Hello,

I'm just getting into astronomy. I have tried to google this question many times but I cannot find an answer. I need to know why gravity makes things spin. I can understand the description of looking at space like a flat membrane. Things with mass, like planets, are heavy. And if you put one on this flat membrane it sinks down and bends the membrane down a little.
If something gets close enough to this depression, it will slide down this slope, and collide. Essentially gravity.

So I can understand this, gravity pulls things together, because the heavier object will have a deeper depression in space, and lighter objects will slide twoard it.

But this doesnt explain why it makes things spin. How does gravity make our planet spin on its axis? How does gravity make the planets spin around the sun? Why does the moon spin around the earth and not just collide with it?

It doesnt make sense!! Someone help please!

Wallace
Try reading http://en.wikipedia.org/wiki/Orbit" [Broken] Wikipedia entry on orbits. The very short answer is that it is not some magical property of gravity per se that causes many things in nature to rotate, spin or orbit but just basic Newtonian physics. Have a read of that entry then come back with any more specific questions you have.

Good luck!

Last edited by a moderator:
Ok, I have read your website. From there I under stand how objects orbit. By looking at this picture, I can understand how an object fired out of this cannon may or may not orbit depending on how fast it is fired. If it is fired with a low velocity, it will end up at point A. With a little more velocity it will end up at point B, and with too much, it will escape the earths gravity, and continue out in space as in point E. You must have the perfect velocity to keep something in a circular orbit.

This may explain why the moon stays in orbit around the earth, and how the planets stay in orbit around the sun.

1) But it doesnt explain why the earth spins on its axis. Also,

2) It doesnt explain what caused objects to move at this perfect velocity the first place. I'm pictureing a huge cloud of dust. I can see that all over the cloud, pieces clumping together. Eventually, large clumps would form spread through the cloud. Nothing would be in a perfect disk. And eventually, the large clumps would suck in everything that was close enough, and you would just have large clumps of matter, spread at large distances from each other, all through out this cloud. They would be sitting still, not close enough to any of the other large clumps to effect each other.

So in order for our solar system to form. Gravity would have to get these large clumps moving again.

Can someone explain 1 and 2?

#### Attachments

• 300px-Newton_Cannon_svg.png
12.3 KB · Views: 627
russ_watters
Mentor
The earth, moon, and planets formed from clouds of gas and dust that were non-uniform and randomly moving. When they collapsed, these motions and non-uniformities induced rotations. Only a perfectly symmetrical, perfectly still cloud of debris could collapse into a non-rotating object.

Ok, If they are all randomly moving, why in the end do they all move the same direction?

Would it make sense to have some of our planets orbit one way, and others in a different way?

Wallace
Try http://en.wikipedia.org/wiki/Solar-system#Formation" wiki page on solar system formation. The key concept here is angular momentum.

Sorry to be handballing your questions to a website, but since your question is so fundamental it requires a somewhat lengthier explanation than a bulletin board post would allow. The wiki page explains this pretty well I think, but again anything your not sure of after reading that post away

Last edited by a moderator:
russ_watters
Mentor
Ok, If they are all randomly moving, why in the end do they all move the same direction?

Would it make sense to have some of our planets orbit one way, and others in a different way?
Add all that random motion together and there is a net motion in one direction.

Chronos
Gold Member
Affirming Wallace and Russ, it's all about conservation of angular momentum. Consider what happens as a star forms from a collapsing gas cloud [forget about planets for now]. Lets also assume the initial gas cloud is not 'rotating'. As the particles are drawn toward the center of gravity, they collide. This deflects their course away from the perfectly straight line they were following toward the center of gravity. If they acquire insufficient velocity to escape the gravitational well of the gas cloud, what path do they take? - a spiral, of course. As the gas cloud densifies, the spirals flatten out. It gets increasingly difficult to approach the center of gravity because collisions become more frequent, hence the paths grow increasingly circular. Please note it is virtually impossible for all the collisions to cancel out. While at first the paths will be purely random, the tiniest of imbalances [like the gravity of the nearest star] will impart a preferred direction of travel.

I like Chronos's explination. Conservation of angular momentum is THE main factor. A byproduct of inertia and velocity. You could also refer to the classic example of sitting in a spinning chair with your arms extended, then pulling your arms inwards to your chest... your rotation speed increases.

A good example of this is the moon slowly drifting away from the Earth. The moon causes fluctuations of the tides which, by friction, is slightly causing the Earths rotation to slow. That rotation slowdown causes the moon to increase in distance... where an increase in rotational speed would cause the moon to drift closer.

