<p>The speed of rotation of Earth is controlled by its angular momentum. And the conservation of angular momentum is a very serious law of physics (perhaps even stricter than conservation of mass). So in the same way that for the Earth to lose mass, that mass have to go somewhere. For the Earth to lose angular momentum, it'd have to go somewhere.</p>

<p>Earth's rotation can vary within a small range by rearranging the distribution of mass (changing the <a href="https://en.wikipedia.org/wiki/Moment_of_inertia" rel="nofollow noreferrer">Moment of inertia</a>). That's why on ice ages the day was a few seconds longer, because large amounts of water that used to be on the ocean was piled in large ice sheets, so like in the classic example of a rotating ice skater, by moving mass (ice on Earth or the arms of the skater) away from the rotation axis, slows down the rotation speed.</p>

<p>The only other way for the Earth to significantly change its rotation speed is by losing <a href="https://en.wikipedia.org/wiki/Angular_momentum" rel="nofollow noreferrer">angular momentum</a>, and any imaginable event that could change significantly Earth's angular momentum over a period of 15 years would be so catastrophic that the change in rotation speed would be secondary. Such event would have to be something like the collision with a massive asteroid, the fly-by of planet-size celestial body, or the ejection of large amounts of mass by the explosion of gigantic natural nuclear reactors (all things that might have happened very early in Earth's history).</p>

<p>There are however processes at work today that are slowly stealing Earth's angular momentum. The most important one is tides. Tides produce bulges of water around Earth that rotate slower than Earth, as in this picture (from <a href="https://en.wikipedia.org/wiki/Tide" rel="nofollow noreferrer">Wikipedia</a>):
<a href="https://i.sstatic.net/0pRS7.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/0pRS7.png" alt="enter image description here"></a></p>

<p>Therefore, to keep the bulges in place the water have to be constantly flowing into those bulges moving against Earth's rotation and slowing it down by friction. The lost angular momentum goes mostly to the moon that accelerate on its orbit, therefore moving away from us <a href="https://www.scientificamerican.com/article/earth-rotation-summer-solstice/" rel="nofollow noreferrer">3.8 centimeters per year</a>.</p>

<p>This process makes the day longer <a href="https://www.scientificamerican.com/article/earth-rotation-summer-solstice/" rel="nofollow noreferrer">1.7 milliseconds per century</a>, something that is not a lot even at geological times scales. It is equivalent to 17 seconds per million of years. But even that small amount adds up to almost one full hour since the start of the Jurassic (201 Ma ago). But the change is so slow that animals would have plenty of time to adapt.</p>

<p>So I would say that any explanation of how the dinosaurs supported their own weight have nothing to do with changes in Earth rotation speed. Also, I think you are overestimating the impact of the rotation speed on the perceived weight. If we make the numbers, the centripetal acceleration due to rotation at the equator is
<span class="math-container">$$a_\mathrm c=\frac{v^2}r=\frac{(462\ \mathrm{m/s})^2}{6\,378\,100\ \mathrm m}=0.0336\ \mathrm{m/s^2}$$</span></p>

<p>Equivalent to a 0.3% of the acceleration of gravity. Therefore, even if the Earth were to stop completely, a dinosaur of 1000 kg would feel only ~3 kg heavier.</p>

<p>Said that, if we go to the very hypothetical case you say, a slow down wouldn't make much of a difference (ignoring any environmental impacts of having a one-year-long day).</p>

<p>And for how fast the can Earth spin and support life? I think nobody can really answer that question, as we don't know what conditions life can endure. But if you make the Earth spin faster, the centripetal acceleration grows as the speed squared, therefore it wouldn't take too much to make the centripetal acceleration equal to the acceleration of gravity, in which case objects in the equator would be weightless. In that case, Earth would lose its atmosphere and oceans, and further speed-up would start ejecting rocks into space. By rearranging the above equation we can get that critical speed, and it would be:
<span class="math-container">$$v_\mathrm c=\sqrt{rg}=\sqrt{6\,378\,100\mathrm m\times9.8\ \mathrm{m/s^2}}=7\,906\ \mathrm{m/s}$$</span></p>

<p>That is 17 times the current speed, equivalent to a day of one hour and 24 minutes.</p>

