<p>In short, the answer is yes -- because of the conservation of the angular momentum.</p>
<p>The orbital angular momentum scales as the square root of the semimajor axis. Therefore, if the satellite's orbit is shrinking, the orbital angular momentum is getting smaller.</p>
<p>Had Triton's orbit been prograde, I would have said that, to compensate for the said decrease in the orbital angular momentum, the rotational angular momentum of the planet must be increasing (i.e., the planet must be rotating faster).</p>
<p>However, Triton's orbit is retrograde. So the sign of the rotational orbital momentum is opposite to that of the planet's spin. (To simplify things, I am neglecting the inclination of the orbit on the equator.) In this situation, our conclusion will be opposite: to compensate for the decrease in the orbital angular momentum, the planet must be rotating slower.</p>

CC BY-SA 4.0
2022-08-17T01:04:29.990Z
2022-08-17T01:22:46.527Z
2022-08-17T01:22:46.527Z
Michael_1812
<p>Just a quick question regarding Neptune. Because Phobos is spiraling in towards Mars, Mars' rotation speeds up. Does the same thing happen with Neptune's rotation?</p>

CC BY-SA 4.0
2019-01-03T16:19:22.850Z
I don't have enough information for an answer, but some things to keep in mind: Neptune has a lot more moons, and Neptune has differential rotation since it is a gas giant.
called2voyage
CC BY-SA 4.0
2019-01-03T16:33:55.117Z
The obvious answer is "yes, because Newton." What you need to consider is the magnitude of the effect, roughly by comparing the mass ratio of Triton/Neptune as compared with Phobos/Mars , to see if it is significant. [ the calculation is left as a homework problem :-) ]
Carl Witthoft
CC BY-SA 4.0
2019-01-03T20:07:51.110Z
Let's just say it  you have a moon and . planet, the moon orbits retrograde. Would it happen?
StellarExile
CC BY-SA 4.0
2019-01-04T02:20:16.007Z
This is an interesting question, and since Neptune is a gas giant rather than a rocky planet, there are some interesting differences. I think this question deserves a good answer that makes this difference clear.
uhoh
<tidal-forces><neptune><tidal-locking><triton>
Does the orbital decay of Triton affect Neptune's rotation?
StellarExile
# Does the orbital decay of Triton affect Neptune's rotation?

(Asked by: username_0 on 2019-01-03)

Just a quick question regarding Neptune. Because Phobos is spiraling in towards Mars, Mars' rotation speeds up. Does the same thing happen with Neptune's rotation?

**Tags:** tidal-forces, neptune, tidal-locking, triton

> **Comments:**
> **username_1**: I don't have enough information for an answer, but some things to keep in mind: Neptune has a lot more moons, and Neptune has differential rotation since it is a gas giant.  
> **username_2**: The obvious answer is "yes, because Newton." What you need to consider is the magnitude of the effect, roughly by comparing the mass ratio of Triton/Neptune as compared with Phobos/Mars , to see if it is significant. [ the calculation is left as a homework problem :-) ]  
> **username_0**: Let's just say it  you have a moon and . planet, the moon orbits retrograde. Would it happen?  
> **username_3**: This is an interesting question, and since Neptune is a gas giant rather than a rocky planet, there are some interesting differences. I think this question deserves a good answer that makes this difference clear.  

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## 1 answers

### Answer by username_4 on 2022-08-17. Score: 4

In short, the answer is yes -- because of the conservation of the angular momentum.

The orbital angular momentum scales as the square root of the semimajor axis. Therefore, if the satellite's orbit is shrinking, the orbital angular momentum is getting smaller.

Had Triton's orbit been prograde, I would have said that, to compensate for the said decrease in the orbital angular momentum, the rotational angular momentum of the planet must be increasing (i.e., the planet must be rotating faster).

However, Triton's orbit is retrograde. So the sign of the rotational orbital momentum is opposite to that of the planet's spin. (To simplify things, I am neglecting the inclination of the orbit on the equator.) In this situation, our conclusion will be opposite: to compensate for the decrease in the orbital angular momentum, the planet must be rotating slower.


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