<p>David Hammen's excellent answer covers the key pieces, but the original question also has this part:</p>
<blockquote>
<p>The literal claim (as it appears in headlines) seems to make no sense given a basic understanding of friction and conservation of angular momentum.</p>
</blockquote>
<p>This isn't all that simple, actually. What is being conserved is the angular momentum of the Earth as a whole, not of the Earth core in isolation. As a consequence, the Earth's inner core can slow down a little bit if the rest of the Earth accelerates a little bit, and the other way around. Of course there needs to be a torque between the two, but that is not hard to fathom: We <em>know</em> that the outer core is moving rapidly (on the order of centimeters per second), due to thermal and/or chemical convection, and so it is not hard to imagine that there is friction between inner and outer core.</p>
<p>This is not so different from the fact that the solid Earth does not rotate at a fixed rate, but that days are fractions of seconds longer or shorter because the average east-west speed of the atmosphere changes from day to day; because the <em>overall</em> angular momentum must stay the same, an accelerating atmosphere implies a slowing down solid Earth, and the other way around. The same kind of thing happens (on slightly longer time scales) with ocean currents that take on more or less angular momentum and, as a consequence, have an influence on the rotation rate of the solid earth.</p>

CC BY-SA 4.0
2023-01-26T14:55:05.503Z
There are seven rotating parts of the Earth: The inner core, outer core, mantle, crust, oceans, cryosphere, and atmosphere. The mantle and crust rotate together over the span of hundreds of thousands of years. All of these parts collectively more or less conserve angular momentum over the span of a few centuries. The gradual transfer of angular momentum to the Moon's orbit is very, very gradual; it takes hundreds to thousands of years to observe this transfer. In the short term (a few centuries is short term), the parts exchange angular momentum with one another.
David Hammen
CC BY-SA 4.0
2023-01-26T17:28:30.890Z
Underlying my statement was the recognition of a few facts and principles of physics, admittedly distilled down somewhat. Bottom line: what would seem in fact to be a subtle phenomenon (mundane physics, but interesting geologically) has been exaggerated to an absurd and nonsensically impossible extreme in the popular media.
Anthony X
CC BY-SA 4.0
2023-01-26T04:12:15.850Z
2023-01-26T04:12:15.850Z
Wolfgang Bangerth
<p>The mantle rotates about 131850 degrees per year. The actual assertion is that the inner core cycles between rotating about 131851 degrees per year versus 131849 degrees per year over the course of 70 year cycle. The paper was only published yesterday, so the scientific consensus is not there yet. The scientific consensus is not there yet on work done by the same authors eight years ago.</p>
<p>The inner core rotates in the same direction as do the mantle and crust, and rotates at almost at the same rate as the mantle and crust. The claim is that this rate varies by a tiny fraction compared to the mantle's rotation rate, plus or minus a degree or so per year compared to the mantle (but keep in mind that is one degree out of 131850 degrees). It does not switch directions.</p>

