What does tidally locked mean? | Explained

If you’re curious about what it means to be tidally locked, then you’re in the right place. Being tidally locked is an inevitability for many moons in our solar system, although some have managed to escape it. Let’s learn what it means to be tidally locked.

What does Tidally Locked mean?

Tidally locked means that a satellite rotates around an object, typically a planet, at the same pace that it orbits it. So, the length of a year and a day on this moon would equal the same, as it takes it the same amount of time to orbit the planet as it does to rotate once fully.

This results in the moon only ever showing one side of its face to the planet. This is the case for the Earth’s moon, and most of the other major moons in our solar system. However, there are some exceptions which have a slightly different tidal locking pattern. Here’s a few examples of planets and dwarfs that are tidally locked.

  • Pluto and Charon – Pluto and Charon are both tidally locked to each other – this is because of the size of Charon, which is not that much smaller than Pluto. This means that Pluto also moves simultaneously to Charon, so its same side is always faced to the moon. However, Pluto’s other moons are not tidally locked to it, and that’s because the gravitational effect of Charon affects their orbital pattern.
  • Mercury and the Sun – Mercury is tidally locked to the Sun, but in a unique way that is different to the other satellites. It is locked in a 3:2 fashion, which means that it rotates 3 times for every 2 full orbits it makes of the Sun.

Questions about Tidal Locking

Is the moon completely tidally locked?

Sometimes, you might hear people say that the moon isn’t really tidally locked because of it’s elliptical orbit pattern. This is true, as the moon orbits the Sun in an ellipse, or oval, shape. So, we do actually end up seeing a little bit more of the moon that just “one side”. However, this only allows us to see around 10% more of the moon that if it was to orbit in a spherical manner

Will the Earth become tidally locked?

If things were to go on forever, then the Earth would eventually become tidally locked. However, the reality is that in a few billion years, the Sun will continue to grow and grow until it is large enough to pull the Earth in with it’s gravitational pull, and end up destroying it.

What causes tidal locking?

Tidal locking is a result of the satellite being transformed into an oval by the larger planet.when a satellite is rotating around a planet, it’s long axis will start to move away from facing the planet; however, the gravity of the planet pulls the satellite back. This slows down the rotation process, until only one side of the satellite is permanently facing the planet. So, then reality is that tidal locking is a result of satellites not being physically rigid and being able to stand up to a planets gravity.

Are any moons not tidally locked?

The vast majority of the moons of our solar system are tidally locked to their planet. The main exception is Hyperion, which is not tidally locked to Saturn – this is caused by the gravitational effect of Titan, Saturn’s largest moon. Also, the moons that orbit Pluto – Styx, Nix, Kerberos, and Hydra – are not tidally locked to the planet either, as they are also affected by Pluto’s moon, Charon.

Is tidal locking a coincidence?

No, tidal locking is not a coincidence. It is a result of the gravitational pull of the larger object, in most cases of the planet.


Hopefully this has taught you all you need to know about tidal locking and how it works. Some different moons in our solar system are not tidally locked, but this is caused by other moons affecting their rotation. In time, all moons would become tidally locked if it were not affected by another object, which has a gravitational interaction with the moon and knocks it off pattern.

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