<p>It is called "Tidal locking", or "gravitational locking" or "captured rotation". According the the Wikipedia page on "<a href="https://en.wikipedia.org/wiki/Tidal_locking#Mechanism" rel="nofollow">tidal locking</a>" (check the references for sources), it is due to Earth's gravity causing a small tidal bulge on the Moon, which affects its rotation. Over time, the Moon's rotation, affect by Earth's gravity, makes it's orbital rotation synchronise with its lunar rotation.</p>

CC BY-SA 3.0
2014-11-26T11:31:14.957Z
2014-11-26T11:31:14.957Z
iantresman
<p>The Earth and Moon have been "together" billions of years and so even realitively weak forces such as gravitation have had a chance to cause profound effects. One of those is the Moon's tidal lock. The Earth too is being driven towards tidal lock - ie at a point in the future only one face of the Earth will face the Moon, but as the Moon's tidal drag on the Earth is relatively lower (compared to the energy contained in the Earth's spin) than Earth's on the Moon, this is not going to happen for very many millions of years.</p>

CC BY-SA 3.0
2014-11-28T20:55:14.620Z
*realitively weak forces such as gravitation* lol. If you meant "gravity", that's the weakest force on small scales, but the only force on large scales. So it doesn't matter that it's "weak".
Walter
CC BY-SA 3.0
2014-11-29T10:21:35.963Z
The only force on large scales? Really? Stand outside in the mid summer sun for half an hour then.
adrianmcmenamin
CC BY-SA 3.0
2014-11-29T17:35:46.657Z
This is not a force, but radiation. Radiation also decays like $r^{-2}$, like gravitational forces (both is simple geometry).
Walter
CC BY-SA 3.0
2014-11-29T21:17:12.337Z
Gravity is also radiation.
adrianmcmenamin
CC BY-SA 3.0
2014-11-27T14:19:45.037Z
2014-11-27T14:19:45.037Z
adrianmcmenamin
<p>We never see the dark side of the moon. It rotates so that the same side always faces us.</p>

<p>I heard two (dinner party) theories on this:
1. The moon was made by smashing out a chunk of the earth, and so somehow preserved some orbital rotation from the earth 
2. The gravitational drag from the earth to the moon is lowest when the moon is in 'sync' to only show one face to the earth </p>

<p>I don't trust either of these.</p>

<p>My question is: "Why does the face of the moon 'sync' with the earth?"</p>

CC BY-SA 3.0
2014-11-26T08:48:45.540Z
Actually, you can see the dark side of the moon every month during the new moon. ;)
David H
CC BY-SA 3.0
2014-11-26T21:06:53.987Z
Lunar Libration: http://apod.nasa.gov/apod/ap010218.html
Wayfaring Stranger
<the-moon>
Why does the face of the moon 'sync' with the earth?
hawkeye
# Why does the face of the moon 'sync' with the earth?

(Asked by: username_0 on 2014-11-26)

We never see the dark side of the moon. It rotates so that the same side always faces us.

I heard two (dinner party) theories on this:
1. The moon was made by smashing out a chunk of the earth, and so somehow preserved some orbital rotation from the earth
2. The gravitational drag from the earth to the moon is lowest when the moon is in 'sync' to only show one face to the earth

I don't trust either of these.

My question is: "Why does the face of the moon 'sync' with the earth?"

**Tags:** the-moon
> **Comments:**
> **username_1**: Actually, you can see the dark side of the moon every month during the new moon. ;)  
> **username_2**: Lunar Libration: http://apod.nasa.gov/apod/ap010218.html  

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## 2 answers

### Answer by username_3 on 2014-11-26. Score: 3 | Accepted answer

It is called "Tidal locking", or "gravitational locking" or "captured rotation". According the the Wikipedia page on "[tidal locking](https://en.wikipedia.org/wiki/Tidal_locking#Mechanism)" (check the references for sources), it is due to Earth's gravity causing a small tidal bulge on the Moon, which affects its rotation. Over time, the Moon's rotation, affect by Earth's gravity, makes it's orbital rotation synchronise with its lunar rotation.


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### Answer by username_4 on 2014-11-27. Score: 1

The Earth and Moon have been "together" billions of years and so even realitively weak forces such as gravitation have had a chance to cause profound effects. One of those is the Moon's tidal lock. The Earth too is being driven towards tidal lock - ie at a point in the future only one face of the Earth will face the Moon, but as the Moon's tidal drag on the Earth is relatively lower (compared to the energy contained in the Earth's spin) than Earth's on the Moon, this is not going to happen for very many millions of years.

> **Comments:**
> **username_5**: *realitively weak forces such as gravitation* lol. If you meant "gravity", that's the weakest force on small scales, but the only force on large scales. So it doesn't matter that it's "weak".  
> **username_4**: The only force on large scales? Really? Stand outside in the mid summer sun for half an hour then.  
> **username_5**: This is not a force, but radiation. Radiation also decays like $r^{-2}$, like gravitational forces (both is simple geometry).  
> **username_4**: Gravity is also radiation.  

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