CC BY-SA 4.0
2023-01-25T10:14:16.577Z
So "backwards" here means it's either slightly slower or slightly faster compared to the mantle?
gerrit
CC BY-SA 4.0
2023-01-25T10:33:39.483Z
@gerrit Exactly. With respect to the rotating mantle, the inner core appears to move forward or backward by up to a degree per year. (This remains to be confirmed.) With respect to the "fixed stars" (one revolution per sidereal day) or with respect to the Sun (one revolution per mean solar day), the various parts of the Earth all rotate at almost the same rate.
David Hammen
CC BY-SA 4.0
2023-01-25T15:15:48.570Z
Sure, go ahead, bust the bubble of this headline-reader.  See if I care!  ;-)
Scott Sauyet
CC BY-SA 4.0
2023-01-25T15:34:26.103Z
And if I understand correctly, proposed explanations for the "back-and-forth" involve exchange of angular momentum between the solid and liquid layers of the core (mantle apparently outside the influence of this exchange), convection of the liquid in the core, "coriolis forces" due to convection-driven motion within the general rotation and forces/torques generated by the magnetic fields created due to movement of the liquid core.
Anthony X
CC BY-SA 4.0
2023-01-25T15:50:26.310Z
@AnthonyX Maybe. The mechanism, if a mechanism is needed, is not known. What is being observed is a tiny variance between the rotation rates of the mantle+crust and the inner core. It might be that what is being observed is due to multidecadal oscillations in the rotation rate of the mantle+crust while the inner core rotates at a much smoother rate. We already know that climate changes the rotation rate of the mantle+crust in the form of the easily observed seasonal oscillations in the Earth's rotation rate.
David Hammen
CC BY-SA 4.0
2023-01-25T15:51:30.860Z
There are multidecadal climate oscillations and multidecadal rotational oscillations, and these appear to be correlated.
David Hammen
CC BY-SA 4.0
2023-01-26T11:55:49.537Z
@gerrit Another user provided a really nice analogy on a related question. Suppose you're driving at 65 mph along a highway and you pass a vehicle going in the same direction but only traveling at 60 mph. Is that other vehicle driving backwards? (That was rhetorical; the answer is obviously "no".)
David Hammen
CC BY-SA 4.0
2023-01-26T14:09:51.747Z
Wow, this is being ***wildly*** misreported in the popular press and media than.
RBarryYoung
CC BY-SA 4.0
2023-01-26T14:15:42.180Z
@RBarryYoung That's standard operating procedure (SOP) for the lay media regarding reporting on science and technology. It's typically quite atrocious, regardless of the subject. Keep in mind that a key reason young adults major in journalism is so that they do not have to take any classes on mathematics or science. And if they do have to take such a class, there's always Physics For Poets (seriously; this is the actual name of a college level class at many colleges and universities in the US).
David Hammen
CC BY-SA 4.0
2023-01-26T14:20:49.633Z
@DavidHammen I am actually quite familiar with that, but this is a particularly egregious instance.
RBarryYoung
CC BY-SA 4.0
2023-01-26T14:37:56.400Z
@RBarryYoung Are you familiar with A/B headline testing? Journalists and editors are. That's mathematics and science that journalists and editors have learned. The calculation is simple: Make the headline as splashy as possible so as to attract more eyeballs, and then do the same with the article. More eyeballs = more money.
David Hammen
CC BY-SA 4.0
2023-01-27T11:03:20.707Z
Is this comparable to the apparent [retrograde motion of Mars](https://mars.nasa.gov/all-about-mars/night-sky/retrograde/), as seen from Earth?
Eric Duminil
CC BY-SA 4.0
2023-01-27T17:23:19.177Z
@EricDuminil: A better analogy might be driving next to another car at 65mph on the highway, and then realizing the other car is actually going 65mph and pulling ahead, so you start screaming because "you're now going backwards on the highway"
Mooing Duck
CC BY-SA 4.0
2023-01-25T03:32:48.573Z
2023-01-25T03:32:48.573Z
David Hammen
<p>Recent news articles related to <a href="https://www.nature.com/articles/s41561-022-01112-z.epdf" rel="noreferrer">this paper</a> report the claim that Earth's solid inner core sometimes rotates backwards. The literal claim (as it appears in headlines) seems to make no sense given a basic understanding of friction and conservation of angular momentum. The matter is discussed in <a href="https://earthscience.stackexchange.com/questions/402/why-does-earths-outer-core-rotate-in-the-opposite-direction-to-the-inner-core">this SE question</a>, but there is also the matter of multi-decadal cycles. What is the actual assertion? Is the inner core thought to be cycling between prograde and retrograde rotation, or is it something more subtle, like a wobble, sometimes slightly leading and sometimes slightly lagging, and at what amplitude? Is it a matter of &quot;gaining&quot; or &quot;losing&quot; a few degrees over some number of years (so would only be rotating &quot;backwards&quot; in the frame of reference of the crust, not non-rotating space)?</p>

