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.
What causes tidal movements?
low tide when the ocean covers slightly less land; the lowest level on the shore that the tide recedes to
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high tide when the ocean covers slightly more land; the highest level that the tide reaches on the shore
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
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Figure 1 The Moon
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.
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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.
Neap tide
Spring 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.)
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OXFORD UNIVERSITY PRESS OXFORD SCIENCE 7: VICTORIAN CURRICULUM No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means.
