<blockquote>
  <p>If a giant object would approach the Earth, would Earth start to turn faster or slower while the object starts to become nearer and nearer?</p>
</blockquote>

<p><strong>tl;dr:</strong> Probably will slow it down a small amount, but may induce a wobble that make a simple answer impossible without further details.</p>

<p>The problem is complicated. I'll leave a short answer.</p>

<p>The question is carefully written in (at least two) ways. It is agnostic to the size of the effect, instead asking only about its sign. So arguments that the effects are too tiny to matter are moot. It also does not specify the nature, distance, or duration of the approach. That makes the question quite difficult to answer definitively. </p>

<h2>induced tidal distortion</h2>

<p>The most straight-forward effect we know about is slowing due to tidal forces. The <a href="https://en.wikipedia.org/wiki/Tidal_acceleration#Theory" rel="nofollow noreferrer">Theory</a> subsection of Wikipedia's article <a href="https://en.wikipedia.org/wiki/Tidal_acceleration" rel="nofollow noreferrer">Tidal acceleration</a> outlines this nicely.</p>

<p>Because the Earth is slightly stretchy, the Moon's or another close body's gravity induces a tidal bulge. Wikipedia's <a href="https://en.wikipedia.org/wiki/Earth_tide" rel="nofollow noreferrer">Earth tide</a> gives several components of a thorough decomposition, but the top two are about 38 cm due to the Moon and 17 cm due to the Sun. These exist on both the near and far side of each body. </p>

<p>Because the Earth responds slowly, these have a "lag". The lumps exist offset from the line between the Earth and the other body, and this allows for a torque. In the case of the Earth-Moon system that torque then tends to slow the Earth's rotation.</p>

<p>If the body is double in mass, the induced bulge's mass will be roughly linearly proportional and since the torque proportional to both it will be quadruple, proportional to <span class="math-container">$m^2$</span>. As shown in that subsection it is also proportional to $r^{-6}. So just for example, a planet about the size of the Earth (~80 Moons) and half the distance of the moon would have a torque 80 x 80 x 64 times larger. </p>

<p>However, it's hard to imagine a scenario where that could last for a thousand years to have a huge effect on Earth's rotation speed. This would be small, but definitely negative.</p>

<h2>interaction with equatorial bulge</h2>

<p>The body could pass perpendicular to the Earth's equator as well, and produce a really huge torque on Earth's equatorial bulge (expressed often as the <span class="math-container">$J_2$</span> term). That bulge is more like <em>30 kilometers</em>, compared to the 30 centimeters of the induced tidal bulge by the Moon. </p>

<p>A perpendicular pass like that would induce a substantial wobble in the Earth's rotation during the time of passage, but I don't think it is easy to say with certainty the sign of the effect on the rotation rate with such complex motion and no further details.</p>

CC BY-SA 4.0
2018-11-13T01:39:00.393Z
:)  Quantative hurts.  So the example is an object the size of Earth, which arrives from outer space and passes us - we'll say as close as you like , 10,000 km separation.  Let's say it's near us (within a million miles) for 10 minutes.  No wait, let's say a day .. no, let's say a month.  How much is the day slowed by tidal effects?
Fattie
CC BY-SA 4.0
2018-11-13T01:42:12.630Z
@Fattie that's a detailed calculation requiring a detailed description of the particular conditions. The beauty of the question is that it does not ask for the magnitude, only the sign.
uhoh
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2018-11-13T01:43:38.623Z
Heh!   :)   I just gave you exact figures, that match the story description in the question.
Fattie
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2018-11-13T01:46:26.787Z
@Fattie you can't ask a new question in comments. You are not new to SE and so you already know what comments are for, and what they are not for.
uhoh
CC BY-SA 4.0
2018-11-13T01:46:29.560Z
"Because the Earth is slightly stretchy, the Moon's or another close body's gravity induces a tidal bulge."  You realize that sentence is totally wrong, old bean?  Tidal bulges happen in the case of two objects >>>>>>> ***orbiting each other***.  <<<<<< So, we study tidal effects in binary stars etc.  Say the Earth and Moon were just "sitting next to each other" in empty space. There would be no tidal effect whatsoever.  (Obviously, the two objects would just fall towards each other en masse.)
Fattie
CC BY-SA 4.0
2018-11-13T01:48:38.243Z
No, [also](https://astronomy.stackexchange.com/questions/28327/if-a-giant-object-would-approach-the-earth-would-earth-start-to-turn-faster-or/28350?noredirect=1#comment50263_28335) wrong. The Earth is turning, and the rotation would result in a lag, and therefore a torque.
uhoh
CC BY-SA 4.0
2018-11-13T01:56:35.497Z
Sure sure.  So imagine a stationary Earth.  This object passes.  For the 10 minutes? a day? a month? that it passes, that is enough to *start the Earth rotating* ...
Fattie
CC BY-SA 4.0
2018-11-13T02:43:18.047Z
@Fattie Earth's oblateness and strong quadrupole moment are a direct result of Earth's rotation. They go hand-in-hand. The question specifically asks about Earth, which has an equatorial radius over 20 km larger than it's polar radius, due to it's rotation.
uhoh
CC BY-SA 4.0
2018-11-13T03:11:50.727Z
Yes my friend, but imagine if a large passing planet, "flew by" Earth.  It would pull *the whole of Earth* towards it ("to the top right") then *the whole of Earth* towards it ("to the bottom left").  Sure, there is a >>>minuscule<<< "rabbit ears" made (but it Could well be NONCONTRARY to Earth's current rotation - it would speed it up!)
Fattie
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2018-11-13T03:12:01.603Z
BTW it is three asterisks front and back for ***this***  :)
Fattie
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2018-11-13T03:13:00.770Z
@Fattie your secret is safe with me, I won't tell a soul! (I've undone my down vote btw)
uhoh
CC BY-SA 4.0
2018-11-12T04:09:04.027Z
2018-11-12T04:09:04.027Z
uhoh
<p>Could be, you'd be better to ask this on the Physics site, by the way.</p>

