Why Does The Earth Spin?

In a previous video, we did the math for just how fast you're moving at the Earth's equator as it spins in space. This time around, we explain exactly "why" the Earth is spinning in the first place. It's all about inertia and the conservation of angular momentum.

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In a classic episode of this video series, I did the calculations for how fast the Earth is spinning.

We know the Earth is rotating, but why? Why is it spinning?

Why is everything in the Solar System spinning? And why is it mostly all spinning in the same direction?

It can't be a coincidence.

Look down on the Earth from above, and you'd see that it's turning in a counter-clockwise direction.

Same with the Sun, Mars and most of the planets.

4.54 billion years ago, our Solar System formed within a cloud of hydrogen not unlike the Orion Nebula, or the Eagle Nebula, with its awesome pillars of creation.

Then, it took some kick, like from the shockwave from a nearby supernova, and this set a region of the cold gas falling inward through its mutual gravity.

As it collapsed, the cloud began to spin.

But why?

It's the conservation of angular momentum.

Think about the individual atoms in the cloud of hydrogen. Each particle has its own momentum as it drifts through the void.

As these atoms glom onto one another with gravity, they need to average out their momentum. It might be possible to average out perfectly to zero, but it's really really unlikely.

Which means, there will be some left over.

Like a figure skater pulling in her arms to spin more rapidly, the collapsing proto-Solar System with its averaged out particle momentum began to spin faster and faster.

This is the conservation of angular momentum at work.

As the Solar System spun more rapidly, it flattened out into a disk with a bulge in the middle.

We see this same structure throughout the Universe: the shape of galaxies, around rapidly spinning black holes, and we even see it in pizza restaurants.

The Sun formed from the bulge at the center of this disk, and the planets formed further out.

They inherited their rotation from the overall movement of the Solar System itself.

Over the course of a few hundred million years, all of the material in the Solar System gathered together into planets, asteroids, moons and comets.

Then the powerful radiation and solar winds from the young Sun cleared out everything that was left over.

Without any unbalanced forces acting on them, the inertia of the Sun and the planets have kept them spinning for billions of years.

And they'll continue to do so until they collide with some object, billions or even trillions of years in the future.

So are you still wondering, why does the Earth spin?

The Earth spins because it formed in the accretion disk of a cloud of hydrogen that collapsed down from mutual gravity and needed to conserve its angular momentum. It continues to spin because of inertia.

The reason it's all the same direction is because they all formed together in the same Solar Nebula, billions of years ago.

[frasercain]

Thanks. That's why I said "almost" everything in the Solar System has the same rotation. :-)

[frasercain]

Well, everything's following the laws of physics, we just don't fully understand them yet. :-) But if there were any objects that really fascinated you, I'd be happy to do a show on them.

[PanamaRose]

Fraser, thank you for making these videos!  I'm with an amateur astronomy club that does lots of public outreach, and these would be great to show, as well as promoting them.

[mattr]

I discovered this one day while peeing in a toilet. Spin Is created from objects falling onto other objects, off center or "angular".

[frasercain]

It's not creating energy, it's converting mass into energy. Remember Einstein's famous e=mc2 equation? That tells you how much energy is contained in mass.

[alexair]

Depends on your perspective, look from the bottom at South Pole the earth is rotating clockwise

[abhineetsharma]

Nicely Explained

Thanks for your video

[amaliaantonopoulou]

Congratulations, I'm lucky I found this channel, it answered me some fundamental questions

[redbeard]

I love your videos Fraser! Entertaining and educational! That being said I am having a wonderful time reading the comments below.

[has]

Okay, Fraser. I have a question, , , what will happen if the the earth spins FASTER or SLOWER, , , eg 23 hours for one day or 25 hours for one day, , Please explain, , ,

[JayTuberTaken]

I thought you can't create nor destroy energy. So how is fusing atoms create energy?

[HaniTiby]

Thanks for your videos.
I have a question, can the earth axis tilt more or even flip upside down ?

[avenuePad]

Great video! But what about Venus? It has a retrograde rotation?

Anyway, you have a new subscriber. :)

[petrlazar]

So it's because of inertia? But that means that one day planets will just stop spin, doesn't it?

[frasercain]

No, the atmosphere is spinning with the Earth. It is slowing down because of its interactions with the Moon. The Moon is slowly drifting away, and the Earth's rotational speed is slowing to compensate for this.

[NOLAMarathon]

I think a short answer -- and not a particularly good one -- is that every entity in the cosmos is changing shape. But typically, objects change on such a scale and over such long time periods that it's not always easy to measure changes in shape. Changes in shape are often inferred from observations of similar, but non-identical, entities.

[gravityspinemassasjeogtera]

If all planets revolve around the sun in one direction. Is this direction cloak wise or anti cloak wise?

[NitinShenoy]

Why does the VENUS spins in opposite direction ?

[hnlkitup]

Thank for the response. We accept that Venus has a retrograde rotation, but do we know why?

[lukaskywalker]

So what started the angular momentum? Wasn’t clearly explained in this video