How do the Tides Work?

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  • Written By: Mary McMahon
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Tides are caused by the gravitational pull of the Sun and the Moon on the Earth's surface. While most people associate them specifically with the ocean, the entire planet is subject to tidal forces, as is the atmosphere, and in fact all celestial bodies are influenced by these forces. The large volume of water on the Earth has made their actions particularly notable and interesting. Discussions of the movements of water can be found in the most ancient writings of the world, suggesting that people have long been intrigued by the once seemingly mysterious rise and fall of water along the shoreline.

As the Earth rotates, it is pulled on by the Moon and Sun. Because the Moon is much closer, its pull is approximately twice as strong as the pull of the Sun, which explains why the tides are so closely linked with the lunar day. As the Moon moves around the Earth, it creates a bulge of water on the Earth's surface which follows its movements. A corresponding bulge appears on the opposite side of the Earth, thanks to the centrifugal forces generated by the Earth's rotation.


There are a number of different types of tides. So-called “spring tides” occur when the Sun, Moon, and Earth are lined up all in a row, in a situation called “syzygy” by astronomers. During syzygy, the Moon and Sun are both pulling in the same direction, creating an especially large bulge and a correspondingly larger tide. In neap tides, the Sun and Moon are at right angles, exerting forces pulling in opposite directions and creating a lower tide. The difference between the high and low points is called the tidal range, and things that are alternately exposed and covered by the water are said to be in the intertidal zone.

Tides vary radically around the world, depending on location and geographical features. As a general rule, they are less noticeable in the open ocean, and more pronounced along the shoreline, with places like the Caribbean and the Mediterranean having generally smaller tidal ranges, while the Canadian Bay of Fundy has an extremely large tidal range which can be as much as 50 feet (15 meters).

They also vary in frequency. In some locations, tides are diurnal, meaning that there is one high and one low tide every day. In other areas, they are semidiurnal, with two high and two low points. By observing tidal patterns in an area and keeping track of the movements of the Earth, Moon, and Sun, it is possible to create tide charts, predictions which list the time and height of various ones. These charts are extremely important for navigation, especially in areas with extreme tidal ranges.


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Post 13

What is the reason for the tidal range being less in the Caribbean?

Post 9

Can someone please explain to me how can two high tides and two low tides occur in one day?

Post 8

Thanks for the explanation, however I'm not convinced it is a fully accurate explanation. I may be wrong, though. If tides are created by the Moon (leave the Sun apart) gravity, and if the Moon is above the Atlantic, then the center of the Atlantic should have a bulge (I'm OK), but the other side of the Earth, the Pacific, should experiment a level decrease, not a smaller bulge (the Moon's gravity is not "pushing", but "pulling"). To me, the rotation of the Earth makes all oceans bulging (centrifugal force) around the equator, but the Moon attraction adds to this bulge on one side (where there is the low tide), and subtract from this bulge on the other side (the

bulge is smaller than the one created by the rotation).

I'm not either convinced by the difference in the Moon gravity field being so different between the opposite sides of the Earth (which in the article explain differences of bulges). Remember the Moon is at 384,000 km from the Earth, and the Earth diameter is only 13,000 km (3 percent). Anyone agreeing?

Post 7

I was under the impression that it had more to do with how the horizontal gravity vector actually pushes the water horizontally to where the furtherest and closest points to the moon are? Thus the water bulges due to being pushed sideways along the Earth to where the both high tides are. Thank you for the valuable information.

Post 4

@ Fiorite- The tidal forces of gravity that you wrote about are due to the properties of gravitational waves. The tidal effects of gravity are controlled by the properties of vertical and horizontal separation.

Gravitational waves pull to the center of an object. This causes objects vertically aligned to the axis of the force of gravity to pull away from each other. This is because the object closest to a gravitational force will accelerate towards its center slightly faster than the object farther away. The ocean closest to the moon accelerates to the moon faster than the ocean on the far side of the earth.

The horizontal separation property of the tidal effect causes two objects that are spaced horizontally

to decrease their horizontal separation as they move towards the gravitational force. This is the phenomenon of tidal contraction. The water pulls horizontally across the earth's surface toward the center of the gravitational force. This causes the ebb and flow of the tides.

Post 3

@ Anon77673- Most tides are semidiurnal because of the physics of gravity and the relationship between the earth and its oceans. When the moon is pulling on one side of the earth it pulls both the earth and everything on it, but the earth is somewhat elastic.

The tidal forces of gravity cause the side of the earth closest to the moon to accelerate faster than the side of the earth farthest from the moon. This is because gravity is not a uniform force, becoming weaker the farther the objects are from each other.

The far side of the earth moves slower in relation to the center side of the earth causing an opposite tidal bulge on an ocean centered

on the far side. This will create a corresponding tide on the opposite side of the earth.

I'll give an example. The Atlantic Ocean is centered below the moon on one side of the earth. This creates a low tide because the center of the ocean is bulged causing the ocean to move away from the shore. The Pacific Ocean will bulge in the center on the opposite side of the earth, creating a corresponding tide. If the edge of an ocean is centered under the moons gravitational force then a high tide forms.

Post 2

I don't get the two tides a day explanation. The Moon (and to a lesser extent the sun) pull a bulge of water creating a tide that follows them around. How does "centrifugal force" create a bulge 180 degrees out from the gravitational pull?

Surely this force pulls in proportion to the mass of the body of water and would exaggerate the gravitational pull and minimize any tendency to have an opposing bulge to the that caused by the moon's gravity?

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