[David Cooper]

If the bearings are good (and they are) you don't need to run  motor to keep the gyro spinning.

Good point - the gyro could have components that aren't connected up at all physically but just float in space while being kept in place and made to rotate by magnetic means. No wear and no friction.

[Bored chemist]

If the bearings are good (and they are) you don't need to run  motor to keep the gyro spinning.

Good point - the gyro could have components that aren't connected up at all physically but just float in space while being kept in place and made to rotate by magnetic means. No wear and no friction.

It seems they come in two flavours; I hadn't realised how complicated it is.
https://en.wikipedia.org/wiki/Control_moment_gyroscope
https://en.wikipedia.org/wiki/Reaction_wheel
But presumably, Alan doesn't believe in them because they rely on the conservation of angular momentum.

[Halc]

A major split of this thread has been made.  New topic is "Can heat affect Earth's rotation?"

https://www.thenakedscientists.com/forum/index.php?topic=80717.0

Please confine posts concerning angular momentum changes without external torque to that topic.

[mikahawkins]

No, it won't. The Earth spins because it formed in the accretion disk of a cloud of hydrogen that collapsed down from mutual gravity and needed to conserve its angular momentum. It continues to spin because of inertia.
Basically the Earth and all the windmills on Earth will constitute a single system.

[hamdani yusuf (OP)]

Conserving angular momentum doesn't necessarily mean conserving angular velocity.

How A Massive Dam in China Slowed the Earth's Rotation

Most of us agree that the answer to the title question is no, as long as the earth can be treated as an isolated system. Otherwise, the answer could be yes.
Here are some phenomena violating earth's isolation :
- Meteor shower, mostly burn in the atmosphere.
- Solar wind, besides the influence through direct impact, is also stripping light gases from upper atmosphere.
- Launching of satellites and other spacecraft, which are mostly eastward, hence tends to slow down earth rotation.

[Bored chemist]

It's always been accepted that there are small effects.
But the origins of the other thread lie in Alan's claim that the weather is powered by the Earth's rotation- which is plainly wrong, and the idea that you can interconvert  energy, angular momentum and linear momentum which is also obviously wrong.

Essentially a windmill will affect the Earth's rotation.
But it will have largely the same effect if it is jammed and can't rotate, or replaced by a tree.

[hamdani yusuf (OP)]

Essentially a windmill will affect the Earth's rotation.
But it will have largely the same effect if it is jammed and can't rotate, or replaced by a tree.

How would it affect the Earth's rotation? Would it accelerate, or rather, decelerate earth rotation?

[Halc]

Essentially a windmill will affect the Earth's rotation.
But it will have largely the same effect if it is jammed and can't rotate, or replaced by a tree.

How would it affect the Earth's rotation? Would it accelerate, or rather, decelerate earth rotation?

Most of this has already been answered in the first reply to this topic.
The erection of a windmill (or the growing of tree) typically involves the movement of mass from here to there, which alters the moment of the system, and thus changes the rotation speed.  Which way depends on where the material is move to and from, so a windmill laying on the ground and then flipped upright will slow the Earth's spin since that is an increase of moment. This change in moment is the reason behind the China dam video you posted. The dam is apparently far enough south that it increases the moment, and thus decreases the spin rate, but even that isn't indefinite since the weight of the project will sink the crust that buoys it up, and thus restores the moment close to where it was without the project. Your bathtub does this, just on a smaller scale.

Secondly, the atmosphere is always more or less in equilibrium with the crust.  If the wind on the surface prevails in one direction, friction slows the one and speeds the other, tending always towards equilibrium.  So imagine I toss a ball to the west. That speeds Earth's spin momentarily and shortens the day, but in moments the ball hits the ground and comes to rest, and the spin is restored. Putting up a windmill (or tree) is like that: Each might put a force on the ground, but that just takes away the same force that would have had the same friction elsewhere, so the net effect is temporary and negligible.

The atmosphere does change momentum due to the weather, and as has been pointed out somewhere, the daily variation (microseconds was it?) of length of day is quite measurable. The energy from the sun does mess with the equilibrium.

Third: A windmill has angular momentum of its own that a tree doesn't.  So a north facing windmill (or Ferris wheel for that matter) will increase the length of day if it spins clockwise, and decrease it if it spins counterclockwise. This has nothing to do with wind, friction, or force. It is just a thing having more than its share of the angular momentum of the system, just like your satellite put into orbit.

