So tides are simultaneously very simple and very complicated.

That's a good question.

What is what causes the tides on Earth?

It's like, What?

Why is the sky blue?

It's one of those questions that kids ask and often stumps.

Grownups, you might naively think, Well, it's just the force of gravity from the moon pulling on the earth.

The earth surface is mostly water, and so that water gets pulled to one side of the earth, and that causes a high tide.

If that were the case, though, we'd only have a high tide once a day.

We actually have it twice a day every 12 hours.

Eso There's something slightly more complicated going on, and in physics, we talk about tidal forces.

This is a generic term on a tidal force is not the main gravitational force, but it's the differential force.

It's caused by the fact that the pull of the moon on this side of the earth it's stronger than the pull of the moon on that side of the Earth.

Which means if you just think about the Earth sitting here with the oceans around on the moon.

Over here, this side gets pull hard this way.

The middle, the earth itself gets pulled a little less.

And then the stuff on the far side, the oceans on the far side gets pulled even less.

And so it's the difference in the force of gravity felt in this position versus that position that leads to the tides.

So the net effect of all where is that?

That sort of stretches the oceans out.

And that's why you end up with two tied two high tides because the near side is getting pulled towards the moon on the far side.

Exactly.

Getting pulled less than the earth years.

So it kind of gets left behind.

We've got the moon over here.

We've got the earth over here.

This side of the earth is being pulled quite strongly.

The middle being pulled a little bit on the far side is being pulled.

Not so much.

So because these air vector forces we can actually subtract one from the other.

And when we do that, we got something that looks like this.

We get a net force in that direction on this side of the Earth, which is a side with the moon, and we get a net force of remaining force in the opposite direction on the other side.

And it even happens at the top and bottom of year that there's a net force inwards as well.

And because the earth is most, the surface of the earth is mostly water.

What happens to the Earth is that it gets slightly squashed and slightly stretched on both sides.

The complicated part is that isn't actually what happens with tides that if you sew a prediction of that, for example, is that would tell you that actually high tidal to occur when the moon's directly overhead because that's when the moon's pulling the ocean that way, or when the moons around the far side of the earth.

If you actually look a tide tables, you find that's not when high tide occurs.

Typically, it could be several hours out on.

That's because the oceans don't instantly respond to this.

They don't actually instantly form the shape because they have to kind of slosh around the earth and rearrange themselves in order to actually produce the ties themselves.

So the reality is that tides on the earth or a complicated mixture of the driving physics, which is the moon kind of tugging the the oceans out.

And then the way the oceans actually respond, which is the complicated part, which is all to do with the topology around them, where the coastlines are, what the sea floors like, how deep the oceans are and so on.

I mean, the moon's far away, which means that the pull of gravity of the moon is actually quite weak, which means that the difference between the pull of gravity on one side and on the other side is even weaker.

So actually, tidal forces a very weak force.

But it's enough.

It's enough to actually distort the ocean if you think about it for a moment.

In fact, the pull of gravity of the sun is a lot stronger than the pull of gravity of the moon.

Because although the moon's a lot closer to us than the sun, the sun is much, much, much more massive.

And so actually, you know, that's the reason why we orbit around the sun.

We don't orbit around the moon.

So, in fact, the gravitational influence over all of the sun is far more important than the moon.

But because the sun is so far away, that means the difference in gravity between one side of the earth.

The other side of the earth is very small because actually, the difference in distance is a tiny fraction of the total distance.

The sun.

Where is the moon?

Because it's a lot closer.

The difference in distance to the moon from one side of the Earth and the other side of the earth is a much bigger factor, and that kind of wins out.

Although the gravitational influence of the moon is much weaker than the sun, the title influence of the moon is actually stronger than some well again in the solar system.

You get really interesting configurations going on when you have large planets and not just one, but lots of moon.

So course Jupiter is the largest planet in the solar system.

It has many, many moons, some of which are almost his biggest planets themselves.

The moon closest to Jupiter orbiting closest to Jupiter, is called Io, and that's a really interesting object.

It's most geologically active body in the solar system.

It's got volcanoes going off its surface is constantly being renewed because of all this volcanic activity.

All these earthquakes, moon quakes that are happening.

And that's because it's being tugged in all different directions in one direction by the huge, massive Jupiter right next door, but also by these other major what we call Galilean moons.

Eames, also in orbit around Jupiter, are pulling in another direction, so its insides are constantly being squashed and distorted.

And it's that geological heating that's driving this Titley induced heating that's driving all of geological activity on its surface in my own particular field, mostly is studying Galaxies on dhe.

Tides have a massive influence on Galaxies because Galaxies don't exist in isolation, they often exist in clusters in groups.

And so once in a while, one galaxy comes quite close to another, and then the gravitational influence off one galaxy on the other can.

Actually, the title effects can then stretch that galaxy out.

There's a very famous pair of Galaxies called the antennae, which are in a very close interaction with one another and the title effects, and have pulled out these long, beautiful title tales from these things.

The leading title tales form sort of a bridge of material, which is kind of obscured by the fact that they're both colliding.

The trailing tales in both cases are being flung off quite spectacularly behind each of the Galaxies.

So what you're seeing in the antenna Galaxies you've had these two Galaxies that came close together, and then so this galaxy fear was the title influence off this galaxy.

So it feels the gravitational pull.

But the gravitational pull on its backside is weaker than the gravitational pull on its front side, which means that the stars at the back kind of get left behind.

So it got stretched out into this long title tail.

And then conversely, exactly the same thing's happening to the other galaxy.

And it's getting stretched out by its companion to because tides just basically the difference in pull of gravity being on one side of the thing.

On the other side of the thing.

Wherever you've got gravitational forces influencing extended objects, then you have tides.

And so, for example, of a famous example in astronomy is if you got a black hole at the pull of gravity of a black hole is incredibly intense, which means that the tidal forces, the difference in pull between one side of something and the other could become very intensive.

A cz Well, if you're at the event horizon, you're what we call the edge of a black hole.

That's a small black hole.

You're very, very close into where most of the masses and that means a small change in your position.

We'll induce enormous changes in the gravitational force that you feel.

So if I'm coming up to a black hole and I'm falling in feet first, my feet will feel an enormous different force compared to my head, simply because the small difference between them and what would happen if I were to be so unfortunate as to cross the event horizon of a fairly small black hole is that my feet would be stretched far away from my head.

And the term for that is called spaghetti ification.

So you're stretched into spaghetti because of the differential force of gravity.