Think of the moon as your arms in that example. Another good one would be using a star as an example. Say you have an red giant star that is slowly rotating. When that giant runs out of fuel and can no longer push outward to resist the pull of gravity... it collapses in to a fast spinning white dwarf.

My astronomy professor taught me that the tendency of bodies to spin in space is an unexplained phenomenon, and he said he attended an exhibit about this at NASA's Glenn Research Center. The links recommended in this forum sidestep the question being asked here, and only Chronos has truly offered an explanation. The conservation of angular momentum does not, however, explain why a pencil will suddenly start to spin if an astronaut lets it float inside a spacecraft without applying any force on it. Furthermore, it does not explain why the spinning motion is uniformly counterclockwise. There seems to be a larger impetus at work, but I don't have any guess as to what it might be. I feel your frustration, Tarnix: I haven't been able to find anything on Google about it either. You'd think there would be more theories out there.

Integral
Staff Emeritus
Gold Member
My astronomy professor taught me that the tendency of bodies to spin in space is an unexplained phenomenon, and he said he attended an exhibit about this at NASA's Glenn Research Center. The links recommended in this forum sidestep the question being asked here, and only Chronos has truly offered an explanation. The conservation of angular momentum does not, however, explain why a pencil will suddenly start to spin if an astronaut lets it float inside a spacecraft without applying any force on it. Furthermore, it does not explain why the spinning motion is uniformly counterclockwise. There seems to be a larger impetus at work, but I don't have any guess as to what it might be. I feel your frustration, Tarnix: I haven't been able to find anything on Google about it either. You'd think there would be more theories out there.

This spontaneous spinning you speak of is news to me. Please link to a verifiable source.

The conservation of angular momentum does not, however, explain why a pencil will suddenly start to spin if an astronaut lets it float inside a spacecraft without applying any force on it.

That's interesting. I wonder if anybody could tell me: Would a perfect ball start spinning as well in this situation? Would this happen even in a cockpit without atmosphere?

Also: Not everething is spinning in space. Space itself for example. And the largest structures are not spinning as I understand it. I do not know whether glaxy clusters spin around each other. I would doubt it as they seem to move away from each other. But the galaxies within the clusters themselves certainly spin - a motion which I would guess is due to their interaction through gravity? And would this galaxy spinning not mean that the motion was already there for any cloud that the solarsystem started out from? Also the idea that the cloud the solarsystem form from is just sitting there in space and then collapses i not right. It was very likely a violent neibourhood in the galaxy where this took place under influence of a very large star sending off very energetic material. And one more argument for the cloud being already in motion: The cloud would be put together from material which all vere in motion already: Gas, pulled in from intergalactic space to the galaxy and dust from past stellar explosions. This was no immobile cloud.

Greetings from interested amateur.

russ_watters
Mentor
If objects would start spontaneously spinning while in orbit, the entire space shuttle and all of our satellites would spin. Quite obviously, they do not.

Some people have proposed a Homopolar motor mechanism to explain rotational motion of bodies in space with an electric current running through them. You are basically converting electrical current into rotational force. Currents run through the earth, and other objects in space, but i'm not sure if they persist for long enough to actually generate rotaional motion. That's a possibility, but i dont think it has been proved yet.

Maybe meteors with a high angle impacts would give sufficient impulse to cause rotation. Their direction should cancel out on average, but that could be another factor.

Last edited:
i might just be hijackin this thread here,but my physics teacher told me a cloud of dust came together and started rotating, and after some steps forms a star. he did mention 'no one knows why the dust cloud rotates' which are forbidden words id expect to hear from him cus he seemed all-knowing.anyone shed light on this?
anyway tarnix my understanding of some of thems things you ask is centripetal force

i cant find a decent article but its things like $$F= \frac {mv^2}{r} = \frac {Gmm}{r^2}$$

Chris Hillman
Well, what happens when a spinning figure skater pulls her arms in? By conservation of angular momentum, her spin rate increases dramatically! In the same way, when a dust cloud collapses, it will usually have some angular momentum at the start of the collapse (wrt "the center"), and the result is that the dust particles typically start to rotate dramatically about "the center" as the cloud collapses.

"This spontaneous spinning you speak of is news to me. Please link to a verifiable source."

Unfortunately, I cannot find one. I'm just going off of what my teacher told me, but he seems like a fairly reputable guy.

"If objects would start spontaneously spinning while in orbit, the entire space shuttle and all of our satellites would spin. Quite obviously, they do not."

