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.
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.
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.
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.