<p>No, the <strong><em>spinning</em></strong> of a perfectly round object is not affected at all, it just <strong><em>keeps spinning</em></strong> identically.</p>

<p>Let's say ..</p>

<p><a href="https://i.sstatic.net/Jls9u.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/Jls9u.png" alt="enter image description here"></a></p>

<p>say another Sun appeared, just like that.</p>

<p>Yes, the <strong><em>path</em></strong> that Earth is taking would go crazy.  We might fly off, head towards this new Sun, starting making a figure of 8, or some crazy path!</p>

<p>But the <strong><em>spinning</em></strong> as such of the Earth would be totally unchanged.</p>

<p>The "way we point" ("North up" so to speak) would be completely unchanged, and the speed we turn at (ie, 24 hours a turn) would be unchanged.</p>

<p>This might seem surprising but, well, you can learn all about it in Newtonian physics!</p>

<p>Once again, no change to the spin as such of the Earth.  the "direction we point" (ie the axis we spin around) and the time taken is totally unchanged if (for some bizarre reason!) we are "moved around in the heavens".</p>

<hr>

<p>Note that there would be some <strong><em>absolutely tiny</em></strong> effects due to things like relativity (<a href="https://www.sciencenews.org/article/einstein-general-relativity-mercury-orbit" rel="nofollow noreferrer">have a read!</a>), the water on our planet sloshing around, and after all our Moon could be moved, bashed, etc.  These effects on the speed we are spinning would be extremely small over the short period of time you're considering, in the "sci fi" scenario you outline, as seen in the diagram.</p>