<p>Finally, for your  question: What is the fastest the Earth has ever spun? It depends. If we consider it after the main change in angular momentum associated to the formation of the Moon <a href="https://en.wikipedia.org/wiki/Moon#Formation" rel="nofollow noreferrer">4.51 billion years ago</a>, we can just extrapolate those 17 seconds per million years (this is not the most precise as there were no oceans then, but it is a fair first approximation), and we get about 21 hours, so that the day at that time was only about <strong>three hours</strong> long.
Careful calculations for the length of the day in the proto-Earth before the formation of the moon lead to a day of 5.8 hours (see <a href="https://arxiv.org/pdf/1001.4243.pdf" rel="nofollow noreferrer">this paper</a>, line 358)</p>

<p>As an anecdotal note, the fact that this number is quite close to the critical velocity, have made some people think that <a href="https://arxiv.org/pdf/1001.4243.pdf" rel="nofollow noreferrer">the explosion of a natural nuclear reactor could have been enough to give the extra kick to eject the whole Moon from the surface</a>. Something that would explain why the composition of the Moon is so remarkably similar to that of Earth by recent analysis, challenging the <a href="https://en.wikipedia.org/wiki/Giant-impact_hypothesis" rel="nofollow noreferrer">theory of formation by a collision</a> with "<a href="https://en.wikipedia.org/wiki/Theia_(planet)" rel="nofollow noreferrer">Theia</a>" a hypothetical Mars-sized body. Although, there are other recent theories with an alternative solution to this problem, such as the <a href="https://www.ted.com/talks/sarah_t_stewart_where_did_the_moon_come_from_a_new_theory#t-658680" rel="nofollow noreferrer">synestia theory</a>.</p>

<p>Finally, it would be wrong to restrict all possible changes of rotation speed only to surface processes or interactions to other celestial bodies. It could also be possible that mass redistribution in the Earth's interior could affect rotation speed. Events like the <a href="https://en.wikipedia.org/wiki/Iron_catastrophe" rel="nofollow noreferrer">iron catastrophe</a> would have increased the rotation speed, and other mass redistribution events could have happened or might be happening, but this is just speculation. Studying such processes is extremely difficult, we know very little about the dynamics of the Earth interior, and it will be extremely hard to prove or falsify theories concerning such internal mass redistribution events. <a href="https://worldbuilding.stackexchange.com/a/139530/61556">This answers</a> and and others in the same question have some ideas of how the angular velocity of a planet could change.</p>

