<blockquote>
<p>Why rotation period of almost all moons are in sync with the orbit period?</p>
</blockquote>
<p>This is not the case.</p>
<p>This appears to be close to the case for the <a href="https://en.wikipedia.org/wiki/Regular_moon" rel="noreferrer">regular moons</a>, but even then there are exceptions. In is definitely not the case for <a href="https://en.wikipedia.org/wiki/Irregular_satellite" rel="noreferrer">irregular moons</a>. Note well: Most moons are irregular.</p>
<p>The regular moons of a planet tend to</p>
<ul>
<li>Orbit closer to the planet than do the irregular moons.<br />
Whether tidal locking will occur, and if so, when, highly depends on orbital radius. All other things being equal, the time it takes for a body to be tidally locked grows with the sixth power of orbital radius.</li>
<li>Be larger than the irregular moons (Hyperion and Triton are marked exceptions).<br />
Smaller objects have a markedly non-spherical shape. Objects much larger than the <a href="https://astronomy.stackexchange.com/a/37048">potato radius</a> tend to pull themselves into a more or less spherical shape over time. Smaller objects cannot do this. This most likely means an inertia tensor with three distinct principal axes, which can result in chaotic rotation. Hyperion is a lumpy potato of a regular moon, and its rotation is very chaotic in part because of its lumpiness and in part because of interactions with Titan. Triton is classified as an irregular moon due to its retrograde orbit. Triton is large and is tidally locked.</li>
<li>Be older than the irregular moons.<br />
Regular moons are widely thought to be almost as old as the planet, having formed with the planet. This gives them over four billion years to become tidally locked. Irregular moons are widely thought to be captured bodies. Even if they might eventually become tidally locked, they haven't yet had enough time to do so.</li>
<li>Have much smaller orbital eccentricities compared to the irregular moons.<br />
A moon in a highly eccentric orbit essentially has no chance of becoming tidally locked. The orbital angular velocity varies widely for a highly eccentric orbit. The rotational angular velocity cannot keep up with those rapid changes. Many of the irregular moons have highly eccentric orbits.</li>
</ul>

CC BY-SA 4.0
2024-10-05T17:17:46.643Z
Your question would be better if you gave the latest numbers for regular and irregular moons.   When I was a boy reading about astronomy there were 31 known moons orbiting 6 of the planets  Today there are nine times as many, 288, known moons orbiting planets, and 473 known moons orbiting small bodies like asteroids and dwarf planets.   And almost all those new moon discoveries have been irregular moons that are not likely to be tidially locked. https://www.livescience.com/space/astronomy/how-many-moons-are-in-the-solar-system
M. A. Golding
CC BY-SA 4.0
2024-10-06T03:40:14.737Z
Excellent comment. Will research and revise
Tony Anderson
CC BY-SA 4.0
2024-10-03T10:13:10.850Z
2024-10-03T10:37:11.587Z
2024-10-03T10:37:11.587Z
David Hammen
<p>Why is the rotation period of regular moons in sync with the orbit period? What is the evidentiary support for tidal lock? END OF QUESTION
NOTES FOR CLARIFICATION: I am asking for actual evidence, not theories. E.g. If a theory says that they lock in x million years what evidence shows they were not before that time. This is not a textbook question, I know what textbooks say. This is a scientific question challenging the common explanation. References to scientific papers will be appreciated.</p>

CC BY-SA 4.0
2024-10-03T04:36:47.497Z
I’m voting to close this question because this can't be answered.  It **is** tidal lock, there is a clear evidence basis in terms of models of rotation that fit the tidal lock hypothesis.  But the question rejects this simple clear and well supported theory, and any other answer would be wrong. This makes this question impossible to answer.
James K
CC BY-SA 4.0
2024-10-03T04:43:05.640Z
interesting question from celestial mechanics
ayr
CC BY-SA 4.0
2024-10-03T14:10:13.917Z
@JamesK What can be addressed is the claim that "the rotation period of almost all moons is in sync with the orbit period" as this is not the case. Since the OP seems to reject the  notion of tidal locking (why???), I'll just regurgitate that well-established concept for people other than the OP.
David Hammen
CC BY-SA 4.0
2024-10-03T21:57:13.800Z
I’m voting to close this question because "tidal lock, that is just a possible explanation w/o evidentiary support." We don't post crazy stuff here.
Miss Understands
CC BY-SA 4.0
2024-10-06T06:27:49.667Z
Miss Understands: What is the crazy stuff?
Tony Anderson
CC BY-SA 4.0
2024-10-08T07:05:59.470Z
Also recently asked with better phrasing: https://astronomy.stackexchange.com/questions/58649/is-there-any-evidence-that-the-rotation-period-of-the-moon-was-different-to-its/58663
Nilay Ghosh
<cosmology>
Why rotation period of almost all moons are in sync with the orbit period?
Tony Anderson
# Why rotation period of almost all moons are in sync with the orbit period?