Most of us agree that the answer to the title question is no, as long as the earth can be treated as an isolated system.

Of course not. There's tides, which put an immense torque on Earth. That effect is many orders of magnitude greater than any other external effect. Angular momentum is never lost, but it is transferred from spin momentum to orbital momentum. The day used to be a single digit of hours long and the year was shorter. Trees and windmills are not responsible for the current 24 hour day.

Here are some phenomena violating earth's isolation :
- Meteor shower, mostly burn in the atmosphere.

Negligible since there is no particular asymmetry to it. There are about as many adding to our momentum as those decreasing it.  A big one is an exception. The biggest was Theia, and that one is responsible for the ~6 hour spin rate way back in the early days. There's no telling what the spin (or tilt, or even orbital distance/length of year) of Earth was before that impact.

- Solar wind, besides the influence through direct impact, is also stripping light gases from upper atmosphere.

Solar wind is symmetrical and applies no torque. Similarly, stripping gas changes the mass and moment and has no effect on rotation rate.

Launching of satellites and other spacecraft, which are mostly eastward, hence tends to slow down earth rotation.

It doesn't change the momentum of the system, but yes, for the reasons mentioned above.

[hamdani yusuf (OP)]

The day used to be a single digit of hours long and the year was shorter.

When the day was shorter, there should be more days in a year.

[hamdani yusuf (OP)]

Negligible since there is no particular asymmetry to it.

I've also mentioned this in my previous post. If the windmills are massive enough to create significant net movement between the atmosphere and earth crust, then small meteors would tend to slow the atmosphere more then earth crust, since they would be burnt down, hence produce net torque to the earth system. Ditto for solar wind.

[Bored chemist]

If the windmills are massive enough to create significant net movement between the atmosphere and earth crust,

What do you mean?

[Halc]

The day used to be a single digit of hours long and the year was shorter.

When the day was shorter, there should be more more days in a year.

More days doesn't necessarily make the year longer if it's because the days are shorter.

The length of the year is increasing. There are four factors, two positive and two negative.  In what I think is the correct order of decreasing significance.  The higher the effect, the greater its influence, but the list seems to be in order of reverse permanence. The bottom one will win in the end, despite it being the smallest effect.

1) Tides are always pushing the Earth to a higher orbit, just like they're pushing the moon to a higher orbit. This effect will end if the Earth would ever become tidal locked, but it cannot while we have a moon.  The day will eventually be very short again if Earth lives long enough for the moon to come back and impact it.  Only then will the day length grow again until it matches the year length at the time.

2) The sun is losing mass by burning it, and the decrease of gravitational pull drives Earth to a higher orbit. It is possible that it will lose enough mass that when the sun grows to a red giant, we'll be outside its radius. It will definitely grow to Earth's current orbit, but the jury is out whether our orbit will be high enough at that time to escape death by:

3) Friction, with dust, or maybe the sun if it grows enough to touch us.

4) Orbital energy and momentum is continuously radiated away at the rate of 200 watts in the form of gravitational waves. Yes, a closed system is not entirely closed, per general relativity.

If the windmills are massive enough to create significant net movement between the atmosphere and earth crust

Windmills don't create any net movement, massive or not. See the example of my throwing a ball. They are effectively trees, and slow the local wind here rather than it being slowed equally somewhere else, just like a dam generates energy from water going downhill, water that would have gone downhill at the same rate even with the dam not there.

then small meteors would tend to slow the atmosphere more then earth crust, since they would be burnt down, hence produce net torque to the earth system.

There's no torque to meteors since they fall equally on one side and the other. The fact that they heat up on the way in makes zero difference. That's like solar energy adding heat but not momentum.
You're apparently ignoring the replies being posted, and just repeating the same things. Reply to the replies to your comments rather than just repeating the original comment with slightly different wording.

[hamdani yusuf (OP)]

If the windmills are massive enough to create significant net movement between the atmosphere and earth crust,

What do you mean?

I mean that when the windmills collectively reduce windspeed in one direction while not significantly reduce windspeed in the opposite direction. So if windspeed in all points on earth surface are measured and vectorially summarized, the result is non-zero.

