacher-note **Answer:** The moon's gravity pulling on the oceans ::: :::keep-together ### Spring or neap **Spring tides or neap tides?** Groups: Spring tides, Neap tides - New moon: {{field-15}} - Full moon: {{field-15}} - First quarter: {{field-15}} - Last quarter: {{field-15}} - The sun and moon pull in line: {{field-15}} - The sun pulls at right angles to the moon: {{field-15}} - Smallest difference between high and low tide: {{field-15}} ::: :::teacher-note **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 ::: :::keep-together ### 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) ::: :::teacher-note **Answer:** No: they happen about every two weeks all year, at new and full moon ('spring' as in springing up) ::: ## Mix it up :::keep-together **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 ::: :::teacher-note **Answer:** Between the Earth and the sun, so the sun, moon and Earth are in a line At new moon the moon is between the Earth and the sun. The sun and moon pull in line, which gives a spring tide. ::: :::typical ## Elaborate: reading a tide chart :::keep-together **On the board's chart, about how many days are there between one spring tide and the next?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 15 About 14.8 days: half the moon's 29.5-day cycle, from new moon to full moon. ::: ::: :::wider ## Elaborate: the best low tide :::keep-together **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 ::: :::teacher-note **Answer:** Near a new or full moon, at low tide: spring tides give the lowest lows Many coastal First Nations communities have long timed fishing and gathering by the moon and tides; ask local knowledge holders about your own coast. ::: ::: ## 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. ## Exit ticket :::keep-together **When do spring tides happen, and why are they bigger?** {{lines-3}} ::: :::teacher-note **Answer:** At new and full moon, when the sun and moon are in line, so their pulls on the oceans add together. :::