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.
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.
You know the moon's phases and positions. This lesson connects them to the oceans.
Engage: Commit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Explore: Try it on the board and compare what happens with your prediction.
Explain: Use the particle model to explain what you saw.
Practise: A few on your own, to check you have it.
Elaborate: Use the same idea in a new context.
Evaluate: Look back at your first ideas and what changed.
WidthMore contexts, same idea.
Make your prediction first. Help opens once you've locked it in.
EngageCommit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Engage: how many high tides?
ExploreTry it on the board and compare what happens with your prediction.
Explore: bulges and a month of tides
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?
Try it yourselfYour edits are saved in this browser.
Play a day at the beach, then find both spring tides and both neap tides in the month.
Use the board. This ticks when you get there.
ExplainUse the particle model to explain what you saw.
Explain: the moon, the sun and the tides
In your book: draw the Earth with its two bulges and the moon, then the sun in line (spring) and at right angles (neap).
Spring tides or 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
PractiseA few on your own, to check you have it.
Mix it up
Elaborate TypicalUse the same idea in a new context.
Elaborate: reading a tide chart
On the board's chart, about how many days are there between one spring tide and the next?
Elaborate WiderUse the same idea in a new context.
Elaborate: the best low tide
EvaluateLook back at your first ideas and what changed.
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.
For teachers
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.
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.
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).
Running the lesson
Engage: count how many chose one high tide a day.
Explore: ask how many high tides the beach passed through.
Explain: the spring/neap sort is the checkpoint.
The hard part (on purpose)
Explaining the second bulge on the far side. When a student predicts one high tide a day, ask how many bulges the board shows and how many the coast passes through.
When students get stuck
A student says there should be one high tide, when the moon is overhead. Ask: How many bulges does the board show? Fix: Two: one towards the moon and one on the far side, so two high tides as the Earth turns.
A student thinks spring tides are a season. Ask: How often does the chart show the biggest tides? Fix: About every two weeks: at new and full moon.
Stretch and support
Stretch: Why are the tides at full moon about as big as those at new moon, even though the sun is on opposite sides?
Support: Give a four-picture strip (new, first quarter, full, last quarter) to label spring or neap.
Where it 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.
What each step is for
Engage: how many high tides?: Students who picture one bulge under the moon predict one high tide a day; the board's two bulges and a played day correct it.
Explore: bulges and a month of tides: Playing a day at the beach shows two highs; moving the moon through its phases and watching the chart shows spring and neap tides follow the moon's position.
Explain: the moon, the sun and the tides: Connecting the moon's gravity, the two bulges and the sun's lining up is the integration this node asks for.
Mix it up: Spring and neap tides depend on where the moon is in its orbit, which the phases lesson laid out.
Elaborate: reading a tide chart: Reading the period of spring tides from a chart connects the model to real data.
Elaborate: the best low tide: Using the moon's phase to predict extreme low tides applies the model, as coastal people have for thousands of years.
Now you can
Explain that the moon's gravity causes the tides.
Explain why most coasts have two high tides a day.