# Why does the world spin?

1. Apr 28, 2013

### mrblint

What makes the world spin?

2. Apr 28, 2013

### Bandersnatch

Hello, mrblint.

Why won't you tell us what do you think it is, so we know where to start explaining?

3. Apr 28, 2013

### mrblint

I think that as the world travels around the sun in an elliptical orbit, there is a force pulling away from the sun (the centrifugal force) and also the sun is also pulling the mass of the earth towards itself (the sun being the more massive body). As the earth moves in the elliptical orbit, the slightly changing orientation of the pull results in spin of the planet on its axis. That's what comes to mind just thinking about it. But I'm wondering now if the earth spins at different speeds as it moves through the elliptical orbit.

4. Apr 28, 2013

### Feodalherren

Yes. The speed increases as earth gets closer to the Sun. The orbit is only very slightly elliptical though so it's not a big change.

5. Apr 28, 2013

### Mordred

The reason the world spins on its axis has to do with the conservation of angular momentum.

As the dust gathered that formed our planet the dust collisions imparted inertia. As the diameter decreased the angular momentum increases much like a spinning figure skater.
Nearby large gravitational bodies can affect the spin in a fashion called "tidally locked"

6. Apr 28, 2013

### rewtnode

Correct. May I add that it would be extremely unlikely to have a huge primordial dust cloud with a very small or zero angular momentum. So the rotation of the entire solar system was pretty much there since it formed. Otherwise one would expect the whole thing collapses in one blob point without rotation. As far as I know, all planets move in the same direction in the disc around the sun and the sun itself rotates in that direction. There are, as far as I know, only a few captured moons for the big planets that rotate in other ways, and there is just one "misbehaved" planet, Uranus, where the axis of rotation is almost parallel to the planetary disk. (Well, Uranus also got a weird name, at least in English ;-).

I still wonder though if there are any planetary systems out there where all this isn't the case, and where the planets are not just in one disk but varying planes of their orbits - and then why that could be?

7. Apr 29, 2013

### Feodalherren

Planets that have been thrown out of their original systems and hurtle through space and end up in other systems could be such cases. Among the gazilions of stars there surely must be numerous examples of such planets.

8. Apr 30, 2013

### HallsofIvy

I thought love made the world go around!

9. Apr 30, 2013

### Mordred

if its any consolation it will still make your head spin lol

10. Apr 30, 2013

### Bandersnatch

11. Apr 30, 2013

### Mordred

Roflmao now there is a correlation I never would have thought of lol

12. May 18, 2013

### tfr000

Could be, but it probably wouldn't last long. The mutual interactions between the planets in all of those strange orbits would tend to either kick some of them out of the system or settle them into one plane.

13. May 18, 2013

### HallsofIvy

Would you really consider it "natural" if the world happened to have exactly 0 angular momentum?