CC BY-SA 4.0
2023-01-25T03:56:55.340Z
A/B headline testing has shown over and over that a more catchy headline attracts more viewers to read an article. More views means more money in the bank for the publisher. What's catchier: That the Earth's inner core is reversing direction, or that its rotation rate perhaps slowly varies by about a thousandth of a percent? The StackExchange network uses a form of this in its list of Hot Network Questions.More eyeballs = more money.
David Hammen
CC BY-SA 4.0
2023-01-25T04:05:15.557Z
@DavidHammen So it would seem. When I first saw the headline, I thought the news team was being punked... evidently they distilled the actual claims down to the simplest possible statement, and then turned it up to 11.
Anthony X
CC BY-SA 4.0
2023-01-25T14:38:00.020Z
Should we say "backwards" or would it be better and more exact to say "the other way"?
RedSonja
CC BY-SA 4.0
2023-01-25T15:32:12.717Z
@RedSonja It would be better to say rotates slightly slower or slightly faster, and by slightly I mean less than a hundredth of a percent. This phraseology caused confusion when it first came out about eight years ago. But it sure did get eyeballs on the articles!
David Hammen
CC BY-SA 4.0
2023-01-25T15:58:52.723Z
@AnthonyX This article just came out two days ago. It is usually highly erroneous to take any newly published scientific article as fact, even if it's from a highly respected one such as *Nature Geoscience*. Getting something published in a peer reviewed journal is where science starts.
David Hammen
CC BY-SA 4.0
2023-01-25T16:29:15.543Z
Actual current CNN headline: "Earth’s inner core may have stopped turning and could go into reverse, study suggests"
Anthony X
CC BY-SA 4.0
2023-01-25T17:03:29.790Z
Great question.  I saw this and just thought "huh?"  It's like saying that cars you pass on a divided highway are driving backwards.  So much sensationalist nonsense out there in the media.  It's exhausting.
JimmyJames
CC BY-SA 4.0
2023-01-25T18:38:12.017Z
@AnthonyX Those stupid headlines are **everywhere**.
David Hammen
CC BY-SA 4.0
2023-01-25T18:41:20.390Z
@JimmyJames That is a perfect analogy.
David Hammen
CC BY-SA 4.0
2023-01-26T05:05:24.733Z
Reminders that the media doesn't always have truth or their viewers best in mind?  Seems a good idea to note which sources can't be bothered to get it right (either intentionally, or because they're just repeating what they heard).  Using semantics to "technically" be right when knowing it's heavily misleading people (and probably alarming quite a few people, given past disaster movies and such?) counts the same.
JeopardyTempest
CC BY-SA 4.0
2023-01-26T14:08:15.813Z
I saw this in the news yesterday and had much the same reaction:  the Earth's inner core has (very) approximately the same angular momentum as the Moon, plus is aligned with the mantle's rotation and nearly matches it in a highly viscous medium (the outer core) that is *also* rotating with both the mantle and the inner core.  How the heck can you just "occasionally reverse" that much angular momentum?!?
RBarryYoung
CC BY-SA 4.0
2023-01-26T17:37:05.307Z
@JeopardyTempest All very well for "most" commercial media, but news organizations (like CNN) offer themselves as bastions of accountability and assert that they actively verify claims and fact-check. Seems the self-declared "source of truth" is falling a bit short.
Anthony X
CC BY-SA 4.0
2023-01-26T21:20:45.323Z
In a CNN on-camera on-set interview (video was embedded [here](https://www.cnn.com/2023/01/25/world/earth-core-turning-scli-scn-intl/index.html) ), [Michio Kaku](https://en.wikipedia.org/wiki/Michio_Kaku) did nothing to mitigate the misrepresentation of the "spinning backwards" headline; and almost seemed to be deliberately (if subtly) trying to preserve it.
Anthony X
CC BY-SA 4.0
2023-01-27T18:24:38.463Z
@AnthonyX Michio Kaku, the [professor of woo](https://rationalwiki.org/wiki/Michio_Kaku)? Color me surprised. (Not really).
David Hammen
CC BY-SA 4.0
2023-01-27T20:03:03.723Z
@DavidHammen Someone who apparently has legitimate credentials from a respectable institution and ought to know better. Whatever his true leanings, he should be living up to those credentials instead of woo-ing out for a little face-time with a national/global audience.
Anthony X
CC BY-SA 4.0
2023-03-08T21:53:00.993Z
At some point, hopefully, everybody who consumes "science news" will come to realize that 99% of it is just as void of substance as 99% of all other news and current event reporting. Sure, there is the occasional actually relevant/ accurate/ informative/ unbiased article/ report that somebody can point to as evidence to the contrary, but 99% of the time if you start looking into the actual underlying data/ history/ assumptions/ characterizations you find out that is a "lie" (or _just inaccurate enough_ when it comes the really relevant points if you want to be generous).
RIanGillis
<earth-rotation>
Does Earth's core actually turn "backwards" at times?
Anthony X
# Does Earth's core actually turn "backwards" at times?