CC BY-SA 3.0
2018-02-24T07:04:18.017Z
@Muze No, by the time of the dinosaur as I said the day was about one hour shorter, so 23 hours day. And why you think the meteorite impact changed the magma flow??
Camilo Rada
CC BY-SA 3.0
2018-02-24T07:06:58.790Z
https://worldbuilding.stackexchange.com/questions/102787/can-the-pitch-and-speed-of-an-asteroid-cause-a-less-catastrophic-results
Muze
CC BY-SA 3.0
2018-02-24T15:09:26.463Z
Mind you, it _would_ explain why all fossilised creatures appear to have been lying down when they died... ;)
Lightness Races in Orbit
CC BY-SA 3.0
2018-02-24T16:42:01.077Z
@Muze: To be clear, I was joking. They appear to have been lying down because people extract and show fossils laterally (or you wouldn't be able to see anything)
Lightness Races in Orbit
CC BY-SA 3.0
2018-02-24T18:11:19.360Z
@Muze No it is not absolutely known that their days were 23 hours, are just models, but those models tend to be very precise. The only hard data I know goes back to eclipses recorded by ancient Chinese, that allows to constrain the actual changes in the day duration. Your comment looks more like another question than a clarification, so better post another question and somebody will try to answer it.
Camilo Rada
CC BY-SA 3.0
2018-02-24T20:59:16.393Z
@CamiloRada  does the models include asteroids or comets in the equation?
Muze
CC BY-SA 3.0
2018-02-24T21:05:56.413Z
@Muze the part of ice asteroids, etc.
Models probably include asteroids and comets, but in an statistical way, because most impact craters have been erased by tectonics, and also angular momentum transfers don't happen only on impacts. Close fly-by can also give/take angular momentum, and those leave no trace at all.
Camilo Rada
CC BY-SA 3.0
2018-02-24T21:34:48.070Z
@CamiloRada my theory is that some passes through the crust into the magma and leaves less scaring than let say the moon.
Muze
CC BY-SA 3.0
2018-02-24T21:50:19.750Z
@Muze but that would be a global cataclysm, and the changes in angular momentum would be a minor problem for life compared with everything else that would happen.
Camilo Rada
CC BY-SA 3.0
2018-02-24T21:54:32.520Z
@CamiloRada most likely. I do like the passing body creating angular momentum scenario.
Muze
CC BY-SA 3.0
2018-02-25T01:20:35.210Z
A passing body doesn't create much angular momentum.   We know that days on the Earth were about 23 hours 100 million years ago because we know the Earth's rotation changes very slowly.  Passing asteroids are close to insignificant.   Impacting asteroids have more effect, but need to be enormous to have much effect.   The lunar effect can be estimated.   There's no magic bullet that significantly changes the model.   4 billion years ago the Earth rotated much faster, but by the time the Dinos got here, not so much.  21-23 hour days from 250 to 66 mya.
userLTK
CC BY-SA 3.0
2018-02-25T03:55:18.183Z
@userLTK A magnetic planet passing close could pull the planet right out of orbit so getting spun by one would be more likely, but there is many unlikely scenarios that could change the Earth rotation. What do you think about why dinosaurs where so big if gravity was not much less then?
Muze
CC BY-SA 3.0
2018-02-25T04:45:59.480Z
Getting spun is less likely, not more likely by a passing planet because planets tend to be round and evenly distrusted.  Some spin by tidal bulges and gravitational variations is possible, but it's small.   By comparison, moving the entire planet is easy because that's a matter of the entire mass, not just an imbalance in the mass.    A collision could change the rotation much more than a pass and even that, the colliding object would have to be very large.
userLTK
CC BY-SA 3.0
2018-02-25T04:48:13.830Z
@Muze and the dinosaurs being big had nothing to do with gravity which was basically the same.   They were big because of evolution and being big helped them survive.
userLTK
CC BY-SA 3.0
2018-03-04T17:48:48.023Z
When the moon was closer the tidal effects would have been greater and thus the loss of angular momentum higher. I was wondering if this was considered when winding the clock back on earth's rotation period.
Jack R. Woods
CC BY-SA 3.0
2018-03-04T18:27:37.190Z
@JackR.Woods That is right and would definitely complicate the calculation, tides are complex and it depends on the rigidity of the body, amount of surface water, etc. I didn't took that into account in my simple calculations. However, if you assume Earth and Moon were together at one point, just adding up the masses and angular momentums would give you the rotation velocity. Look at the paper I mention in the second to last paragraph.
Camilo Rada
CC BY-SA 4.0
2019-04-12T19:48:39.747Z
@Muze I made some minor additions. Have a look.
Camilo Rada
CC BY-SA 4.0
2019-04-12T20:11:04.750Z
Looks great....
Muze
CC BY-SA 4.0
2018-02-24T06:50:20.710Z
2019-04-12T19:46:50.210Z
2019-04-12T19:46:50.210Z
Camilo Rada
<p>How fast can the Earth rotate and support life?</p>

<p>In prehistoric times, dinosaurs were so massive that archeologists wonder how they were not crushed under their own weight. Could a faster spinning Earth allow larger animals to exist?</p>

<p>Could the slowing of Earth's rotation cause extinction among the larger animals but not the smaller animals?</p>

<p><a href="https://physics.stackexchange.com/questions/278117/how-fast-can-the-earth-spin-and-support-life">https://physics.stackexchange.com/questions/278117/how-fast-can-the-earth-spin-and-support-life</a></p>

<p>*Ignoring any environmental impacts</p>

<atmosphere><earth-history><planetary-science><earth-rotation><gravity>
What is the fastest the Earth has ever spun?
Muze
# What is the fastest the Earth has ever spun?

(Asked by: username_0 on 2018-02-24)

How fast can the Earth rotate and support life?

In prehistoric times, dinosaurs were so massive that archeologists wonder how they were not crushed under their own weight. Could a faster spinning Earth allow larger animals to exist?

Could the slowing of Earth's rotation cause extinction among the larger animals but not the smaller animals?

<https://physics.stackexchange.com/questions/278117/how-fast-can-the-earth-spin-and-support-life>

\*Ignoring any environmental impacts

**Tags:** atmosphere, earth-history, planetary-science, earth-rotation, gravity


---

## 1 answers

### Answer by username_1 on 2018-02-24. Score: 24 | Accepted answer

The speed of rotation of Earth is controlled by its angular momentum. And the conservation of angular momentum is a very serious law of physics (perhaps even stricter than conservation of mass). So in the same way that for the Earth to lose mass, that mass have to go somewhere. For the Earth to lose angular momentum, it'd have to go somewhere.