CC BY-SA 4.0
2018-11-11T10:12:28.167Z
Can you *support* "But the spinning as such of the Earth would be totally unchanged" with some science based explanation? Also, tidal distortion is a reality of planetary rotation, and to [just say](https://astronomy.stackexchange.com/questions/28327/if-a-giant-object-would-approach-the-earth-would-earth-start-to-turn-faster-or#comment50232_28327) "The tidal effects would be absolutely tiny" and "this question is simply about simple Newtonian physics" and then give a wrong answer is unhelpful and unscientific.
uhoh
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2018-11-11T10:15:33.230Z
The tidal effects would be absolutely tiny.  If you can prove otherwise, go for it.   This question *is about basic Newtonian physics*.  It is foolish to answer in a different *domain*.  It's not clever or pedantic, it's just incorrect to answer in the wrong domain.
Fattie
CC BY-SA 4.0
2018-11-11T14:30:57.947Z
tidal effects on  Earth alone may be tiny because it is "very round" - but what about tidal effects on the Earth-Moon combo, which is much less symmetric? I suppose the moon orbit could be changed,  even drastically,  even be "stolen". In the *long run* this would influence the earth rotation because  predictions how  Earth rotation changes by 2.5 ms per century needs to be adjusted. Then again, all this is still either negligible or going in unpredictable directions - not to mention that the influx on the Earth orbit (as demonstrated in the highly accurate image) might be a lot more drastic
Hagen von Eitzen
CC BY-SA 4.0
2018-11-11T15:14:33.927Z
As you say, if the moon *disappeared* (!!) instantly right now, the only affect on the *length of the day* would be on the order of 2ms *in a century*.  Can you say "!!!!!!" ?!  The question is about what happens to the spin speed of Earth when ("in a scifi movie!") another huge star astoundingly comes up to us (as in the accurate diagram).  ***There are NO tidal effects***, for goodness sake.   And *even if there were*, tidal effects take *millions of years* to have effect on rotation !!!!!!!!!  Gosh!
Fattie
CC BY-SA 4.0
2018-11-11T17:10:52.907Z
per Wikipedia [The solar tidal force is 46% as large as the lunar.](https://en.wikipedia.org/wiki/Tide#Forces) Moon: 1.1 × 10−7 g, Sun: 0.52 × 10−7 g, so here you are just saying wrong stuff that no amount of **bold**, *italics* or "!!!!!!!!! Gosh!" can right. However, I sure would like to know how you were able to produce the [bold italics](https://astronomy.stackexchange.com/questions/28327/if-a-giant-object-would-approach-the-earth-would-earth-start-to-turn-faster-or#comment50259_28335)!
uhoh
CC BY-SA 4.0
2018-12-12T09:16:45.757Z
@uhoh *italics* is one asterisk either side of the expression, **bold** needs two asterisks, ***bold italics*** is three :-)
Chappo Hasn't Forgotten
CC BY-SA 4.0
2018-11-11T07:38:23.267Z
2018-11-11T10:57:58.297Z
2018-11-11T10:57:58.297Z
Fattie
<p>I am asking myself this question: if a giant celestial object would approach the Earth, would Earth start to turn faster (then day duration would be diminished) or slower (day duration increased) while the object starts to become nearer and nearer?
Thank you!</p>

CC BY-SA 4.0
2018-11-11T02:27:09.573Z
I believe it depend on which direction and speed it is coming toward the earth. Of it is a fly by then it depend otherwise, it is going to slow down if its is headed straight for the center of the earth
Arvin Kushwaha
CC BY-SA 4.0
2018-11-11T02:53:53.397Z
[How closely did When Worlds Collide authors pursue the physics of the catastrophic damage and planet trajectories?](https://scifi.stackexchange.com/q/198302/51174)
uhoh
CC BY-SA 4.0
2018-11-11T07:36:32.903Z
hi @ipmp .  Welcome new user.   **The tidal effects would be absolutely tiny.**  There are plenty of other effects as well (relativistic!  quantum! etc)   But this question is simply about simple Newtonian physics of bodies.  It is completely inappropriate to confuse the various domains.  OP is just asking about how a point mass angular momentum is affected by being moved about.
Fattie
CC BY-SA 4.0
2018-12-24T12:24:15.057Z
It can be both depending if the approach is kind of prograde or or retrograde as well as depending on the actual position of the passing body.  Earth has already gravitational "handles" as it is oblate and everything will depend on the relative position of the giant object. If it is massive enough it can even made its own handles and eventually things might get stripped apart.
Alchimista
<earth><rotation>
If a giant object would approach the Earth, would Earth start to turn faster or slower while the object starts to become nearer and nearer?
ipmp
# If a giant object would approach the Earth, would Earth start to turn faster or slower while the object starts to become nearer and nearer?

(Asked by: username_0 on 2018-11-10)

I am asking myself this question: if a giant celestial object would approach the Earth, would Earth start to turn faster (then day duration would be diminished) or slower (day duration increased) while the object starts to become nearer and nearer?
Thank you!

**Tags:** earth, rotation

> **Comments:**
> **username_1**: I believe it depend on which direction and speed it is coming toward the earth. Of it is a fly by then it depend otherwise, it is going to slow down if its is headed straight for the center of the earth  
> **username_2**: [How closely did When Worlds Collide authors pursue the physics of the catastrophic damage and planet trajectories?](https://scifi.stackexchange.com/q/198302/51174)  
> **username_3**: hi @username_0 .  Welcome new user.   **The tidal effects would be absolutely tiny.**  There are plenty of other effects as well (relativistic!  quantum! etc)   But this question is simply about simple Newtonian physics of bodies.  It is completely inappropriate to confuse the various domains.  OP is just asking about how a point mass angular momentum is affected by being moved about.  
> **username_4**: It can be both depending if the approach is kind of prograde or or retrograde as well as depending on the actual position of the passing body.  Earth has already gravitational "handles" as it is oblate and everything will depend on the relative position of the giant object. If it is massive enough it can even made its own handles and eventually things might get stripped apart.  