(Asked by: username_0 on 2023-01-25)

Recent news articles related to [this paper](https://www.nature.com/articles/s41561-022-01112-z.epdf) report the claim that Earth's solid inner core sometimes rotates backwards. The literal claim (as it appears in headlines) seems to make no sense given a basic understanding of friction and conservation of angular momentum. The matter is discussed in [this SE question](https://earthscience.stackexchange.com/questions/402/why-does-earths-outer-core-rotate-in-the-opposite-direction-to-the-inner-core), but there is also the matter of multi-decadal cycles. What is the actual assertion? Is the inner core thought to be cycling between prograde and retrograde rotation, or is it something more subtle, like a wobble, sometimes slightly leading and sometimes slightly lagging, and at what amplitude? Is it a matter of "gaining" or "losing" a few degrees over some number of years (so would only be rotating "backwards" in the frame of reference of the crust, not non-rotating space)?

**Tags:** earth-rotation

> **Comments:**
> **username_1**: A/B headline testing has shown over and over that a more catchy headline attracts more viewers to read an article. More views means more money in the bank for the publisher. What's catchier: That the Earth's inner core is reversing direction, or that its rotation rate perhaps slowly varies by about a thousandth of a percent? The StackExchange network uses a form of this in its list of Hot Network Questions.More eyeballs = more money.  
> **username_0**: @username_1 So it would seem. When I first saw the headline, I thought the news team was being punked... evidently they distilled the actual claims down to the simplest possible statement, and then turned it up to 11.  
> **username_2**: Should we say "backwards" or would it be better and more exact to say "the other way"?  
> **username_1**: @username_2 It would be better to say rotates slightly slower or slightly faster, and by slightly I mean less than a hundredth of a percent. This phraseology caused confusion when it first came out about eight years ago. But it sure did get eyeballs on the articles!  
> **username_1**: @username_0 This article just came out two days ago. It is usually highly erroneous to take any newly published scientific article as fact, even if it's from a highly respected one such as *Nature Geoscience*. Getting something published in a peer reviewed journal is where science starts.  
> **username_0**: Actual current CNN headline: "Earth’s inner core may have stopped turning and could go into reverse, study suggests"  
> **username_3**: Great question.  I saw this and just thought "huh?"  It's like saying that cars you pass on a divided highway are driving backwards.  So much sensationalist nonsense out there in the media.  It's exhausting.  
> **username_1**: @username_0 Those stupid headlines are **everywhere**.  
> **username_1**: @username_3 That is a perfect analogy.  
> **username_4**: Reminders that the media doesn't always have truth or their viewers best in mind?  Seems a good idea to note which sources can't be bothered to get it right (either intentionally, or because they're just repeating what they heard).  Using semantics to "technically" be right when knowing it's heavily misleading people (and probably alarming quite a few people, given past disaster movies and such?) counts the same.  