[hamdani yusuf (OP)]

Secondly, the atmosphere is always more or less in equilibrium with the crust.  If the wind on the surface prevails in one direction, friction slows the one and speeds the other, tending always towards equilibrium.  So imagine I toss a ball to the west. That speeds Earth's spin momentarily and shortens the day, but in moments the ball hits the ground and comes to rest, and the spin is restored. Putting up a windmill (or tree) is like that: Each might put a force on the ground, but that just takes away the same force that would have had the same friction elsewhere, so the net effect is temporary and negligible.

That's true if you only throw the ball once and then stop throwing, also assuming the ball doesn't reach orbital velocity. 
If you continuously throwing many balls, and the balls just fallen are immediately thrown again by other ball throwers, then you will have a stream layers of balls rotating in one direction, and the earth crust rotating in the opposite direction (relative to without ball throwings).

[hamdani yusuf (OP)]

There's no torque to meteors since they fall equally on one side and the other. The fact that they heat up on the way in makes zero difference. That's like solar energy adding heat but not momentum.

Let's have a still water in a pool. A rotating basket ball is dropped and float on it. Water molecules hit the ball equally on one side and the other, but nonetheless the ball's rotation slows down, and finally stop.

[Bored chemist]

If the windmills are massive enough to create significant net movement between the atmosphere and earth crust,

What do you mean?

I mean that when the windmills collectively reduce windspeed in one direction while not significantly reduce windspeed in the opposite direction. So if windspeed in all points on earth surface are measured and vectorially summarized, the result is non-zero.

If the wind doesn't push the windmill, the wind pushes the earth.
If the wind does push the windmill, the windmill pushes the earth.

The effects cancel and there's no net effect.

on one side and the other, but nonetheless the ball's rotation slows down, and finally stop.

And the water is set spinning.
Eventually, it pushes on the earth and alters the spin of the earth (very slightly).

But when you set the ball spinning you also pushed on the earth and altered it's spin slightly.

When all these process have finished they cancel out and the earth spins like it did before.

This was all pointed out earlier.
Have you actually read the thread?

[hamdani yusuf (OP)]

And the water is set spinning.
Eventually, it pushes on the earth and alters the spin of the earth (very slightly).

Analogously, the gas stripped by solar wind leaving the earth taking away some of the angular momentum.

But when you set the ball spinning you also pushed on the earth and altered it's spin slightly.

When all these process have finished they cancel out and the earth spins like it did before.

Let's start with the ball not spinning before dropped from a drone above the pool. The drone spin the ball clockwise, consequently the drone spin counterclockwise, hence maintaining zero angular momentum. The ball is then dropped while still spinning clockwise, and the drone keeps spinning counterclockwise until the end of the experiment.

[Bored chemist]

The drone spin the ball clockwise,

In doing so the ball spins the drone the other way.
The drone pushes the air.
The air pushes the earth and everything cancels out.

Analogously, the gas stripped by solar wind leaving the earth taking away some of the angular momentum.

It's not an analogy because, in this case, something actually leaves the earth.
Launching the Voyager probe will have made a difference too.
This is one of the real, but tiny, effects.

But nothing that happens entirely "on Earth" can affect the angular momentum of the Earth.

[Petrochemicals]

, just like a dam generates energy from water going downhill, water that would have gone downhill at the same rate even with the dam not there.

Technically not true, take the hoover, lower transportation of sediment due to the removal of energy from the fast flowing colorado has led to life expectancy of the dam to be only 100 years before it is sedimented to the cusp of the dam wall, same at other dams. The lake upstream, lake Powell will act more as a sediment block rather than a acompanying dam. Also now lower excavation rates and flow in the lower colorado.

I mean that when the windmills collectively reduce windspeed in one direction while not significantly reduce windspeed in the opposite direction. So if windspeed in all points on earth surface are measured and vectorially summarized, the result is non-zero.

All that has happened is windmills are taking the place of  trees,  mountains oceans in the particular direction but the wind in the opposite direction will still hit the above listed features, thus cancelling out. Wind energy makes waves waves make sound and heat, wind bends trees, trees make sound heat etc.

[hamdani yusuf (OP)]

It's not an analogy because, in this case, something actually leaves the earth.

In my analogy above,  the ball represents the earth,  while the pool water represents solar wind.
The water molecules that once touch the ball will move away from it, taking away some of the momentum.