Why does the Earth spin?

My 4-year old just surprised me with this question and I realized that not only I didn’t have an age appropriate answer, but that my adult answer was not really satisfactory.

I am thinking that when objects form from gravity pulling stuff together, most matter doesn’t fall straight to the center of mass, but overshoot and make an orbit and all that rotational momentum is somehow conserved in the final object. Still, this answer vague at best, if not flat out wrong.

Wouldn’t the momentum of stuff approaching from all different directions more or less cancel out?

The answer my kid got was along the lines of all the stars, planets spin but I don’t know why they do. He didn’t look satisfied. Here is hoping I will have a better answer for when I pick him up from school.

It was an eddy in the accretion of matter. You can see similar in debris in a stream.

Because the primordial gas cloud had angular momentum. All of that spin had to go somewhere.

In a nutshell: The planets formed from spinning gases and the planet travels through vacuum (nothing to slow down the spin) and thus the spin does not stop.

Well at least that is my attempt at a version for a 4 year old. I let a physics expert give you the real science.

My favorite answer: Because if it didn’t, you wouldn’t be here to ask the question.

In response to “Why is our orbit circular?” my eighth grade math teacher said “Cuz it had to choose between being cool or being square.” Best. Pun. Evah.

An age appropriate answer would be great, but I am afraid it might be impossible. This is one of the moments when I wish I were deeply religious and could get away with “because God wanted it to”.

Still, on the adult side, I understand the proximate answer of “because it formed from spinning stuff”. What I would like is an ultimate answer to “why was that spinning stuff spinning on the first place?”

I think your vague answer is correct, and the reason that everything doesn’t average out to zero is that, on all scales where mass is coalescing, there were once nonuniformities in mass and velocity. No matter how small these are, if they are nonzero then some degree of coalescence will turn them into significant rotational velocities.

In other words, it is the concentrating effect of coalescence that turned seemingly insignificant nonuniformity of momentum into noticeable rotations.

This cornell site will help.

I thought it was the collision that we had by that other planet (name?). The result was a big chunk that came off and formed the moon. It is also the reason for our tilted axis. i.e. The equator does not line up with the ecliptic.

Sorry, I just got up and am too undercaffeinated for cites.

[velikovski] That would be Venus. [/velikovski]

Love makes the world go 'round, y’all.


No, this isn’t it because all the other planets rotate at various rates and have various tilted axes.

Here’s why. The gas cloud that made up the proto-solar system wasn’t moving uniformly. Some bits were moving this way, some were moving that way. As matter condensed into stars and planets, the bits that condensed were moving and spinning and therefore the bodies they condensed into had the sum of their movement and spin.

Or to think about it another way…wouldn’t it be weird if a planet DIDN’T spin? There would have to be some sort of mechanism to make it stop. Even bodies that are tidally locked with a primary body spin…they spin on their axis once every rotation about the primary.

The simple answer is: “Because it hasn’t stopped yet”, and that isn’t as trite as it sounds.

A tiny amount of angular momentum in a big cloud of dust turns into a relatively fast rate of spin as it contracts and coalesces (think ice-skater bringing her arms in, in that old favourite physics analogy). And there’s really nothing much to stop the planet spinning, apart from tidal forces and (presumably) friction from the solar wind.

Great link! Thanks.

So it is the case that a little non-uniformity got somehow amplified into a big spin. Right? After that I can easily see how all the processes result in spinning galaxies, systems, bodies. It is just the very beginning that is tripping me.

Am I right to assume that a spinning cloud will force bodies approaching it into an orbit and that that will increase its spin? Or am I inventing perpetual motion machines?

:smiley: you are right !

My attempt at a kid’s version:
The Earth was formed from a bunch of debris orbiting the Sun. All the little bits also tugged on each other with their own gravity. Now, when one bit tugs on another, the paths they follow both curve; like when you throw a baseball, the Earth tugs down on it. If the two bits are close enough, they will curve around each other over and over until they join together. At that point the curving becomes spin.
That combined bit attracts other bits, and the same thing happens. The little spinning rock that kept adding other little bits over and over eventually became the Earth. Since there isn’t anything out there to stop it, it’s still spinning.

More or less, yes. The remainder of what didn’t exactly cancel is what gives us our spin.

I can see that you’re thinking of the Sun already existing with a bunch of stuff around it randomly coalescing, but instead think of the whole Solar System coalescing at the same time. This huge amount of material is drawn together, and settles into a disc kind of configuration, whose rotation is the leftover angular momentum that didn’t exactly cancel out.

The Sun coalesces in the center, and that already-spinning disc of material coalesces into all the planets. Since the disc they were formed of was already spinning, the planets that get made spin as well, in the same direction. All the planets were formed spinning the same way, but one now spins upside-down due to a serious collision.

Because spinning is so much cooler than not spinning.

(with apologies to Stargate: SG1 Episode 200)