why does the Earth spin?
confused Capt.Stan@verizon.com
It’s been asked before:
http://itss.raytheon.com/cafe/qadir/q2679.html
Here’s another one:
http://www.physlink.com/ae201.cfm
Oh yeah, welcome to the SDMB!
Nancy: “The whole world is spinning.”
Nick Danger: “Why, that’s lucky for us, Nancy. If it were flat all the Chinese would fall off.”
The theory from the Fireside Theater.
Sorry, I couldn’t resist.
Welcome to the SDMB. I changed the title of this thread from never asked question to the more descriptive Why does the earth spin?. Please try to choose descriptive titles in the future.
bibliophage
moderator GQ
You had to ask.
And you other folks had to post links.
I had the earth thing sorted out.
Now, the moon!
“It is incorrect to say that the Moon does not spin. It rotates on its axis in synch with its revolution around the Earth, keeping the same face always pointed toward us. If it did not rotate at all, we would see all of its surface over the course of one month’s orbit. This condition was created by tidal forces between the Earth and Moon, gradually slowing its initial rotation until the current more stable situation evolved. This same “tidal lock” condition has been observed for satellite closely orbiting other planets. Because of its elliptical orbit, a slight “wobble” allows us to actually see slightly more than half of the Moon’s surface over the course of a month.”
I don’t get the “tidal forces between the Earth and Moon, gradually slowing its initial rotation” part…
This is one of those questions you read, say to yourself “that’s simple, I know that”, and go on to find something a little more challenging (such as, “why does Windows always…”). After all, lack of spin in an astronomical object is an extremely unlikely and extremely precarious (or metastable) state. You can drop a pencil from now until the end of time onto a table and there is no reason it can’t land balanced on its point but the odds are very much against it. And even if it does, it is not going to stay that way very long.
But out of minor curiosity, I followed Philistine’s two links. Neither of which actually answers the question. One dismissively says, “all things in the universe ‘spin’” and the other says, “Stars and planets form as a result of the gravitational collapse of accreting material. Any net translational motion of that initial material is accelerated as its radius of rotation decreases.” Well yes, but where does the initial spin come from?
After an hour searching the web and another hour pulling books off shelves, I’ve come to one definite conclusion: my search skills are not as finely honed as I had believed.
The snotty answer to the question is the earth spins because the stuff that came together to form it was spinning. This can be shown statistically: almost everything in the solar system is spinning (to keep it simple) anti-clockwise. If the stuff had no spin, the solar system would consist of only the sun. (Here an expert in fluid dynamics, which I am definitely not, could show how unlikely it is for star formation to occur without resulting angular momentum [spin].)
OK, but why was that stuff spinning? Now things get a little interesting. The galaxy as a whole spins but it spins pretty much clockwise, so it did not directly cause the spin of the primordial solar system stuff. So the direction (or vector) of that stuff’s spin is a more local phenomenon. Here we could go shooting off into star formation theory but the answer is no one really knows. Was our particular spin imparted only to our solar system or was there a whole nursery of stars born collectively with similar characteristics? How do the internal structure and dynamics of the galaxy influence our initial spin? I say check back in fifty years, maybe…
[A slight rant. On a message board for science teachers, I found this: “In space everything moves and everything rotates. An object doesn’t need a “reason” to move or spin,” And I guess we don’t need a reason to explain the abysmal state of education in America.]
Why shouldn’t it ?
When you walk, you think you’re pushing yourself forward against a solid surface - but in fact you’re pushing the earth backwards. The combined steps of everyone on earth create the spin. It just so happens that the great majority of people walking are always doing so in the same direction and at the same speed: home from work, very quickly. The earth does not register our journeys TO work because we don’t seem particularly eager to get there, wander around and stop and start a lot more, which dissipates the effect.
And I almost forgot…
Although it has nothing to do with Earth’s spin, inquiries into the origins of spin in astronomy have produced interesting results. Vera Rubin’s work on galactic spin has led to theorizing on the preponderance of dark matter in the universe. And the work of my only hero, Kurt Gödel (how can you not hero worship someone who runs into the basement when there is a knock on the front door?), on the rotation of the universe is fascinating, to say the least.
(I also apologize profusely and profoundly for the cheap shot at science teachers in retaliation for those of their profession who so completely and miserably ill-informed me from the sandbox through quantum mechanics.)
OK, let me see if I can do this without any math.
Suppose you have a very large group of particles, including dust specks, crystals, molecules, and individual atoms, mostly of hydrogen, and helium. The group of objects comprises a cloud, roughly isolated from other objects, and very lightly bound together by mutual gravity. The individual velocity of each particle is random, or very nearly so.
For each particle it is either true that it has a velocity which will take it out of the cloud against the common gravity, or it will not. We will consider only those that are fated to remain behind, and assume the other objects are not a part of our system. All of the remaining objects have orbits around the center of the cloud, which are the result of their initial motion, and the gravity of the cloud as an object.
Now comes the very interesting part. There is a center of gravity for the cloud. Initially it might be somewhat variable, but it will be much smaller in its locus of motion than most of the elements of the cloud. It is just the “middle” of the cloud, nothing more. Now, if you draw an axis through the center, and calculate each and every single particle’s orbit, various orbits will cancel or enhance each other in terms of the cloud’s total angular momentum. The numbers change for each axis described. The inescapable fact is that there will be one axis for which the resultant angular momentum is greatest, unless every possible axis produces an angular momentum of zero.
How unlikely that is could be very difficult to describe without mathematics. I won’t try. Most clouds do have an axis of greatest angular momentum.
Now while the cloud is a few hundred astronomical units in diameter, that angular momentum might seem like a very small amount of spin. The main reason it matters is that orbital mechanics cause different objects to affect each other in certain ways. One of the ways that matter in our investigation is that whenever a particle pair interacts, one will gain radial momentum, and the other will gain angular momentum. One molecule escapes the cloud, and the other spins around it faster. (There is another complicated mechanical relationship that flattens the cloud into a disk, but we will gloss over that one, for now.) The loss of matter and the increase in angular momentum cause the cloud to spin faster as it draws closer to its center.
Even a very nearly stationary cloud will spin somewhat when it is entirely collapsed into a solar sized cloud. However, there are other effects that increase the likelihood of spin in collapsing clouds. The most likely source of the material in such a cloud is a collision between two shells of matter ejected from distant energetic systems, such as Supernovas. The collisions of such clouds are not likely to be perfectly centered on each other’s center of gravity. Because of that the two component portions of the cloud will begin coalescing with a significant angular momentum, derived from the differences in original vectors.
So, spinning turns out only to be the most likely result of the interaction of forces and objects that make up stellar systems. It’s not odd. It’s ordinary! Sitting still would be odd.
Tris
“It should be possible to explain the laws of physics to a barmaid.” ~ Albert Einstein ~
“You should see the place where Einstein used to drink!” ~ Triskadecamus ~
You should see what Einstein used to drink!