> **username_5**: I saw this in the news yesterday and had much the same reaction:  the Earth's inner core has (very) approximately the same angular momentum as the Moon, plus is aligned with the mantle's rotation and nearly matches it in a highly viscous medium (the outer core) that is *also* rotating with both the mantle and the inner core.  How the heck can you just "occasionally reverse" that much angular momentum?!?  
> **username_0**: @username_4 All very well for "most" commercial media, but news organizations (like CNN) offer themselves as bastions of accountability and assert that they actively verify claims and fact-check. Seems the self-declared "source of truth" is falling a bit short.  
> **username_0**: In a CNN on-camera on-set interview (video was embedded [here](https://www.cnn.com/2023/01/25/world/earth-core-turning-scli-scn-intl/index.html) ), [Michio Kaku](https://en.wikipedia.org/wiki/Michio_Kaku) did nothing to mitigate the misrepresentation of the "spinning backwards" headline; and almost seemed to be deliberately (if subtly) trying to preserve it.  
> **username_1**: @username_0 Michio Kaku, the [professor of woo](https://rationalwiki.org/wiki/Michio_Kaku)? Color me surprised. (Not really).  
> **username_0**: @username_1 Someone who apparently has legitimate credentials from a respectable institution and ought to know better. Whatever his true leanings, he should be living up to those credentials instead of woo-ing out for a little face-time with a national/global audience.  
> **username_6**: At some point, hopefully, everybody who consumes "science news" will come to realize that 99% of it is just as void of substance as 99% of all other news and current event reporting. Sure, there is the occasional actually relevant/ accurate/ informative/ unbiased article/ report that somebody can point to as evidence to the contrary, but 99% of the time if you start looking into the actual underlying data/ history/ assumptions/ characterizations you find out that is a "lie" (or _just inaccurate enough_ when it comes the really relevant points if you want to be generous).  

---

## 2 answers

### Answer by username_1 on 2023-01-25. Score: 26 | Accepted answer

The mantle rotates about 131850 degrees per year. The actual assertion is that the inner core cycles between rotating about 131851 degrees per year versus 131849 degrees per year over the course of 70 year cycle. The paper was only published yesterday, so the scientific consensus is not there yet. The scientific consensus is not there yet on work done by the same authors eight years ago.

The inner core rotates in the same direction as do the mantle and crust, and rotates at almost at the same rate as the mantle and crust. The claim is that this rate varies by a tiny fraction compared to the mantle's rotation rate, plus or minus a degree or so per year compared to the mantle (but keep in mind that is one degree out of 131850 degrees). It does not switch directions.