---

## 2 answers

### Answer by username_2 on 2018-11-12. Score: 1

> If a giant object would approach the Earth, would Earth start to turn faster or slower while the object starts to become nearer and nearer?

**tl;dr:** Probably will slow it down a small amount, but may induce a wobble that make a simple answer impossible without further details.

The problem is complicated. I'll leave a short answer.

The question is carefully written in (at least two) ways. It is agnostic to the size of the effect, instead asking only about its sign. So arguments that the effects are too tiny to matter are moot. It also does not specify the nature, distance, or duration of the approach. That makes the question quite difficult to answer definitively.

## induced tidal distortion

The most straight-forward effect we know about is slowing due to tidal forces. The [Theory](https://en.wikipedia.org/wiki/Tidal_acceleration#Theory) subsection of Wikipedia's article [Tidal acceleration](https://en.wikipedia.org/wiki/Tidal_acceleration) outlines this nicely.

Because the Earth is slightly stretchy, the Moon's or another close body's gravity induces a tidal bulge. Wikipedia's [Earth tide](https://en.wikipedia.org/wiki/Earth_tide) gives several components of a thorough decomposition, but the top two are about 38 cm due to the Moon and 17 cm due to the Sun. These exist on both the near and far side of each body.

Because the Earth responds slowly, these have a "lag". The lumps exist offset from the line between the Earth and the other body, and this allows for a torque. In the case of the Earth-Moon system that torque then tends to slow the Earth's rotation.

If the body is double in mass, the induced bulge's mass will be roughly linearly proportional and since the torque proportional to both it will be quadruple, proportional to $m^2$. As shown in that subsection it is also proportional to $r^{-6}. So just for example, a planet about the size of the Earth (~80 Moons) and half the distance of the moon would have a torque 80 x 80 x 64 times larger.

However, it's hard to imagine a scenario where that could last for a thousand years to have a huge effect on Earth's rotation speed. This would be small, but definitely negative.

## interaction with equatorial bulge

The body could pass perpendicular to the Earth's equator as well, and produce a really huge torque on Earth's equatorial bulge (expressed often as the $J\_2$ term). That bulge is more like *30 kilometers*, compared to the 30 centimeters of the induced tidal bulge by the Moon.

A perpendicular pass like that would induce a substantial wobble in the Earth's rotation during the time of passage, but I don't think it is easy to say with certainty the sign of the effect on the rotation rate with such complex motion and no further details.

> **Comments:**
> **username_3**: :)  Quantative hurts.  So the example is an object the size of Earth, which arrives from outer space and passes us - we'll say as close as you like , 10,000 km separation.  Let's say it's near us (within a million miles) for 10 minutes.  No wait, let's say a day .. no, let's say a month.  How much is the day slowed by tidal effects?  
> **username_2**: @username_3 that's a detailed calculation requiring a detailed description of the particular conditions. The beauty of the question is that it does not ask for the magnitude, only the sign.  
> **username_3**: Heh!   :)   I just gave you exact figures, that match the story description in the question.  
> **username_2**: @username_3 you can't ask a new question in comments. You are not new to SE and so you already know what comments are for, and what they are not for.  
> **username_3**: "Because the Earth is slightly stretchy, the Moon's or another close body's gravity induces a tidal bulge."  You realize that sentence is totally wrong, old bean?  Tidal bulges happen in the case of two objects >>>>>>> ***orbiting each other***.  <<<<<< So, we study tidal effects in binary stars etc.  Say the Earth and Moon were just "sitting next to each other" in empty space. There would be no tidal effect whatsoever.  (Obviously, the two objects would just fall towards each other en masse.)  
> **username_2**: No, [also](https://astronomy.stackexchange.com/questions/28327/if-a-giant-object-would-approach-the-earth-would-earth-start-to-turn-faster-or/28350?noredirect=1#comment50263_28335) wrong. The Earth is turning, and the rotation would result in a lag, and therefore a torque.  
> **username_3**: Sure sure.  So imagine a stationary Earth.  This object passes.  For the 10 minutes? a day? a month? that it passes, that is enough to *start the Earth rotating* ...  
> **username_2**: @username_3 Earth's oblateness and strong quadrupole moment are a direct result of Earth's rotation. They go hand-in-hand. The question specifically asks about Earth, which has an equatorial radius over 20 km larger than it's polar radius, due to it's rotation.  
> **username_3**: Yes my friend, but imagine if a large passing planet, "flew by" Earth.  It would pull *the whole of Earth* towards it ("to the top right") then *the whole of Earth* towards it ("to the bottom left").  Sure, there is a >>>minuscule<<< "rabbit ears" made (but it Could well be NONCONTRARY to Earth's current rotation - it would speed it up!)  
> **username_3**: BTW it is three asterisks front and back for ***this***  :)  
> **username_2**: @username_3 your secret is safe with me, I won't tell a soul! (I've undone my down vote btw)  

