<p>With a more eccentric orbit, tidal control of the rotation does not lead to a librating lock. <a href="https://en.wikipedia.org/wiki/Mercury_(planet)" rel="noreferrer">Mercury</a> is in a 3:2 spin-orbit resonance. <a href="https://en.wikipedia.org/wiki/Hyperion_(moon)#Rotation" rel="noreferrer">Hyperion</a>'s spin is chaotic: it gets a quasi-random kick each close approach to Saturn. So, your question is a good one, and the phenomena are rich. The Moon is merely an example of the simplest case.</p>

CC BY-SA 4.0
2020-08-13T03:06:39.803Z
Mercury's rotation is such that the Sun stands nearly still in its sky *at perihelion* when the tide is strongest.
Anton Sherwood
CC BY-SA 4.0
2020-08-09T14:35:26.943Z
2020-08-09T19:39:20.813Z
2020-08-09T19:39:20.813Z
John Doty
<p>You are right, that would be weird if the Moon speeds up and slows down this way to always show the exact same side to the Earth. That's why it doesn't.</p>
<p>At some point in the orbit the Moon's rotation (its phase) lags behind and at some point it is too fast. It is (pretty much) constant and does not adjust to the varying orbital velocity. That's (one reason) why it wobbles. And this wobble is what's called <a href="https://en.wikipedia.org/wiki/Libration" rel="noreferrer">libration</a>.</p>
<p>But the Moon is still tidally locked as its rotation period is still equal to its orbital period.</p>
<p><a href="https://i.sstatic.net/Zed3L.gif" rel="noreferrer"><img src="https://i.sstatic.net/Zed3L.gif" alt="Lunar libration" /></a></p>

CC BY-SA 4.0
2020-08-09T03:16:23.277Z
`+1`! Possibly of interest: [Roughly express how much of our Moon's apparent libration amplitude seen from the Geocenter is due to different effects?](https://space.stackexchange.com/q/45366/12102) and also [How “locked” are Pluto and Charon? How much does each librate as seen from the other?](https://space.stackexchange.com/q/45342/12102)
uhoh
CC BY-SA 4.0
2020-08-08T23:26:51.890Z
2020-08-10T11:13:26.500Z
2020-08-10T11:13:26.500Z
SpaceCore
<p>This popped up into my mind just now. The Moon is tidally locked to Earth, and also has a significantly eccentric orbit. This means that its orbital velocity near periapsis is considerably faster than its orbital velocity at apoapsis.</p>
<p>Thus, for the Moon to always show one side to Earth, its rotation around its own axis has to slow down and speed up depending on where it is in its orbit, but doesn't that violate the law of conservation of angular momentum? And if its rotation around its axis was constant then it would get out of sync and not be tidally locked, no?</p>

CC BY-SA 4.0
2020-08-09T14:22:56.833Z
Just for a reference point, "considerably faster" here means [about 10% faster](https://en.wikipedia.org/wiki/Orbital_speed#Tangential_velocities_at_altitude). That's not negligible, but also not the kind of orders-of-magnitude speed difference that you'd see with a _really_ eccentric orbit like e.g. that of a comet or a satellite in a [Molniya orbit](https://en.wikipedia.org/wiki/Molniya_orbit).
Ilmari Karonen
CC BY-SA 4.0
2020-08-10T15:03:43.217Z
The moon is tidally-locked anyway because moons don't care what astrophysicists say is possible.
Comic Sans Seraphim
CC BY-SA 4.0
2020-08-10T19:29:37.617Z
silence contrarian
ChristieToWin
<tidal-locking>
Shouldn't tidal locking be impossible for a satellite that has a considerably eccentric orbit?
ChristieToWin
# Shouldn't tidal locking be impossible for a satellite that has a considerably eccentric orbit?

(Asked by: username_0 on 2020-08-08)

This popped up into my mind just now. The Moon is tidally locked to Earth, and also has a significantly eccentric orbit. This means that its orbital velocity near periapsis is considerably faster than its orbital velocity at apoapsis.

Thus, for the Moon to always show one side to Earth, its rotation around its own axis has to slow down and speed up depending on where it is in its orbit, but doesn't that violate the law of conservation of angular momentum? And if its rotation around its axis was constant then it would get out of sync and not be tidally locked, no?

**Tags:** tidal-locking
> **Comments:**
> **username_1**: Just for a reference point, "considerably faster" here means [about 10% faster](https://en.wikipedia.org/wiki/Orbital_speed#Tangential_velocities_at_altitude). That's not negligible, but also not the kind of orders-of-magnitude speed difference that you'd see with a _really_ eccentric orbit like e.g. that of a comet or a satellite in a [Molniya orbit](https://en.wikipedia.org/wiki/Molniya_orbit).  
> **username_2**: The moon is tidally-locked anyway because moons don't care what astrophysicists say is possible.  
> **username_0**: silence contrarian  

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## 2 answers

### Answer by username_5 on 2020-08-08. Score: 27 | Accepted answer

You are right, that would be weird if the Moon speeds up and slows down this way to always show the exact same side to the Earth. That's why it doesn't.

At some point in the orbit the Moon's rotation (its phase) lags behind and at some point it is too fast. It is (pretty much) constant and does not adjust to the varying orbital velocity. That's (one reason) why it wobbles. And this wobble is what's called [libration](https://en.wikipedia.org/wiki/Libration).

But the Moon is still tidally locked as its rotation period is still equal to its orbital period.

[![Lunar libration](https://i.sstatic.net/Zed3L.gif)](https://i.sstatic.net/Zed3L.gif)

> **Comments:**
> **username_6**: `+1`! Possibly of interest: [Roughly express how much of our Moon's apparent libration amplitude seen from the Geocenter is due to different effects?](https://space.stackexchange.com/q/45366/12102) and also [How “locked” are Pluto and Charon? How much does each librate as seen from the other?](https://space.stackexchange.com/q/45342/12102)  

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### Answer by username_3 on 2020-08-09. Score: 8

With a more eccentric orbit, tidal control of the rotation does not lead to a librating lock. [Mercury](https://en.wikipedia.org/wiki/Mercury_(planet)) is in a 3:2 spin-orbit resonance. [Hyperion](https://en.wikipedia.org/wiki/Hyperion_(moon)#Rotation)'s spin is chaotic: it gets a quasi-random kick each close approach to Saturn. So, your question is a good one, and the phenomena are rich. The Moon is merely an example of the simplest case.

> **Comments:**
> **username_4**: Mercury's rotation is such that the Sun stands nearly still in its sky *at perihelion* when the tide is strongest.  

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