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"What will happen if Earth stopped rotating?" have been answered multiple times with a lot of informative and interesting answers. Continuing this hypothetical question, I have another one in mind.

If Earth stopped, how will it start rotating again? I've read that Gravity assist/pull from other planets and Moon will make Earth start rotating again. I would like to know what else can play a role in Earth's re-rotation.

I will also appreciate if you could help me know how much time would it take to Earth regain its rotating speed.

Edit1: This is clearly a curious question from a made up situation. I am quite familiar with the fact that Earth can't and won't stop rotating. It's just a fictional question coming in mind after reading the similar topic "What will happen if Earth stopped rotating?".

Edit2: Earth's course around the sun is called Revolution. The day and night cycle is the result of Earth's rotation. That's what I'm curious about. I only want to know the factors that will make Earth "Rotate" again and I haven't considered anything related to Revolution.

I'm sorry I wasn't able to make myself clear. Please let me know if we still need some more clarifications.

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closed as unclear what you're asking by ACuriousMind, CuriousOne, Bill N, Kyle Oman, dmckee May 5 '16 at 14:34

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

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    $\begingroup$ The Earth, as you know it, will neither stop rotating, nor will it start again, all of that is pure science fiction. $\endgroup$ – CuriousOne May 5 '16 at 9:20
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    $\begingroup$ This question is not useful. It is pure speculation. While "What will happen if Earth stopped rotating?" can be interpreted as "What effect does rotation have on weight?", I don't see any similar useful interpretation here. Except perhaps "What factors effect changes in the Earth's rate of rotation and to what extent?" $\endgroup$ – sammy gerbil May 5 '16 at 9:56
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    $\begingroup$ Sometimes, a person is just curious to know something but too naive to understand the triviality. Please dont demotivate their curiosity but make them understand why its not a good question. $\endgroup$ – seeking_infinity May 5 '16 at 10:06
  • $\begingroup$ @sammygerbil I don't think that makes it a bad question. It certainly made me think about tidal effects and realise that you can answer the question as to whether the Moon or Sun have a greater effect just by looking at the sea. Counterfactual questions can be interesting so long as the physics is not silly (or the silliness is contained). $\endgroup$ – tfb May 5 '16 at 19:49
  • $\begingroup$ @tbf : Tidal effects come under my qualification : "factors effecting changes in rate of rotation." $\endgroup$ – sammy gerbil May 5 '16 at 20:50
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I think the two big factors would be that the Earth would 'want to' become tidally-locked to the Moon and the Sun. The Moon would win here, which is easy to see because tides are caused more strongly by the Moon than by the Sun. So in due course the Earth would end up tidally-locked to the Moon with a rotation period which would be the same as a lunar month. The lunar month however would be shorter than it is now as angular momentum would have been transferred from the Moon to the Earth. This would take a very long time to happen: billions of years I think. There would also be some wobble in the day length due to the influence of solar tides.

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  • $\begingroup$ Note that as the tidal lock proceeds the Moon moves out and it's tidal strength drops. I don't know if that would effect the eventual balance, but it makes the off-hand observation that the Moon's tidal effect is stronger in this epoch less useful than it seems at first. $\endgroup$ – dmckee May 5 '16 at 14:35
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    $\begingroup$ @dmckee I think it moves in: it is moving out currently as Earth transfers AM to it, so as it transfers AM to Earth it will move in. $\endgroup$ – tfb May 5 '16 at 15:16
  • $\begingroup$ @tfb That is correct. The relative motion between Earth's surface and the Moon is in one direction now and would be the other way if Earth didn't rotate, so all the tidal effects reverse direction as well. $\endgroup$ – Asher May 5 '16 at 19:43
  • $\begingroup$ @tfb This is exactly what I was asking. Thanks for pointing out the fact that Lunar month will get shorter than it is now. Do you think that weak forces from other planets may affect the earth's rotation too? $\endgroup$ – Shabaz Khan May 9 '16 at 8:41
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    $\begingroup$ @ShabazKhan Yes, but I think the effects would be rather (probably very) small. Probably there are solar-system dynamics people who can quantify this sort of thing $\endgroup$ – tfb May 9 '16 at 8:53
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In an isotropic space (which means it's equal in all directions), Noether's theorem tells us that angular momentum is conserved. For the Earth this means that it keeps the angular momentum that it has now. This can be calculated from $$L = I \omega$$ where $I$ is the moment of inertia and $\omega$ the angular speed. You'll see that if $L$ remains constant (and $I$ of course too, since it only depends on the distribution of mass in the body), also $\omega$ will stay constant. That means that in effect, Earth won't stop spinning.

To change the angular momentum you need to apply a torque, for example from a (non-frontal) collision with another object. $$M = \dot{L}$$

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  • $\begingroup$ Let we applied that torque. Will it restart rotating again after that? $\endgroup$ – Anubhav Goel May 5 '16 at 15:13
  • $\begingroup$ Of course, applying a torque implies that the angular speed changes, and (if you apply it in the right direction) it will rotate faster then afterwards. $\endgroup$ – ahemmetter May 5 '16 at 15:17
  • $\begingroup$ I meant we applied external torque to stop planet. Will outer celestial bodies re-apply a torque, to re-rotate planet. $\endgroup$ – Anubhav Goel May 5 '16 at 16:11

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