The tides

0:00- [Narrator] Have you ever been to the beach? Maybe you spent hours building the perfect sand castle only to watch in horror as the ocean slowly crept closer and washed it away. Or maybe you were relaxing on a towel and after a while, notice that the water had pulled way back.

0:18What's going on over here? That rise and fall of the ocean? That's called the tide. And this cycle repeats every day. It takes roughly 12 hours to go from one high tide to the next. But what causes it? And why do we see tides in the ocean, but not in most lakes or ponds or swimming poles?

0:42The answer might surprise you. It's not the wind or the waves, it's actually the moon. But how? How does the moon cause tides on earth from all the way up there? Let's find out. To understand tides, we need to first understand gravity. Gravity is a force of attraction between any two objects with mass, like the earth and the moon, for example.

1:07And the amazing thing is gravity doesn't require objects to touch. Masses can attract each other across space. Now, there are two key things that affect how strong this gravitational pull is. First, the mass. The more mass something has, the stronger it's gravitational pull.

1:25So you and your friend actually do attract each other with gravity. But since both of you have pretty tiny masses, the pull is so small, it's basically negligible. You never notice it. To feel a noticeable gravitational pull, at least one of the objects need to be very, very massive.

1:44And that's why we can feel the pull of the earth, because the earth is enormously massive. It's the earth's gravity that keeps you, me, the oceans and even the atmosphere close to the surface. It's also what keeps the moon in orbit around the earth. But the second thing is distance.

2:04The farther apart the two objects are, the weaker the gravitational pull between them. So if the earth and moon were farther apart, for example, their gravitational attraction would be weaker. The closer they are, the stronger the pull. But remember, earth and moon both pull on each other with exactly the same amount of force.

2:24And it's the moon's pull on the earth that causes the tides on the earth. But how exactly? Even though this image is not drawn to scale, you can still see that not all parts of the earth are the same distance from the moon. Some parts are closer and some parts are farther away.

2:45So the force of gravity from the moon is stronger on the near side and weaker on the far side. Now, to help us picture the effect of this, let's place four identical rocks on different parts of the earth. Now, this rock over here, which is closest to the moon, it'll experience the strongest force of gravity.

3:05And these two rocks are slightly farther away from the moon. And so, they experience slightly weaker force of gravity. And these forces are also in different direction. They're slightly angled, as you can see. And this rock, which is the farthest, experiences the weakest force.

3:20So what's the effect of this? Well, let's just concentrate it on the rocks for now. When the moon pulls on them, look, they don't get pulled equally. Because of this, the distance between the rocks changes. It's as if it got stretched a little along this direction and squeezed along this one.

3:45Why did this happen? It's not just because the moon is pulling on the rocks. It's because the strength and the direction of that pull varies with distance. I mean, if the moon pulled equally on every part in exactly the same direction, then look, everything would move together.

4:02No stretching, no squeezing. But since the pull on these two rocks are different, they get pulled apart. And since the pull on these two rocks are slightly angled inwards, they get pulled together. So from the perspective of the rocks, they will experience a stretching and a squeezing force.

4:24This is what we call the tidal force. Now, if we bring back the earth, the earth will also experience the same stretching and squeezing tidal forces. Now, of course, we only showed the forces on four points, but we could draw them at every location. Just imagine placing those rocks everywhere.

4:48Now, these forces are gonna get very complicated, but you can kind of imagine as we go from here to here, the tidal force will start pointing more sidewards. And so, this is what it would look like. This is the tidal force pattern across the planet. Now, looking at this, we might wrongly think that tidal forces can lift things off the surface over here, making them levitate, right?

5:17But of course, that doesn't happen because these tidal forces are very, very tiny. And the earth's own gravity, oh, it's much, much stronger. So the tidal forces have absolutely no effect on most things that you see around you, except for the ocean, because the ocean is huge and moveable, and these tiny forces add up.

5:44So because the water spreads over such a large area, the tidal forces can add up and create a noticeable effect. What's the effect? Well, on the sides over here, the water gets squeezed inwards, and in the middle, it gets pushed upwards. Kind of like squeezing a water balloon.

6:07When you press the sides, the water bulges up the middle. So as a result of this squeezing, the water starts piling up on two places, one on the side that's facing the moon, and one on the side that's farthest away from the moon. We call this the tidal bulge. It's greatly exaggerated over here, okay?

6:30And so, these are the locations where we'll end up with high tide and the areas being squeezed, those experience low tides. But remember, the earth is spinning, so you can imagine the solid earth rotating underneath those bulging ocean layers. Now, right now we are looking at this from the side, but let's switch to a 3D model of the earth to get a top down view.

6:55There we go. If you now, say, focus on a person that's standing over here, let's see what he experiences in one full rotation. Okay, here we go. Low tide, then about six hours later, high tide, another is six hours later, low tide, six hours later, high tide. And there you have it.

7:16We can look at the same thing from this perspective. Here we go, low tide, high tide. And then again, a low tide, and then again, a high tide. And there you have it. That's why in a single day, you experience about two high tides and too low tides. So now we can go back to our original question.

7:39Why do we see tides in the ocean, but not in lakes or swimming pools? Well, it's because lakes and pools are too small. Even though the tidal forces do exist, they're not strong enough to make a noticeable difference in small bodies of water because they have smaller area.

7:54But there are some large lakes and rivers that do experience small tides. It all depends on their size and shape and how they're connected to the oceans. And so, yeah, it's a little more complicated, but when it comes to ocean, we are talking about like a giant planet-sized body, and that's why the tidal forces all add up and we can see its effects.

8:15Okay, so we've seen how the moon causes the tides, but what about the sun? Well, the sun is way more massive than the moon, but it's also much, much farther away compared to the moon. Now, this drawing is not to scale, but if I try to do that, this is what it will look like.

8:33I mean, even this is not to scale, because if this is how big the sun is, we wouldn't be able to see the earth or the moon at all. But it kind of gives you the sense of how far the sun is. So because the sun is so far away, its tidal force on the earth is much weaker compared to the moon.

8:50That's why the tides on earth are mostly driven by the moon. But when the sun, earth and the moon are in a straight line like this, the sun's tidal effects adds to the moons. They both squeeze from here, increasing the tidal bulge, making high tides higher and low tides lower.

9:12We call this a spring tide. And this doesn't have anything to do with spring season, by the way, the word spring here means tides are springing forth, stretching farther than usual. This happens twice a month, once during the full moon and once during the new moon.

9:31Okay, now, what do you think is gonna happen when the moon is somewhere over here, when the earth sun line is perpendicular to the moon earth line? Well, remember, the sun's tidal forces are still squeezing from here, but now the moon's tidal forces will squeeze from here.

9:46And so, look, their effects now start to partly cancel out. That leads to lower high tides and higher low tides. So everything is more gentle now. These are called neap tides. The word neap comes from an old English word meaning neaping, like the extremes are being neaped or trimmed down.

10:08Neap tides also happen twice a month during the first and third quarter moons. So the next time you're at a beach, maybe enjoying a peaceful sunset, remember the moon is out there shaping the shoreline and setting the gentle rhythm of the tides.