What Causes the Oceans’ Tides?

If you have ever spent time at the beach, you probably noticed that the water rises slowly on the shore for six hours, then slowly recedes, or falls back, for another six hours. This movement, which happens twice a day, is called tides.

Tides are caused by the pull of the moon when it is directly overhead. That gravity is actually pulling the water towards the moon. In doing that, it causes the water to rise on the surface of the earth.

Then why, you may wonder, do we have high tides when the sun is out as well? What is actually happening is that while the moon’s gravity is pulling the water on the opposite side of the earth, the side that is having night while you are having day, that same gravity is pulling the earth away from the water on the side on which you are standing.

At the shore, tides may rise from 6 to 8 feet, but in the middle of the ocean, the rise is much less, and so it is barely noticed.

Tides are useful to man. They keep harbors and seaports clean by carrying waste material’ from the land out to sea, where it sinks to the bottom. Tides also make harbor entrances deep enough for fishing vessels and ocean liners to sail through.

Because of the shape, size, and depth of the ocean, there are a few places on earth, parts of Alaska and parts of countries on the Gulf of Mexico, that have only one high and low tide a day!

About Karen Hill

Karen Hill is a freelance writer, editor, and columnist. Born in New York, her work has appeared in the Examiner, Yahoo News, Buzzfeed, among others.

1 thought on “What Causes the Oceans’ Tides?”

  1. I-Search Paper.

    I have chosen to write my paper on Tides, honestly I know absolutely nothing about them. When choosing my topic for this paper I made the foolish mistake of picking a topic I’m not particularly interested in. I hope that as I learn more I become more interested. I am curious to find out the basics, what causes them, when and where they happen, and also what effect they have on the world.

    I know a few very simple facts about tides, I know they occur in the ocean, and also gravity and the moon play some part in the process, but im not really sure why or how. I think they effect the world more then I know. I think its unfortunate I don’t know more about tides, but I guess since I’ve never lived by the ocean I have never felt it necessary to learn about them. I imagine they effect the ocean, like the animals, beaches, and maybe even the weather im not really sure though, so I hope I can find more out about that.

    I have done some research and learned quite a bit about tides, I learned that Gravity is the key to the Earth’s rising and falling tides. The combined gravitational effects of the Sun and the Moon constantly pull the world’s oceans in different directions and create tidal effects. But there are several other things that complicate this basic process. The Earth’s rotation, the tilt of its axis and the gravitational pull given off by the Sun and Moon that affects Earth’s atmosphere. These forces together conspire to make our planet’s oceans into a battleground. These forces tug the oceans this way and that way around the globe, and this creates high tides and low tides.

    The Moon’s gravity stretches the earth into an oval but there isn’t much of an effect its so tiny that the solid parts of the planet are only distorted by a little more than eight inches. The effect on the oceans is more noticeable. At the point on the Earth directly beneath the Moon, the ocean is tugged into a bulge of high water. At the same time, a second tidal bulge is formed on the opposite side of the planet. This is kind of happens because of the centrifugal force (“the outward force on a body moving in a curved path around another body”)
    created by the Moon and Earth’s combined rotation around their common center of mass, a point called the bar center.

    Because the Earth spins on its axis once every 24 hours, the two bulges go around the planet in waves, creating two high tides per day at every point on the globe. But the twice daily cycle is complicated by the fact that the Earth is tilted, which puts the Moon alternately to the north and south of the equator. This creates a little differences between the two tide everyday.
    another complication is added by the Sun, whose gavational pull on the earth also affects the tides. The tidal force of the Sun and Moon together is almost a third more than that of the Moon alone, At the new and full moons, when the two bodies are in line, they combine to create extra high spring tides. When the Moon is in its first and last quarters, the Sun is at right angles to it, and their gravitational pulls work against each other to create extra low neap tides.

    The definition of a neap tide is, “A tide that occurs when the difference between high and low tide is least; the lowest level of high tide. Neap tide comes twice a month, in the first and third quarters of the moon.” And the definition of a spring tide is, “A tide in which the difference between high and low tide is the greatest. Spring tides occur when the Moon is either new or full, and the Sun, the Moon, and the Earth are aligned. When this is the case, their collective gravitational pull on the Earth’s water is strengthened.”

    I must say I have learned a lot, but unfortunately I am no more interested in tides then I was when I started this paper. Almost everything I learned was for lack of a better word boring. It was basically the science of tides and that’s not really what I was looking for. I wanted to find out more about how tides affect the world and the ocean but during my search I found little information on those subjects and more about what they are and how they happen. But the good news is I can confidently tell you how a tide occurs and where. Honestly I wish I would of picked a subject I was more interested in then this would of probably been a more fun paper to write, research, and for you read.

    Last Updated: May 15, 2001
    Website Author and Curator: Richard Ray

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