> **Comments:**
> **username_8**: So "backwards" here means it's either slightly slower or slightly faster compared to the mantle?  
> **username_1**: @username_8 Exactly. With respect to the rotating mantle, the inner core appears to move forward or backward by up to a degree per year. (This remains to be confirmed.) With respect to the "fixed stars" (one revolution per sidereal day) or with respect to the Sun (one revolution per mean solar day), the various parts of the Earth all rotate at almost the same rate.  
> **username_9**: Sure, go ahead, bust the bubble of this headline-reader.  See if I care!  ;-)  
> **username_0**: And if I understand correctly, proposed explanations for the "back-and-forth" involve exchange of angular momentum between the solid and liquid layers of the core (mantle apparently outside the influence of this exchange), convection of the liquid in the core, "coriolis forces" due to convection-driven motion within the general rotation and forces/torques generated by the magnetic fields created due to movement of the liquid core.  
> **username_1**: @username_0 Maybe. The mechanism, if a mechanism is needed, is not known. What is being observed is a tiny variance between the rotation rates of the mantle+crust and the inner core. It might be that what is being observed is due to multidecadal oscillations in the rotation rate of the mantle+crust while the inner core rotates at a much smoother rate. We already know that climate changes the rotation rate of the mantle+crust in the form of the easily observed seasonal oscillations in the Earth's rotation rate.  
> **username_1**: There are multidecadal climate oscillations and multidecadal rotational oscillations, and these appear to be correlated.  
> **username_1**: @username_8 Another user provided a really nice analogy on a related question. Suppose you're driving at 65 mph along a highway and you pass a vehicle going in the same direction but only traveling at 60 mph. Is that other vehicle driving backwards? (That was rhetorical; the answer is obviously "no".)  
> **username_5**: Wow, this is being ***wildly*** misreported in the popular press and media than.  
> **username_1**: @username_5 That's standard operating procedure (SOP) for the lay media regarding reporting on science and technology. It's typically quite atrocious, regardless of the subject. Keep in mind that a key reason young adults major in journalism is so that they do not have to take any classes on mathematics or science. And if they do have to take such a class, there's always Physics For Poets (seriously; this is the actual name of a college level class at many colleges and universities in the US).  
> **username_5**: @username_1 I am actually quite familiar with that, but this is a particularly egregious instance.  
> **username_1**: @username_5 Are you familiar with A/B headline testing? Journalists and editors are. That's mathematics and science that journalists and editors have learned. The calculation is simple: Make the headline as splashy as possible so as to attract more eyeballs, and then do the same with the article. More eyeballs = more money.  
> **username_10**: Is this comparable to the apparent [retrograde motion of Mars](https://mars.nasa.gov/all-about-mars/night-sky/retrograde/), as seen from Earth?  
> **username_11**: @username_10: A better analogy might be driving next to another car at 65mph on the highway, and then realizing the other car is actually going 65mph and pulling ahead, so you start screaming because "you're now going backwards on the highway"  

---

### Answer by username_7 on 2023-01-26. Score: 10

username_1 username_1's excellent answer covers the key pieces, but the original question also has this part:

> The literal claim (as it appears in headlines) seems to make no sense given a basic understanding of friction and conservation of angular momentum.

This isn't all that simple, actually. What is being conserved is the angular momentum of the Earth as a whole, not of the Earth core in isolation. As a consequence, the Earth's inner core can slow down a little bit if the rest of the Earth accelerates a little bit, and the other way around. Of course there needs to be a torque between the two, but that is not hard to fathom: We *know* that the outer core is moving rapidly (on the order of centimeters per second), due to thermal and/or chemical convection, and so it is not hard to imagine that there is friction between inner and outer core.

This is not so different from the fact that the solid Earth does not rotate at a fixed rate, but that days are fractions of seconds longer or shorter because the average east-west speed of the atmosphere changes from day to day; because the *overall* angular momentum must stay the same, an accelerating atmosphere implies a slowing down solid Earth, and the other way around. The same kind of thing happens (on slightly longer time scales) with ocean currents that take on more or less angular momentum and, as a consequence, have an influence on the rotation rate of the solid earth.

> **Comments:**
> **username_1**: There are seven rotating parts of the Earth: The inner core, outer core, mantle, crust, oceans, cryosphere, and atmosphere. The mantle and crust rotate together over the span of hundreds of thousands of years. All of these parts collectively more or less conserve angular momentum over the span of a few centuries. The gradual transfer of angular momentum to the Moon's orbit is very, very gradual; it takes hundreds to thousands of years to observe this transfer. In the short term (a few centuries is short term), the parts exchange angular momentum with one another.  
> **username_0**: Underlying my statement was the recognition of a few facts and principles of physics, admittedly distilled down somewhat. Bottom line: what would seem in fact to be a subtle phenomenon (mundane physics, but interesting geologically) has been exaggerated to an absurd and nonsensically impossible extreme in the popular media.  

---

