9.7 The Moon’s gravity causes tidal movements

In this topic, you will learn that:

• the Earth’s pull force holds the Moon in orbit • the relationship between the Moon and the tides was recognised by early Aboriginal and Torres Strait Islander peoples.

Figure 1 The Moon

high tide when the ocean covers slightly more land; the highest level that the tide reaches on the shore

low tide when the ocean covers slightly less land; the lowest level on the shore that the tide recedes to The relationship between the Moon and the tides was recognised by early Aboriginal and Torres Strait Islander peoples. Arnhem Land stories tell of high tides fi lling the Moon. As the tide falls, the Moon is left empty for three days before fi lling once more.

What causes tidal movements?

The effect of gravity between two objects is related to the size of each of these objects and how far apart they are from each other. The Earth’s pull force holds the Moon in orbit. The Moon has its own pull force that causes the Earth’s oceans to bulge towards the Moon. This causes the oceans to cover slightly more land, which we see on the Earth as a high tide . The Earth is also being pulled towards the Moon (and away from the water on the opposite side), so another high tide occurs on the opposite side of the Earth. As the Moon travels around the Earth and as both bodies travel around the Sun, the combined pull force from gravity causes the world’s oceans to rise to high tides and fall to low tides . Because the Earth is rotating while this is happening, two high tides occur each day, approximately 11 hours apart. High tides happen when the land at the beach turns towards the water being pulled by the Moon or Sun. Low tides happen when the land turns away from the water bulge. Worked example 9.7 shows how to calculate tides.

Spring tide Neap tide

Spring tide: the gravity of the Sun and the Moon pull together Neap tide: the gravity of the Moon and the Sun pull at right angles to each other

Figure 2 The Moon’s pull on the oceans creates spring and neap tides. (The bulges shown here have been exaggerated so that they are easier to see.)

When the Sun, Moon and Earth are aligned, the combined pull of the Moon and the Sun causes very high tides in some parts of the Earth. These are known as spring tides (Figure 2). Smaller neap tides occur during the Moon’s quarter phases. At these times, the Sun and Moon are at right angles to the Earth, causing the sea tides to be pulled in both directions at once.

World’s biggest tides

The biggest change in the depth of water between low and high tide occurs at the Bay of Fundy in Nova Scotia, Canada. Twice a day, 100 billion tonnes of sea water fl ow in and out of the Bay of Fundy. This is more than the combined fl ow of the world’s freshwater rivers. The narrow, funnelshaped inlet causes changes in depth of up to 17 m, making life interesting for boating.

Figure 3 The Bay of Fundy in Nova Scotia, Canada

9.7 Check your learning

Remember and understand

1 Explain why the Moon has a greater effect on the tide levels than the Sun.

Apply and analyse

2 Referring to Table 1, calculate the difference between:

a the last high tide on Saturday and the next one on Sunday b one high tide and the following low tide on Saturday. 3 Use the data in Table 1 to predict the times and heights of the tides for

Monday.

Worked example 9.7: Calculating tides a

Table 1 shows the times of high tide at Williamstown, in Melbourne, Victoria, over three days in December 2020. Calculate the difference in height between the two high tides on the Friday.

Solution

One high tide is 0.88 m at 6.25 am and the other high tide is 0.72 m at 5.46 pm.

Subtract the smaller tide from the larger tide: 0.88 – 0.72 = 0.16 m

a b

b Figure 4 a High tide and b low tide at the Bay of Fundy, Nova Scotia, Canada

Table 1 High tides in Williamstown, Melbourne, in December 2020

Friday 4 December 2020 Saturday 5 December 2020 Sunday 6 December 2020

Time Height (m) Time Height (m) Time Height (m) 12:18 pm 0.10 12.55 am 0.09 1.31 am 0.09 6.25 am 0.88 7.00 am 0.88 7.35 am 0.87

1.14 pm 0.35 1.14 pm 0.34 2.24 pm 0.33 5.46 pm 0.72 6.24 pm 0.72 7.03 pm 0.72

Evaluate and create

4 For one week, graph the high and low tide levels of a beach in your state. Compare this against the times the

Moon rises and sets. Describe the relationship between the Moon’s position and tide levels.

5 Gravity is not considered a force.

Instead, it is the distortion of space and time caused by a large object. This allows the large object to have a pull force. Rewrite the following sentence so that it is correct.

The force of the Moon’s gravity pulls the water on Earth to cause high tides .

spring tide when there is maximum difference between high and low tides; caused by the combined pull of the Moon and the Sun

neap tide when the difference between high and low tides is smallest; occurs during the Moon's quarter phase, when the Sun and the Moon pull in different directions