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View Full Version : Two part question on rotation

Salty
2008-Sep-09, 10:11 AM
Part I: What started the Earth to rotate around its axis?

Part II: What keeps the Earth rotating around its axis?

Jens
2008-Sep-09, 10:16 AM
Part I: What started the Earth to rotate around its axis?

Part II: What keeps the Earth rotating around its axis?

Very simply, I think the answer to Part II is simply: momentum. Just as an object moving in a straight line will continue to do so until some force intervenes, a rotating object will continue to do so unless stopped.

The answer to Part I may sound somewhat tricky, but I think basically it's like this: All objects in the universe must either be non-rotating or must be rotating to some degree. The chance of an object being totally non-rotating is almost infinitely small, because it is just one of an infinity of possible states. So nearly everything in the universe seems to have some rotation.

Jeff Root
2008-Sep-09, 12:23 PM
I have a speculation about the cause of planet rotation:

Particles falling toward a forming planet from an orbit slightly outside
the planet's orbit will be moving a bit faster than circular orbital speed,
while particles rising from an orbit slightly inside the planet's orbit will
be moving a bit slower than circular orbital speed. If the orbits of the
particles were very nearly circular, like the particles which compose
Saturn's rings, then the slightly-faster-moving particles would mostly
hit the night side of the planet, while the slightly-slower-moving
particles would mostly hit the day side of the planet. The planet ends
up rotating in the same sense that it orbits.

If, on the other hand, the orbits were relatively non-circular, then it
would be about equally likely for a particle to hit either the day side
or the night side, nomatter whether it is coming from a superior or
inferior orbit.

Now somebody tell me this is the mainstream theory...

-- Jeff, in Minneapolis

Hornblower
2008-Sep-09, 12:40 PM
I have a speculation about the cause of planet rotation:

Particles falling toward a forming planet from an orbit slightly outside
the planet's orbit will be moving a bit faster than circular orbital speed,
while particles rising from an orbit slightly inside the planet's orbit will
be moving a bit slower than circular orbital speed. If the orbits of the
particles were very nearly circular, like the particles which compose
Saturn's rings, then the slightly-faster-moving particles would mostly
hit the night side of the planet, while the slightly-slower-moving
particles would mostly hit the day side of the planet. The planet ends
up rotating in the same sense that it orbits.

If, on the other hand, the orbits were relatively non-circular, then it
would be about equally likely for a particle to hit either the day side
or the night side, nomatter whether it is coming from a superior or
inferior orbit.

Now somebody tell me this is the mainstream theory...

-- Jeff, in Minneapolis

Your line of thought looks totally mainstream to me, as in conservation of angular momentum, which has been well understood for a long time.

I would expect the rotation of the primordial contracting nebula to have been largely circular.

Note that among the eight major planets, only Venus and Uranus have oddball rotation, and that may be the result of collisions with other large bodies early on.

Ken G
2008-Sep-09, 02:06 PM
Now somebody tell me this is the mainstream theory...Actually, I think the basic premise is flawed. Slightly perturbed circular orbits simply oscillate in little circles at the orbital period in the co-orbiting frame. There's no asymmetry to lowest order. I would think the source of the angular momentum of a forming planet comes from the gradient in the orbital speed, not of orbits that cross at a point, but rather parallel circular orbits that are drawn together by the gravity of the forming object. That might also explain why larger planets rotate faster-- they draw from a wider range of Keplerian orbital speeds as they form. But I don't know the mainstream theory on that.

Stuart van Onselen
2008-Sep-09, 02:17 PM
IIRC, the dust-cloud that coalesced to form the sun and the planets was itself rotating (albeit very slowly) and when the sun and planets formed, they still had all that angular momentum.

Don't ask me why the cloud was spinning. I have not the faintest idea.

Ken G
2008-Sep-09, 08:11 PM
Don't ask me why the cloud was spinning.I think that's the basic issue Jens raised, that it's hard to have no rotation, and when objects contract whatever initial rotation they had becomes more apparent.

pzkpfw
2008-Sep-09, 08:33 PM
Part II: What keeps the Earth rotating around its axis?

Not the Moon.

(Smiley here)

Senor Molinero
2008-Sep-10, 12:38 AM
The initial rotation rate of the Earth is simply te sum of all of the vectors of all of the particles that formed it. That speed has slowed a little since then, dropping from about 380 days per year to the present 365.2425~ and continues, hence the need for leap seconds.

Jeff Root
2008-Sep-10, 03:27 AM
I would think the source of the angular momentum of a forming
planet comes from the gradient in the orbital speed, not of orbits
that cross at a point, but rather parallel circular orbits that are
drawn together by the gravity of the forming object. That might
also explain why larger planets rotate faster-- they draw from a
wider range of Keplerian orbital speeds as they form. But I don't
know the mainstream theory on that.
If it were the gradient in orbital speed, I get that the resulting
rotation would be in the opposite direction: Particles in circular
orbit superior to a forming planet move slower than the planet,
and particles in circular orbit inferior to the planet move faster.