---

### Answer by username_3 on 2018-11-11. Score: 1

Could be, you'd be better to ask this on the Physics site, by the way.

No, the ***spinning*** of a perfectly round object is not affected at all, it just ***keeps spinning*** identically.

Let's say ..

[![enter image description here](https://i.sstatic.net/Jls9u.png)](https://i.sstatic.net/Jls9u.png)

say another Sun appeared, just like that.

Yes, the ***path*** that Earth is taking would go crazy. We might fly off, head towards this new Sun, starting making a figure of 8, or some crazy path!

But the ***spinning*** as such of the Earth would be totally unchanged.

The "way we point" ("North up" so to speak) would be completely unchanged, and the speed we turn at (ie, 24 hours a turn) would be unchanged.

This might seem surprising but, well, you can learn all about it in Newtonian physics!

Once again, no change to the spin as such of the Earth. the "direction we point" (ie the axis we spin around) and the time taken is totally unchanged if (for some bizarre reason!) we are "moved around in the heavens".

---

Note that there would be some ***absolutely tiny*** effects due to things like relativity ([have a read!](https://www.sciencenews.org/article/einstein-general-relativity-mercury-orbit)), the water on our planet sloshing around, and after all our Moon could be moved, bashed, etc. These effects on the speed we are spinning would be extremely small over the short period of time you're considering, in the "sci fi" scenario you outline, as seen in the diagram.

> **Comments:**
> **username_2**: Can you *support* "But the spinning as such of the Earth would be totally unchanged" with some science based explanation? Also, tidal distortion is a reality of planetary rotation, and to [just say](https://astronomy.stackexchange.com/questions/28327/if-a-giant-object-would-approach-the-earth-would-earth-start-to-turn-faster-or#comment50232_28327) "The tidal effects would be absolutely tiny" and "this question is simply about simple Newtonian physics" and then give a wrong answer is unhelpful and unscientific.  
> **username_3**: The tidal effects would be absolutely tiny.  If you can prove otherwise, go for it.   This question *is about basic Newtonian physics*.  It is foolish to answer in a different *domain*.  It's not clever or pedantic, it's just incorrect to answer in the wrong domain.  
> **username_5**: tidal effects on  Earth alone may be tiny because it is "very round" - but what about tidal effects on the Earth-Moon combo, which is much less symmetric? I suppose the moon orbit could be changed,  even drastically,  even be "stolen". In the *long run* this would influence the earth rotation because  predictions how  Earth rotation changes by 2.5 ms per century needs to be adjusted. Then again, all this is still either negligible or going in unpredictable directions - not to mention that the influx on the Earth orbit (as demonstrated in the highly accurate image) might be a lot more drastic  
> **username_3**: As you say, if the moon *disappeared* (!!) instantly right now, the only affect on the *length of the day* would be on the order of 2ms *in a century*.  Can you say "!!!!!!" ?!  The question is about what happens to the spin speed of Earth when ("in a scifi movie!") another huge star astoundingly comes up to us (as in the accurate diagram).  ***There are NO tidal effects***, for goodness sake.   And *even if there were*, tidal effects take *millions of years* to have effect on rotation !!!!!!!!!  Gosh!  
> **username_2**: per Wikipedia [The solar tidal force is 46% as large as the lunar.](https://en.wikipedia.org/wiki/Tide#Forces) Moon: 1.1 × 10−7 g, Sun: 0.52 × 10−7 g, so here you are just saying wrong stuff that no amount of **bold**, *italics* or "!!!!!!!!! Gosh!" can right. However, I sure would like to know how you were able to produce the [bold italics](https://astronomy.stackexchange.com/questions/28327/if-a-giant-object-would-approach-the-earth-would-earth-start-to-turn-faster-or#comment50259_28335)!  
> **username_6**: @username_2 *italics* is one asterisk either side of the expression, **bold** needs two asterisks, ***bold italics*** is three :-)  

---

