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How Gravity, the Moon & the Sun Influence Tides

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  • 0:02 Tidal Forces & Tides
  • 1:09 Moon's Effects on Tides
  • 3:55 Sun's Effects on Tides
  • 5:41 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov
This lesson will define tides, the tidal force, neap tide, and spring tide. It will also talk about how the sun, moon, and Earth interact to produce tidal bulges.

Tidal Forces and Tides

Have you ever helped Mom or Dad water the garden when you were a little kid? Maybe you still do now. You probably had to lift a bucket of water in order to do so. Lifting a big bucket of water certainly takes strength. But can you imagine the forces necessary to lift an entire ocean many feet?

Those forces are not going to be produced by human power by any means. You'd need something even more powerful. The force that changes the levels of the oceans is known as a tidal force, which is technically the difference in the strength of gravity between two points on a body. The deformations of a body stemming from tidal forces are called tides.

If you've ever lived near the ocean, you've certainly experienced ocean tides stemming from tidal forces. But tidal forces are responsible for much more than just the changing levels of the oceans! They can cause mountains to rise and fall, distort galaxies, and pull a person apart as they fall into a black hole.

But for this lesson, we'll just focus on more familiar, Earth-based tides and how the sun, moon, and gravity interact to cause them.

The Moon's Effects on Tides

Gravity is a mutual force. Earth attracts the moon, and the moon will attract the Earth. This is something I'm sure you already knew. Similarly, it is Earth's gravity that you have to thank for your falling down after you trip. But did you know that the moon attracts your body as well? Why then, don't you 'fall' down towards the moon when you trip? Well, since the moon is much less massive than Earth and it's so far away, it attracts you with only a tiny force, one that you don't even notice.

However, because the oceans are so much larger than your body, they are definitely affected by the moon. There's so much more to be affected to begin with compared to tiny little you. Anyways, here's how these oceans are affected by the moon's gravitational pull in more detail.

the Earth and the moon

Let's look at the picture above. On the left is Earth and on the right in the moon. I know you know how magnets work, so let's pretend that the moon is like a magnet and the Earth is a little metal ball. What would happen? The ball would be attracted to the magnet. If the ball is closer to the magnet, it will be more strongly attracted to the magnet than it would be if it was farther away.

Keeping that very simple metaphor in mind, you can then understand that the side of the Earth that is nearest the moon is the side where the moon's gravity pulls a bit more strongly. Meaning, it pulls the oceans on the near side of Earth more strongly than on Earth's center because Earth's center is farther away from the moon. Thus, the oceans flow into a bulge of water on the side of the Earth that's facing the moon, just as shown below.

The gravitational pull of the moon causes tidal bulges
diagram depicting tidal bulges

At the same time, you know that the Earth's center is closer to the moon than the far side of Earth. Just like before, this means the moon pulls Earth away from the oceans, and they flow into a bulge on the far side of Earth. These bulges are known as tidal bulges.

Since there are two bulges on opposite sides of Earth, that means normally tides will rise and fall twice in 24 hours. But this is just a general rule of thumb because factors such as latitude, winds, and the shape of the shore can alter this.

While all this is happening, the moon pulls the solid parts of Earth into an elongated shape, not just the oceans. But you know that rock is more rigid than water and thus the oceans will deform much more pronouncedly than the solid mountains and planes of our planet. As a result, the maximum tidal distortion of the solid parts of Earth is only about 20 cm.

Compare this to the oceans. For example, in deeper parts of the ocean, far away from land, tidal bulges are about 3 feet in height. Closer to shore, in the Bay of Fundy, tides can be over 40 feet.

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