Earth's rotation can vary within a small range by rearranging the distribution of mass (changing the [Moment of inertia](https://en.wikipedia.org/wiki/Moment_of_inertia)). That's why on ice ages the day was a few seconds longer, because large amounts of water that used to be on the ocean was piled in large ice sheets, so like in the classic example of a rotating ice skater, by moving mass (ice on Earth or the arms of the skater) away from the rotation axis, slows down the rotation speed.

The only other way for the Earth to significantly change its rotation speed is by losing [angular momentum](https://en.wikipedia.org/wiki/Angular_momentum), and any imaginable event that could change significantly Earth's angular momentum over a period of 15 years would be so catastrophic that the change in rotation speed would be secondary. Such event would have to be something like the collision with a massive asteroid, the fly-by of planet-size celestial body, or the ejection of large amounts of mass by the explosion of gigantic natural nuclear reactors (all things that might have happened very early in Earth's history).

There are however processes at work today that are slowly stealing Earth's angular momentum. The most important one is tides. Tides produce bulges of water around Earth that rotate slower than Earth, as in this picture (from [Wikipedia](https://en.wikipedia.org/wiki/Tide)):
[![enter image description here](https://i.sstatic.net/0pRS7.png)](https://i.sstatic.net/0pRS7.png)

Therefore, to keep the bulges in place the water have to be constantly flowing into those bulges moving against Earth's rotation and slowing it down by friction. The lost angular momentum goes mostly to the moon that accelerate on its username_2, therefore moving away from us [3.8 centimeters per year](https://www.scientificamerican.com/article/earth-rotation-summer-solstice/).

This process makes the day longer [1.7 milliseconds per century](https://www.scientificamerican.com/article/earth-rotation-summer-solstice/), something that is not a lot even at geological times scales. It is equivalent to 17 seconds per million of years. But even that small amount adds up to almost one full hour since the start of the Jurassic (201 Ma ago). But the change is so slow that animals would have plenty of time to adapt.

So I would say that any explanation of how the dinosaurs supported their own weight have nothing to do with changes in Earth rotation speed. Also, I think you are overestimating the impact of the rotation speed on the perceived weight. If we make the numbers, the centripetal acceleration due to rotation at the equator is
$$a\_\mathrm c=\frac{v^2}r=\frac{(462\ \mathrm{m/s})^2}{6\,378\,100\ \mathrm m}=0.0336\ \mathrm{m/s^2}$$

Equivalent to a 0.3% of the acceleration of gravity. Therefore, even if the Earth were to stop completely, a dinosaur of 1000 kg would feel only ~3 kg heavier.

Said that, if we go to the very hypothetical case you say, a slow down wouldn't make much of a difference (ignoring any environmental impacts of having a one-year-long day).

And for how fast the can Earth spin and support life? I think nobody can really answer that question, as we don't know what conditions life can endure. But if you make the Earth spin faster, the centripetal acceleration grows as the speed squared, therefore it wouldn't take too much to make the centripetal acceleration equal to the acceleration of gravity, in which case objects in the equator would be weightless. In that case, Earth would lose its atmosphere and oceans, and further speed-up would start ejecting rocks into space. By rearranging the above equation we can get that critical speed, and it would be:
$$v\_\mathrm c=\sqrt{rg}=\sqrt{6\,378\,100\mathrm m\times9.8\ \mathrm{m/s^2}}=7\,906\ \mathrm{m/s}$$

That is 17 times the current speed, equivalent to a day of one hour and 24 minutes.

Finally, for your question: What is the fastest the Earth has ever spun? It depends. If we consider it after the main change in angular momentum associated to the formation of the Moon [4.51 billion years ago](https://en.wikipedia.org/wiki/Moon#Formation), we can just extrapolate those 17 seconds per million years (this is not the most precise as there were no oceans then, but it is a fair first approximation), and we get about 21 hours, so that the day at that time was only about **three hours** long.
Careful calculations for the length of the day in the proto-Earth before the formation of the moon lead to a day of 5.8 hours (see [this paper](https://arxiv.org/pdf/1001.4243.pdf), line 358)

As an anecdotal note, the fact that this number is quite close to the critical velocity, have made some people think that [the explosion of a natural nuclear reactor could have been enough to give the extra kick to eject the whole Moon from the surface](https://arxiv.org/pdf/1001.4243.pdf). Something that would explain why the composition of the Moon is so remarkably similar to that of Earth by recent analysis, challenging the [theory of formation by a collision](https://en.wikipedia.org/wiki/Giant-impact_hypothesis) with "[Theia](https://en.wikipedia.org/wiki/Theia_(planet))" a hypothetical Mars-sized body. Although, there are other recent theories with an alternative solution to this problem, such as the [synestia theory](https://www.ted.com/talks/sarah_t_stewart_where_did_the_moon_come_from_a_new_theory#t-658680).

