Year 7 Science · Unit 4 · Lesson 9
Master Worksheet
Learning Goals
1. The Sun is far bigger than the Moon. Why does the Moon, not the Sun, have the bigger effect on the tides?
Find the error
Leo’s explanation
Line 1: A tide is the regular daily rise and fall of the level of the sea.
Line 2: Because there are two bulges, a beach gets two high tides every hour.
Line 3: At a neap tide the Sun and the Moon are at right angles, so their pulls partly cancel.
Line 4: The biggest tidal range of the month comes when the Moon looks half lit.
2. Two of Leo’s four lines contain an error. Name each wrong line and rewrite it correctly.
Scenario: planning a fishing trip
A half-lit Moon tonight
Tonight Aunt Kim sees a first quarter Moon, exactly half lit. She will go fishing this week, and again seven days from now. She wants to know whether to expect big or small tides each time.
3. Make both predictions for Aunt Kim. For each trip, say where the Sun and Moon sit compared with the Earth.
Read the data: a month of tides
One port, one month
A harbour master recorded the tidal range at an Australian port every few days for one month. The Moon’s phase is given in brackets on the days it was new, full or at a quarter. Read down the first pair of columns, then the second.
| Day | Tidal range (m) | Day | Tidal range (m) |
|---|---|---|---|
| 1 (new Moon) | 1.9 | 19 | 1.4 |
| 4 | 1.5 | 23 (last quarter) | 0.8 |
| 8 (first quarter) | 0.9 | 27 | 1.4 |
| 12 | 1.4 | 30 (new Moon) | 1.9 |
| 16 (full Moon) | 1.8 | – | – |
Source: made-up but realistic records for this worksheet.
4. Plot the tidal range against the day as a line graph. Label both axes with units.
5. List the days of the spring tides and of the neap tides in Table 1. Then say why the range rises and falls twice in a month.
6. Predict the tidal range at this port on about day 37. Explain your prediction.
7. Evaluate this claim: “Because the sea is always moving, the tides are far too messy to predict.” Use Table 1 and an Australian example.
Question 1
The Sun is much further away than the Moon (1), so its pull on the tides is weaker (1). [2 marks]
Find the error
2. Line 2: two high tides each day, about every 12 hours (1). Line 4: a half-lit (quarter) Moon gives neap tides, the smallest range; the biggest comes at new and full Moon (1). Lines 1 and 3 are correct. [2 marks]
Scenario: planning a fishing trip
3. This week: small tides, neap (1), because at first quarter the Sun and Moon are at right angles and their pulls partly cancel (1). In seven days: big tides, spring, because at full Moon the Sun, Earth and Moon are in a line and the pulls add (1). [3 marks]
Read the data: a month of tides
4. Axes labelled (day; tidal range in m) with even scales (1); nine points plotted correctly (1); points joined and a title (1). [3 marks]
5. Spring: days 1, 16 and 30 (1.8–1.9 m). Neap: days 8 and 23 (0.8–0.9 m) (1). At new and full Moon the three line up and the pulls add (1); at the quarters the Sun and Moon are at right angles and the pulls partly cancel (1). [3 marks]
6. About 0.8–1.0 m, a neap tide (1): day 37 is about a week after the day 30 new Moon, so it is near first quarter (1). [2 marks]
Question 7
Marking guide: 1 position (disagree); 1 tides are caused by the gravity of the Moon and Sun; 1 their positions change in a regular, repeating way that can be calculated; 1 Table 1 shows a repeating pattern: biggest at new and full Moon, smallest at the quarters; 1 the day 30 range matches day 1, so the cycle repeats; 1 Australian example: the Bureau of Meteorology prints tide times years ahead, even for Broome’s 9 m tides. [6 marks]
Model answer: I disagree. Tides are caused by the gravity of the Moon and Sun, whose positions repeat in a regular way. Table 1 shows it: biggest ranges at new and full Moon, smallest at the quarters, and day 30 matches day 1. That is why the Bureau of Meteorology can print tide times for Broome years ahead.
Common error: linking spring tides to the season of spring.