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Tides

The level of the sea rises and falls about two times each day. We call these changes tides.

The gravitational pull of the Moon creates tides on Earth . The Moon's gravity pulls the Earth's oceans towards the Moon. This causes a high tide on the side of the Earth nearest the Moon. However, on the opposite side of the Earth, furthest from the Moon we also see a high tide!

The Sun's gravitational pull also has an effect on Earth's tides. When the gravity of the Sun and Moon are pulling in the same direction, the tides are especially high. We call these 'Spring tides'. Spring tides happen at full Moon and at new Moon.

The image shows waves from the ocean crashing onto a sandy beach.
This work by Dan Gold is licensed under Creative Commons Zero v1.0 Universal
Waves coming towards the shore.
Diagram showing the Earth, Moon, and Sun alignment during spring and neap tides, illustrating how their positions affect tidal ranges during different phases of the Moon
This work by The Schools' Observatory (based on User:Club Yachting) is licensed under Creative Commons Attribution 4.0 International
Creation of the tides through the position of the Sun and Moon
 Why Do We Have Tides? 

 Stage 1: The strength of the Moon's gravitational pull varies across the Earth

Diagram showing the Earth and Moon with an arrow indicating the gravitational pull exerted by the Moon on the Earth. Distance is not to scale
This work by The Schools' Observatory is licensed under All rights reserved

 

The orange arrows in the diagram show the strength of the Moon's gravitational force across the Earth. The length of the arrows tells you how strong it is. The longer the arrow, the stronger the Moon's gravitational pull. Remember the Earth is constantly spinning on its axis. 

The arrows show the force is the strongest on the surface of the Earth facing towards the Moon. It is weaker at the centre of the Earth. It is the weakest at the surface of the Earth facing away from the Moon. 

 

Stage 2: The tidal force also varies across the Earth

Diagram showing the Earth and Moon with blue arrows indicating the difference in gravitational forces on the Earth caused by the Moon's pull. Distance is not to scale
This work by The Schools' Observatory is licensed under All rights reserved

 

It is the difference in the forces at the surface and centre of the Earth that causes the tides. We call this 'tidal force'. The blue arrows in the diagram show where there is a tidal force and the direction it is acting. You can see a tidal force on the surface of the Earth facing towards the Moon. But there is also a tidal force on the opposite side of the Earth, on the surface facing away the Moon.

 

Stage 3: Water flows towards the peaks of the tidal force

Diagram showing the Earth with two tidal bulges on opposite sides, caused by the Moon's gravitational force. Distance to the Moon is not to scale
This work by The Schools' Observatory is licensed under All rights reserved

 

The tidal force is not strong enough to change the solid rock in the Earth. But the tidal force is strong enough to move liquid water. The water in the Earth's oceans flows to form a bulge where the tidal force is strongest. The result is two bulges on opposite sides of the Earth. We get two tides each day because the Earth is always spinning. We get one high tide when our part of the Earth turns through the bulge nearest the Moon. And we get another high tide when our part of the Earth turns through the bulge furthest from the Moon.


Part of
Earth's Moon
Gravity
Life and Living things