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[smjjames]

Here's the thing: You propose slowing down the orbit by using the "drag" inherent in the atmosphere.

Except, does the Earth get slowed down by pulling its atmosphere along?

And if it doesn't, but your hypothetical planet *would* be, why does the Earth not have 1000mph east winds as mentioned above?


It's late, so sorry if this doesn't really make sense right now. Trust me, it makes sense in my head and I'll explain it all later.

Um, the jet stream? It's not 1000mph or whatever silly speed, but it is a powerful current of air and east direction in the northern hemisphere.

[Reelya]

However, the point was that there wouldn't be rotational movement. The energy from that is provided by the planet.

The Earth does get slowed. The atmosphere requires energy to keep rotating around the planet. If you track the motion of the air particles only, they lack orbital velocity for the altitude they are at, so they cannot just move around the planet via gravitational falling. Hence, they need a constant, extra force applied to them in addition to gravity to maintain the motion they are in.

Earth is slowing down, we know that. Freely spinning bodies in space don't slow down. The moon is part of that, with tidal effects, however, friction is part of that too, because of conservation of energy: the air needs energy to make it change direction, which is what rotating with the planet entails. If the air was in free-fall then it wouldn't need any more energy, however any deviation from that requires a constant energy input.

https://physics.stackexchange.com/questions/1193/why-does-the-atmosphere-rotate-along-with-the-earth

[Maximum Spin]

Um, the jet stream? It's not 1000mph or whatever silly speed, but it is a powerful current of air and east direction in the northern hemisphere.

It's not "whatever silly speed". The Earth's rotation, tangentially, is actually 1000mph at the equator. So if that was what we were looking for, it would be.
(ETA: If you were wondering, it's the Coriolis force that moves the jet stream.)

Reelya: at this point you have essentially taken every position along the spectrum of possible positions and I think nobody knows what you're actually arguing for anymore.

[Reelya]

What? I've taken exactly one position. Show me where I contradicted myself?

My main points were:

- atmosphere's don't "spin" by themselves, so if you had a no-friction planet, the atomsphere would not rotate.
   - the only way that the air could spin itself around the planet would be if the air was at orbital velocities (which are higher closer to the ground in fact)
- hence, the atmosphere needs energy to force the particles to change direction
- hence that energy came from somewhere
- the source of the needed energy is the planet spinning
- hence the planet loses energy by needing to keep the atmosphere moving around it

All of those add up to the same thing, that having more atmosphere or more drag between said atmosphere and a planet will slow it's spinning.

[smjjames]

Um, the jet stream? It's not 1000mph or whatever silly speed, but it is a powerful current of air and east direction in the northern hemisphere.

It's not "whatever silly speed". The Earth's rotation, tangentially, is actually 1000mph at the equator. So if that was what we were looking for, it would be.

Reelya: at this point you have essentially taken every position along the spectrum of possible positions and I think nobody knows what you're actually arguing for anymore.

You said wind speed, not tangential rotating speed of the rocky orb. Jet Stream winds are still pretty fast https://en.wikipedia.org/wiki/Jet_stream

[Maximum Spin]

What? I've taken exactly one position. Show me where I contradicted myself?

I can't because you keep editing your posts to substantively change what you're saying.

You said wind speed, not tangential rotating speed of the rocky orb. Jet Stream winds are still pretty fast

I didn't say wind speed, but if you look at what I *did* say that the person who said wind speed was referencing, you'd see that we were talking about winds produced by the rotation of the rocky orb if the atmosphere was hypothetically not moving along with it.

[Culise]

More practically and back to the original idea, the idea of using an atmosphere to slow down a planet falls into the same trap as "enough nukes to start a planet's rotation" which was brought up earlier in the thread: while technically accurate, the amount of mass you need is completely impractical.  You may as well argue that enough people jumping up and down can be used to change a planet's orbit, for all the actual value such a statement contains.  Even a mere Earth-sized planet is still quite large, with the Earth itself having a moment of inertia of about...oh, glancing around since I'm too lazy to do the calculus for myself, around 8*10³⁷ kg*m², give or take a few 3-5*10³⁵ kg*m² for the fact that the Earth is technically an oblate spheroid with varying polar and equatorial radii rather than a perfect sphere.  Assuming you want an Earth-normal atmosphere for people to actually live on this world, a few 10¹⁸ kg of gas is not going to be a very large lever to bring a planet's rotation up to a meaningful speed within any reasonable human-usable timeframe. 