I'll be the first to admit that I don't know what I'm talking about, but my guess is that the space shuttle is either already in motion which offsets the spinning, or a stationary spacecraft would have sufficient mass to be orbiting the Earth, which is essentially a large scale spinning of sorts. I guess what I'm referring to is when an object is not affected by gravity or any other force and just idly resting in space, supposedly it will spin. Perhaps I've been misled, but I tend to think that the topic has been brushed aside because sometimes people don't like admitting that they can't figure something out. I saw another forum about this topic and somebody explained it by saying something to the effect that: "I spin, you spin, we all spin: it's just a reference thing." I, however, would argue that it's a tangible, measurable motion that warrants an explanation - an explanation that could perhaps open up a new field of study.

DaveC426913
Gold Member
If objects would start spontaneously spinning while in orbit, the entire space shuttle and all of our satellites would spin. Quite obviously, they do not.
Well they sort of will.

Orbiting objects, if left to their own devices, will (eventually) orient themselves so that their long axis points radially (tidal force). Of course, once there, they'll stop spinning, so it's sortta short-lived. And it takes along time.

Last edited:
DaveC426913
Gold Member
...a pencil will suddenly start to spin if an astronaut lets it float inside a spacecraft without applying any force on it. Furthermore, it does not explain why the spinning motion is uniformly counterclockwise...
I think you're going to have to back this up.

This sounds a lot like some rather specific circumstances being blown up, distorted and mangled into some sort of myth, like the toilet-flushing myth.

Furthermore, it does not explain why the spinning motion is uniformly counterclockwise.

Clockwise and counterclockwise in space would simply be a matter of viewpoint, wouldn't it? If you want to decide which way the Earth rotates you first have to make up your mind which way is up, isn't that right?

DaveC426913
Gold Member
Clockwise and counterclockwise in space would simply be a matter of viewpoint, wouldn't it? If you want to decide which way the Earth rotates you first have to make up your mind which way is up, isn't that right?
That was the first thing I was going to raise. The fact that someone would even try to make this claim hints at myth-creation.

Now, that being said, it seems to me, that a pen, if placed floating in the cabin with (magically) no initial rotation, should actually spin - competing one rotation every 90 minutes or so. The reason for this apparent rotation should be fairly obvious if we think about the frame of reference of the shuttle and observer.

Counterclockwise may be an arbitrary term but we need an arbitrary term to have a frame of reference. The thing I'm getting at is that you can't deny the prevalence of "counterclockwise" spinning, or whatever you wish to call it. All semantics aside, the motion is consistent even though it appears different to each individual observer. In whichever arbitrary perspective of choice, it is consistent within that perspective. It just seems like an unnecessary diversion to get philosophical about terms of choice when the real question is "why do things rotate in the same direction?"

SpaceTiger
Staff Emeritus
Gold Member
"why do things rotate in the same direction?"

In general, they don't -- there is no evidence to suggest that the universe has net angular momentum. In the confines of the solar system, we do see consistent axes of rotation because, as already explained, the cloud from which the solar system formed had angular momentum.

Things do not spontaneously start spinning unless a torque is applied. Gravity can apply this torque in some situations -- for example, the cloud from which the solar system formed was thought to be torqued by nearby masses -- but a pencil suspended above the surface of the earth would be much too small to experience a noticeable torque from gravity alone.

DaveC426913
Gold Member
Counterclockwise may be an arbitrary term but we need an arbitrary term to have a frame of reference. The thing I'm getting at is that you can't deny the prevalence of "counterclockwise" spinning
Why can't we? Where is there evidence that this happens at all? (in the space shuttle scenario)

Last edited:
Counterclockwise may be an arbitrary term but we need an arbitrary term to have a frame of reference. The thing I'm getting at is that you can't deny the prevalence of "counterclockwise" spinning, or whatever you wish to call it. All semantics aside, the motion is consistent even though it appears different to each individual observer. In whichever arbitrary perspective of choice, it is consistent within that perspective. It just seems like an unnecessary diversion to get philosophical about terms of choice when the real question is "why do things rotate in the same direction?"