(Asked by: username_0 on 2024-10-03)

Why is the rotation period of regular moons in sync with the orbit period? What is the evidentiary support for tidal lock? END OF QUESTION
NOTES FOR CLARIFICATION: I am asking for actual evidence, not theories. E.g. If a theory says that they lock in x million years what evidence shows they were not before that time. This is not a textbook question, I know what textbooks say. This is a scientific question challenging the common explanation. References to scientific papers will be appreciated.

**Tags:** cosmology

> **Comments:**
> **username_1**: I’m voting to close this question because this can't be answered.  It **is** tidal lock, there is a clear evidence basis in terms of models of rotation that fit the tidal lock hypothesis.  But the question rejects this simple clear and well supported theory, and any other answer would be wrong. This makes this question impossible to answer.  
> **username_2**: interesting question from celestial mechanics  
> **username_3**: @username_1 What can be addressed is the claim that "the rotation period of almost all moons is in sync with the orbit period" as this is not the case. Since the OP seems to reject the  notion of tidal locking (why???), I'll just regurgitate that well-established concept for people other than the OP.  
> **username_4**: I’m voting to close this question because "tidal lock, that is just a possible explanation w/o evidentiary support." We don't post crazy stuff here.  
> **username_0**: username_4 username_4: What is the crazy stuff?  
> **username_5**: Also recently asked with better phrasing: https://astronomy.stackexchange.com/questions/58649/is-there-any-evidence-that-the-rotation-period-of-the-moon-was-different-to-its/58663  

---

## 1 answers

### Answer by username_3 on 2024-10-03. Score: 9

> Why rotation period of almost all moons are in sync with the orbit period?

This is not the case.

This appears to be close to the case for the [regular moons](https://en.wikipedia.org/wiki/Regular_moon), but even then there are exceptions. In is definitely not the case for [irregular moons](https://en.wikipedia.org/wiki/Irregular_satellite). Note well: Most moons are irregular.

The regular moons of a planet tend to

* Orbit closer to the planet than do the irregular moons.  
  Whether tidal locking will occur, and if so, when, highly depends on orbital radius. All other things being equal, the time it takes for a body to be tidally locked grows with the sixth power of orbital radius.
* Be larger than the irregular moons (Hyperion and Triton are marked exceptions).  
  Smaller objects have a markedly non-spherical shape. Objects much larger than the [potato radius](https://astronomy.stackexchange.com/a/37048) tend to pull themselves into a more or less spherical shape over time. Smaller objects cannot do this. This most likely means an inertia tensor with three distinct principal axes, which can result in chaotic rotation. Hyperion is a lumpy potato of a regular moon, and its rotation is very chaotic in part because of its lumpiness and in part because of interactions with Titan. Triton is classified as an irregular moon due to its retrograde orbit. Triton is large and is tidally locked.
* Be older than the irregular moons.  
  Regular moons are widely thought to be almost as old as the planet, having formed with the planet. This gives them over four billion years to become tidally locked. Irregular moons are widely thought to be captured bodies. Even if they might eventually become tidally locked, they haven't yet had enough time to do so.
* Have much smaller orbital eccentricities compared to the irregular moons.  
  A moon in a highly eccentric orbit essentially has no chance of becoming tidally locked. The orbital angular velocity varies widely for a highly eccentric orbit. The rotational angular velocity cannot keep up with those rapid changes. Many of the irregular moons have highly eccentric orbits.

> **Comments:**
> **username_6**: Your question would be better if you gave the latest numbers for regular and irregular moons.   When I was a boy reading about astronomy there were 31 known moons orbiting 6 of the planets  Today there are nine times as many, 288, known moons orbiting planets, and 473 known moons orbiting small bodies like asteroids and dwarf planets.   And almost all those new moon discoveries have been irregular moons that are not likely to be tidially locked. https://www.livescience.com/space/astronomy/how-many-moons-are-in-the-solar-system  
> **username_0**: Excellent comment. Will research and revise  

---

