Year 7 Science · Unit 4 · Lesson 9
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Learning Goals
1. What is the tidal range? Name the place in Western Australia where it can be more than 9 metres.
Order the steps: two high tides in a day
Number the steps from 1 (first) to 5 (last).
| Order (1–5) | Step |
|---|---|
| The Earth keeps turning, and the beach moves into the lower water between the bulges: low tide. | |
| The Moon’s gravity pulls on the ocean water. | |
| About 12 hours after its first high tide, the beach reaches the other bulge: a second high tide. | |
| The water forms two bulges: one on the side facing the Moon and one on the far side. | |
| As the Earth spins, the beach is carried into one of the bulges: high tide. |
Predict the tide from the Moon
Complete the table. One mark for each row with both cells correct.
| Moon phase | How the Sun, Earth and Moon are arranged | Spring or neap tide? |
|---|---|---|
| Full Moon | ||
| Last quarter Moon | ||
| New Moon | ||
| First quarter Moon |
Cause and effect: a spring tide
Fill the two empty boxes. Each box is caused by the one before it, and row 2 carries on from the end of row 1.
Read the data: two tide charts
Scenario: a harbour on the coast
The Bureau of Meteorology works out charts like these years ahead. Chart A is for a full-Moon day. Chart B is for the same harbour seven days after.
| Chart | Morning high | Midday low | Evening high | Night low |
|---|---|---|---|---|
| A (full Moon) | 5:10 am, 1.8 m | 11:30 am, 0.3 m | 5:45 pm, 1.7 m | 11:55 pm, 0.4 m |
| B (one week later) | 10:40 am, 1.3 m | 4:55 pm, 0.8 m | 11:05 pm, 1.2 m | 5:20 am next day, 0.7 m |
Source: made-up but realistic charts for this worksheet.
2. For Chart A, subtract the midday low from the morning high. Do the same for Chart B. Show your working.
A m B m
3. What type of tide does Chart B show? Which Moon phase was it probably, and how were the Sun and Moon arranged? Use your answers to question 2.
4. A skipper needs the deepest water to bring a boat into this shallow harbour on the Chart A day. Which time should she aim for, and why?
5. At Broome in Western Australia the sea can rise and fall by more than 9 metres in a day, yet the Bureau of Meteorology can predict its tides years ahead. Explain why this is possible, and why it matters to people who use the coast.
Question 1
The difference in height between high tide and low tide (1). Broome (accept King Sound or Derby) (1). [2 marks]
Order the steps: two high tides in a day
Reading down the table as printed, the correct step numbers are 4, 1, 5, 2, 3. In order: gravity pulls the water; two bulges form; the beach enters a bulge (high); it moves between bulges (low); it reaches the other bulge (high).
Predict the tide from the Moon
Full Moon: in a line; spring (1). Last quarter: Sun and Moon at right angles; neap (1). New Moon: in a line; spring (1). First quarter: at right angles; neap (1). [4 marks]
Cause and effect: a spring tide
Box 1: the Sun’s pull and the Moon’s pull act in the same direction and add together (1). Box 2: the tidal bulges become extra large (1). [2 marks]
Read the data: two tide charts
2. A: 1.8 − 0.3 = 1.5 m (1). B: 1.3 − 0.8 = 0.5 m (1). [2 marks]
3. A neap tide: its range is much smaller, 0.5 m against 1.5 m (1). About a week after full Moon: the last quarter Moon (1). The Sun and Moon were at right angles, so their pulls partly cancelled (1). [3 marks]
4. 5:10 am (1): the highest high tide of the day, 1.8 m, gives the deepest water (1). [2 marks]
Question 5
Marking guide: 1 tides are caused by the Moon’s (and Sun’s) gravity; 1 their positions change in a regular, repeating way that can be calculated years ahead; 1 so the tides, although large, are predictable, not random; 1 spring tides at new and full Moon, neap at the quarters; 1 each for two uses (boats entering harbour at high tide; surfers; fishers; safety on rock platforms and mud flats), maximum 2. [6 marks]
Model answer: Tides are caused by the gravity of the Moon and Sun, whose positions repeat in a regular way that can be calculated years ahead, so even huge tides are predictable. Spring tides come at new and full Moon and neap tides at the quarters. Skippers use the predictions to enter harbours, and people on mud flats use them to stay safe.
Common error: thinking large or fast tides must be random.