I would guess that the reason for the larger planets' faster
rotation is that the bulk of their masses have been compressed
more than those of smaller planets, reducing the uh... what's
the right term? Moment arm?

-- Jeff, in Minneapolis

Ken G
2008-Sep-10, 03:54 AM
If it were the gradient in orbital speed, I get that the resulting
rotation would be in the opposite direction: Particles in circular
orbit superior to a forming planet move slower than the planet,
and particles in circular orbit inferior to the planet move faster.Hmm, that's a good point, but let's not let something so insignificant as the sign of the effect spoil a perfectly reasonable explanation. But it does raise a disturbing issue-- it seems to me the planets should spin the opposite way! Clearly, they don't, so something is being overlooked.

I would guess that the reason for the larger planets' faster
rotation is that the bulk of their masses have been compressed
more than those of smaller planets, reducing the uh... what's
the right term? Moment arm?They haven't necessarily been compressed farther, if they start from the same density and end up with the same density as a smaller planet, that's the same amount of compression (measured in terms of the factor of decrease in the moment arm), the factor being the cube root of the density compression factor. A complicating issue is that the large planets form from the hydrogen, whereas the small planets have to sift out the rocks and metals, so they are both probably sampling from a similar amount of stuff in all.

I think the simple truth is, I have no idea how the planets got their spin, and how much they got!

Ken G
2008-Sep-10, 04:28 AM
More on this: I looked up a review article, http://articles.adsabs.harvard.edu//full/1993ARA%26A..31..129L/0000161.000.html, and sure enough, the problem of planetary rotation is quite a difficult one! If I read it correctly, there are theories that say you can get prograde (normal) rotation from gas accretion when you consider how a planet that is accreting gas alters its own gas environment. A key aspect of gas accretion is that the planet is usually moving faster than the gas, because the gas is partially supported by a pressure gradient from the direction of the star, so needs lower Keplerian speeds to satisfy the uncancelled portion of the gravity. That means the planet sees a "headwind" from the gas it accretes. If the planet is also spiralling inward toward the Sun due to that drag, it will tend to encounter that headwind more on the side closer to the Sun, creating a torque from the drag and spinning the planet up in a prograde way.

There seem to be some other models as well for getting prograde rotation, but it also appears that one respected idea is that the rotation of a planet is controlled not by gas accretion, but rather by major impacts that are essentially completely stochastic in nature. If so, then it is sheer coincidence that so many planets have a relatively low obliquity in their axes, though maybe the preference for prograde rotation is left over from these gas accretion effects. Bottom line: the issue remains unsolved!

Jeff Root
2008-Sep-10, 10:40 AM
Thanks for that info, Ken!

There seem to be some other models as well for getting prograde
rotation, but it also appears that one respected idea is that the
rotation of a planet is controlled not by gas accretion, but rather by
major impacts that are essentially completely stochastic in nature.
If so, then it is sheer coincidence that so many planets have a
relatively low obliquity in their axes, though maybe the preference
for prograde rotation is left over from these gas accretion effects.
Bottom line: the issue remains unsolved!
I have long suspected that the direction of rotation might possibly
be random, as a result of random major impacts, but at the same
time, I think the orientations of the axes is too big a coincidence.
The direction of rotation being random I might be able accept, since
there are pretty much only two possibilities. But combined with the
fact that all except Uranus have their axes nearly perpendicular to
their orbital planes, I smell a conspiracy.

I believe that large impacts have played a huge role in the planets'
formation, yet the uniformly low oblquity of the axes seems to imply
otherwise. I don't know how to reconcile those two notions.

Does a particle in circular orbit of the Sun have more or less angular
momentum than an identical particle in a slightly smaller orbit?

-- Jeff, in Minneapolis

Salty
2008-Sep-10, 11:32 AM
Gentle Beings,

I have read all the posts, to this time. It's so refreshing, to find that I'm not alone in not knowing the final answer to these two questions.:whistle:

Seriously, I thank each and all of you, for responding to my question. I had no idea of all the different considerations, about what I once thought was a simple matter. Discovering these varied considerations was a learning experience, in itself.

Best regards.

Jeff Root
2008-Sep-10, 12:11 PM
In reply to Stuart,

I think the overall rotation of the molecular cloud (or portion thereof)
which would later collapse to become the protosolar nebula was not
something which could be detected and measured even by advanced
observers. The individual particles moved in random directions, and
at random speeds consistent with the virialization to the average
temperature. (Which actually must have varied quite a lot from one
part of the cloud to another, so I guess I mean "average" in any one
particular part of the cloud.) So looking at the cloud, even in very
minute detail, it would be impossible to tell that on average, more
particles were moving in one direction than any other.

Only when the cloud became dense enough for collisions to preciptate
collapse did the predominant direction of motion become apparent.

-- Jeff, in Minneapolis

Ken G
2008-Sep-10, 03:54 PM
Does a particle in circular orbit of the Sun have more or less angular
momentum than an identical particle in a slightly smaller orbit?

Circular orbits have more angular momentum the farther out you go. Keplerian speed scales as 1/sqrt(r), so angular momentum scales as sqrt(r).