<p>Consider the case of similar-mass, close binary systems. If the orbital periods are less than a few days they are expected to become tidally locked on a timescale much shorter than the stellar lifetimes.</p>

<p>In many cases the synchronisation can be established by comparing the rotation periods of the stars using modulation by starspots with the orbital period from eclipses e.g. <a href="http://adsabs.harvard.edu/abs/1983A%26A...123..316E" rel="nofollow">http://adsabs.harvard.edu/abs/1983A%26A...123..316E</a></p>

<p>In other cases you can find the projected equatorial velocities of eclipsing binary star components and show that these are consistent with the orbital period, the measured radii and the orbital inclination e.g. <a href="http://adsabs.harvard.edu/abs/2009ApJ...691.1400M" rel="nofollow">http://adsabs.harvard.edu/abs/2009ApJ...691.1400M</a> ; <a href="http://adsabs.harvard.edu/abs/2002ApJ...567.1140T" rel="nofollow">http://adsabs.harvard.edu/abs/2002ApJ...567.1140T</a></p>

CC BY-SA 3.0
2015-01-02T00:11:46.667Z
2015-01-02T00:11:46.667Z
ProfRob
<p>Pluto and its largest moon <a href="http://en.wikipedia.org/wiki/Charon_(moon)#Orbital_characteristics" rel="noreferrer">Charon</a> are tidally locked to each other.</p>

<blockquote>
  <p>Charon and Pluto revolve about each other every 6.387 days. The two objects are both gravitationally locked to the other, so each keeps the same face towards the other. This is a mutual case of tidal locking . . .  </p>
</blockquote>

<p>Because of Charon's large size compared to Pluto, and because its barycenter lies above the primary's surface, some astronomers have referred to the <a href="http://en.wikipedia.org/wiki/Pluto#Charon" rel="noreferrer">Pluto/Charon system</a> as a dwarf double planet.  </p>

<blockquote>
  <p>The system is also unusual among planetary systems in that each is tidally locked to the other: Charon always presents the same face to Pluto, and Pluto always presents the same face to Charon: from any position on either body, the other is always at the same position in the sky, or always obscured.<a href="http://www.boulder.swri.edu/~layoung/projects/talks03/IfA-jan03v1.ppt" rel="noreferrer">(110)</a></p>
</blockquote>

CC BY-SA 3.0
2015-01-02T01:37:23.413Z
2015-01-02T07:00:18.627Z
2015-01-02T07:00:18.627Z
Jerard Puckett
<p>Is there any known pair of objects tidally locked with each other?  I know the moon is tidally locked with the earth (e.g. one side always facing earth), but I don't know of any pair of objects that are both tidally locked with each other.  For example, the planet would be tidally locked with the moon, and the moon would be tidally locked with the planet.</p>

<orbit><tidal-forces>
Are there any known objects in a "dual" tidally locked orbit?
Jonathan
# Are there any known objects in a "dual" tidally locked orbit?

(Asked by: username_0 on 2015-01-01)

Is there any known pair of objects tidally locked with each other? I know the moon is tidally locked with the earth (e.g. one side always facing earth), but I don't know of any pair of objects that are both tidally locked with each other. For example, the planet would be tidally locked with the moon, and the moon would be tidally locked with the planet.

**Tags:** orbit, tidal-forces

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## 2 answers

### Answer by username_2 on 2015-01-02. Score: 9 | Accepted answer

Pluto and its largest moon [Charon](http://en.wikipedia.org/wiki/Charon_(moon)#Orbital_characteristics) are tidally locked to each other.

> Charon and Pluto revolve about each other every 6.387 days. The two objects are both gravitationally locked to the other, so each keeps the same face towards the other. This is a mutual case of tidal locking . . .

Because of Charon's large size compared to Pluto, and because its barycenter lies above the primary's surface, some astronomers have referred to the [Pluto/Charon system](http://en.wikipedia.org/wiki/Pluto#Charon) as a dwarf double planet.

> The system is also unusual among planetary systems in that each is tidally locked to the other: Charon always presents the same face to Pluto, and Pluto always presents the same face to Charon: from any position on either body, the other is always at the same position in the sky, or always obscured.[(110)](http://www.boulder.swri.edu/~layoung/projects/talks03/IfA-jan03v1.ppt)


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### Answer by username_1 on 2015-01-02. Score: 3

Consider the case of similar-mass, close binary systems. If the orbital periods are less than a few days they are expected to become tidally locked on a timescale much shorter than the stellar lifetimes.

In many cases the synchronisation can be established by comparing the rotation periods of the stars using modulation by starspots with the orbital period from eclipses e.g. <http://adsabs.harvard.edu/abs/1983A%26A...123..316E>

In other cases you can find the projected equatorial velocities of eclipsing binary star components and show that these are consistent with the orbital period, the measured radii and the orbital inclination e.g. <http://adsabs.harvard.edu/abs/2009ApJ...691.1400M> ; <http://adsabs.harvard.edu/abs/2002ApJ...567.1140T>


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