Finally, it would be wrong to restrict all possible changes of rotation speed only to surface processes or interactions to other celestial bodies. It could also be possible that mass redistribution in the Earth's interior could affect rotation speed. Events like the [iron catastrophe](https://en.wikipedia.org/wiki/Iron_catastrophe) would have increased the rotation speed, and other mass redistribution events could have happened or might be happening, but this is just speculation. Studying such processes is extremely difficult, we know very little about the dynamics of the Earth interior, and it will be extremely hard to prove or falsify theories concerning such internal mass redistribution events. [This answers](https://worldbuilding.stackexchange.com/a/139530/61556) and and others in the same question have some ideas of how the angular velocity of a planet could change.

> **Comments:**
> **username_1**: @username_0 No, by the time of the dinosaur as I said the day was about one hour shorter, so 23 hours day. And why you think the meteorite impact changed the magma flow??  
> **username_0**: https://worldbuilding.stackexchange.com/questions/102787/can-the-pitch-and-speed-of-an-asteroid-cause-a-less-catastrophic-results  
> **username_2**: Mind you, it _would_ explain why all fossilised creatures appear to have been lying down when they died... ;)  
> **username_2**: @username_0: To be clear, I was joking. They appear to have been lying down because people extract and show fossils laterally (or you wouldn't be able to see anything)  
> **username_1**: @username_0 No it is not absolutely known that their days were 23 hours, are just models, but those models tend to be very precise. The only hard data I know goes back to eclipses recorded by ancient Chinese, that allows to constrain the actual changes in the day duration. Your comment looks more like another question than a clarification, so better post another question and somebody will try to answer it.  
> **username_0**: @username_1  does the models include asteroids or comets in the equation?  
> **username_1**: @username_0 the part of ice asteroids, etc. Models probably include asteroids and comets, but in an statistical way, because most impact craters have been erased by tectonics, and also angular momentum transfers don't happen only on impacts. Close fly-by can also give/take angular momentum, and those leave no trace at all.  
> **username_0**: @username_1 my theory is that some passes through the crust into the magma and leaves less scaring than let say the moon.  
> **username_1**: @username_0 but that would be a global cataclysm, and the changes in angular momentum would be a minor problem for life compared with everything else that would happen.  
> **username_0**: @username_1 most likely. I do like the passing body creating angular momentum scenario.  
> **username_3**: A passing body doesn't create much angular momentum.   We know that days on the Earth were about 23 hours 100 million years ago because we know the Earth's rotation changes very slowly.  Passing asteroids are close to insignificant.   Impacting asteroids have more effect, but need to be enormous to have much effect.   The lunar effect can be estimated.   There's no magic bullet that significantly changes the model.   4 billion years ago the Earth rotated much faster, but by the time the Dinos got here, not so much.  21-23 hour days from 250 to 66 mya.  
> **username_0**: @username_3 A magnetic planet passing close could pull the planet right out of username_2 so getting spun by one would be more likely, but there is many unlikely scenarios that could change the Earth rotation. What do you think about why dinosaurs where so big if gravity was not much less then?  
> **username_3**: Getting spun is less likely, not more likely by a passing planet because planets tend to be round and evenly distrusted.  Some spin by tidal bulges and gravitational variations is possible, but it's small.   By comparison, moving the entire planet is easy because that's a matter of the entire mass, not just an imbalance in the mass.    A collision could change the rotation much more than a pass and even that, the colliding object would have to be very large.  
> **username_3**: @username_0 and the dinosaurs being big had nothing to do with gravity which was basically the same.   They were big because of evolution and being big helped them survive.  
> **username_4**: When the moon was closer the tidal effects would have been greater and thus the loss of angular momentum higher. I was wondering if this was considered when winding the clock back on earth's rotation period.  
> **username_1**: @JackR.username_4 That is right and would definitely complicate the calculation, tides are complex and it depends on the rigidity of the body, amount of surface water, etc. I didn't took that into account in my simple calculations. However, if you assume Earth and Moon were together at one point, just adding up the masses and angular momentums would give you the rotation velocity. Look at the paper I mention in the second to last paragraph.  
> **username_1**: @username_0 I made some minor additions. Have a look.  
> **username_0**: Looks great....  

---