IMO Chronos, Chris Hillman, Skeptic101 (and now SpaceTiger) have explained the angular spin issue about as well as one might hope to answer your question, and they've covered most of the spectrum of planetary formation theories. In pretty much every solar system formation theory, the inflow of the inbound gases begins to flow around the solar system in a regular pattern. That movement pattern of matter in the system is transfered to all the mass in the system and to the planets as they form. As Skeptic101 pointed out, there are other theories that would use an external or internal EM force to generate (or influence) the inflow pattern, but even those theories would tend to work essentially the same way at some point in the process. The link below will explain the math and the process in some depth.

http://www.sjsu.edu/faculty/watkins/solarspin.htm

This link talks about some of the various solar system formation theories and their relative strength's and weaknesses.

http://abyss.uoregon.edu/~js/ast121/lectures/lec24.html

You might also note that Birkeland's work demonstrated that an EM field would tend to concentrate mass around the sun's equator. You might also checkout the concept of a Parker Spiral and note that it too would tend to influence matter concentration patterns over time.

Last edited:

Well, what happens when a spinning figure skater pulls her arms in? By conservation of angular momentum, her spin rate increases dramatically! In the same way, when a dust cloud collapses, it will usually have some angular momentum at the start of the collapse (wrt "the center"), and the result is that the dust particles typically start to rotate dramatically about "the center" as the cloud collapses.

That explains a FASTER spin, but why does it spin in the first place?

russ_watters
Mentor

That explains a FASTER spin, but why does it spin in the first place?
That's covered in posts 4 through 8.

DaveC426913
Gold Member

The short version:

1 Take two objects in an otherwise empty region.
2 Give them random positions and random velocities.
3 Let them interact gravitationally.
(They will eventually go flying off, never to interact again. Trust me on this one. I've spent a lot of time trying to debug orbital simulator algorithms. :grumpy:)

...BUT...

Repeat steps 1-3 a zillion times and at some point you will give then juuuuuuust the right positions and velocities that they will interact and stay interacting for a reasonable duration.

Now, the chances of these two objects having zero proper motion wrt to each other is very, very small. They will fall toward each other and miss. But since they'e bound they'll start orbitting each other.

And now you've got a rotating system.

hmm just fell upon this article and i'm not gonna lie i've always wondered about this as well... sooo i'm just gonna put what i think is going on (from what i've just read) and it would be cool if someone could say i understand whats happening :D

soo if we take random particles.. lets say they start with zero velocity... and you allow them to begin interacting with each other they will move towards each other and collide and miss each other and this will continue and if this continues enough times then whatever the net momentum is for the particles will be the direction of spin? and then it will eventually overcome ALL opposite spin particles (after what i assume would be quite a long period of time) and this can be influenced by say a larger particle off in space giving more momentum to one direction?

if this is all correct then why the heck does venus spin opposite to other planets did one of the opposite spinning objects hit it hard enough to change how it spun... wouldn't OTHER objects come and hit it back into spin with all the other planets though since the NET momentum is in one direction... will it ever happen? and what about uranus spinning side ways..? :| :| :|

or could both of these planets be made say somewhere else and just happen to wander into our solar system? hmmmm i have no idea :D just curious..

as well tw43 said something about there being a possible position where an object is not affected by gravity.. is this not impossible.. i always thought it was. some gravity SOMEWHERE will eventually get u. lol

Last edited:
DaveC426913
Gold Member

soo if we take random particles.. lets say they start with zero velocity
If they're random, then the chances of them having zero velocity wrt each other is extremely small. You can count on them having a net residual velocity.

... and you allow them to begin interacting with each other they will move towards each other and collide and miss each other and this will continue and if this continues enough times then whatever the net momentum is for the particles will be the direction of spin?

I'm not sure how they can both collide and miss, but the rest of the logic is good.

if this is all correct then why the heck does venus spin opposite to other planets did one of the opposite spinning objects hit it hard enough to change how it spun...
That is one of the prevailing theories, yes.

wouldn't OTHER objects come and hit it back into spin with all the other planets though since the NET momentum is in one direction... will it ever happen? and what about uranus spinning side ways..? :| :| :|
At some point in a star system's life, almost all gas and dust has been swept up into planets. Their rotational direction is fixed. Collisions between large bodies become much more rare. But they do happen, and once they do, there's nothing left to undo them.

or could both of these planets be made say somewhere else and just happen to wander into our solar system? hmmmm i have no idea :D just curious..
There are some who hold such theories, but the evidence is against them.

ok thanks dave :) oh and i was talkinga bout having more than 2 random particles should have clarified.. i'm sure it takes more than 2 particles to say create a planet ... or a solar system

Who said Space does not spin ???

Drakkith
Staff Emeritus

Who said Space does not spin ???

Science?

FtlIsAwesome
Gold Member

Who said Space does not spin ???
Maybe you're talking about the frame-dragging effect in GR?