EDIT: Grammar pass. >_<

[smjjames]

What? I've taken exactly one position. Show me where I contradicted myself?

My main points were:

- atmosphere's don't "spin" by themselves, so if you had a no-friction planet, the atomsphere would not rotate.
- hence, the atmosphere needs energy to force the particles to change direction
- hence that energy came from somewhere
- the source of the needed energy is the planet spinning
- hence the planet loses energy by needing to keep the atmosphere moving around it

All of those add up to the same thing, that having more atmosphere or more drag between said atmosphere and a planet will slow it's spinning.

We are also assuming that there is no heat from other sources, like a sun.... Which means that our hypothetical atmosphere is now completely frozen. But if you want to break the laws of thermodynamics, then yes, that thought experiment works.

[Reelya]

However, it's notable that Venus has an extremely slow rotational period, along with a really dense atmosphere. that took billions of years however. But not 10^18 years.

We are also assuming that there is no heat from other sources, like a sun.... Which means that our hypothetical atmosphere is now completely frozen. But if you want to break the laws of thermodynamics, then yes, that thought experiment works.

This is a complete non-sequiter. Heat from the sun doesn't make the atmosphere rotate around the planet, it just makes it less dense. In fact heat from the sun increases motion, thus it increases friction effects. It doesn't have any positive effects on the planet's rotation, only extra frictional effects.

High temperature means higher air pressure, making any drag from the air more effective. Purely going off basic laws of physics here. Hotter air exerts more pressure.

[Maximum Spin]

Just curious, where do you think the atmosphere is dumping its angular momentum to?

Even though it's not in a stable orbit, if the friction from the earth stopped instantly right now, the temperature of the atmosphere would keep it from collapsing, so it'd keep spinning.

[smjjames]

However, it's notable that Venus has an extremely slow rotational period, along with a really dense atmosphere. that took billions of years however. But not 10^18 years.

The main theory on how it got its slow rotation though is that it got smacked with something early on in its history, not atmosphere.

Just curious, where do you think the atmosphere is dumping its angular momentum to?

Even though it's not in a stable orbit, if the friction from the earth stopped instantly right now, the temperature of the atmosphere would keep it from collapsing, so it'd keep spinning.

The Earth is not in a stable orbit? Huh, I didn't know Jupiter was looming.  Edit: I think you meant atmospher is not in a stable orbit, but the wording is a little wierd.

[Reelya]

No ... it would slow down. Because if you take any one air particle, and you plot it's trajectory in free-fall it's not around the planet, it's into the ground. You need a sustained force that makes it travel in a circle around the planet.

Also, I think you're using the concept of "angular momentum" a little wrong there. Free particles don't have angular momentum around some arbitrary point that's outside their center of mass. Free particles travel in a straight line according to Newtonian physics, unless that line deviates because of a force.

We can treat a large, solid object as a single thing with angular momentum around a center of mass, however that's kinda artificial and requires the particles to be joined together by a strong force. By themselves, every atom only "knows" about it's straight-line trajectory. Free moving elementary particles don't have "angular momentum" around some arbitrary position, because "angular momentum" in that sense isn't one of the fundamental forces of physics.

[Maximum Spin]

The Earth is not in a stable orbit? Huh, I didn't know Jupiter was looming.

"it" here refers to the atmosphere, in keeping with the normal rules for pronouns in that situation.

Reelya: Okay. Here's the thing. If the atmosphere was exerting drag on the planet, we'd all be able to feel it. There would indeed be 1000mph winds around the equator. This is not the case. Therefore you can probably guess your analysis is wrong. And also the rotating atmosphere does have angular momentum.

[Trekkin]

Just curious, where do you think the atmosphere is dumping its angular momentum to?

Space, actually, as it boils off.

It has no significant effect on the momentum of the planet, for the reasons of mass differential that Culise outlined, but insofar as it technically happens, that's where the momentum goes.

[smjjames]

Has anybody done a calculation showing how long it would take for an atmosphere the density of Venus's to slow Earths rotation down by say, one minute?

The Earth is not in a stable orbit? Huh, I didn't know Jupiter was looming.

"it" here refers to the atmosphere, in keeping with the normal rules for pronouns in that situation.

I realized that a moment later, somehow misread that sentence.