Why Are Some Planets and Satellites Tidally Locked?

by Carson

Do you know why you can only see one side of the Moon when you’re on Earth? That’s because of tidal locking. Therefore, we’ll explain how it works and its effects in this article.

How Does Tidal Locking Work?

Think of Earth and Moon as an example. When Earth pulls the Moon in with its gravity, it will cause the surface of the Moon to slightly bulge outward towards and away from the Earth, while the other regions are pulled inward to preserve its volume. The same tidal bulge powered by the Sun’s gravity causes tides on Earth.

However, the Moon’s rotational period was shorter than that of its orbital period. Therefore, some regions are pulled outward and inward throughout the process where the Moon orbits Earth. This creates a phenomenon where the bulged areas are out of sync with the actual demand, which is stronger than the phenomenon that retains the bulged regions at the same place. As a result, the rotational period of the Moon was affected, and it got slower and slower until it reached a point where the rotational period and the orbital period of an object were equal. We call this the process of tidal locking.

Note that this effect works for any group of orbiting bodies. For instance, the Sun and the planets are all affected by this phenomenon. Planets are commonly tidally locked if they are very close to their host star, and satellites are often seen tidally locked to their planet in the Solar System. In fact, the Moon is trying to tidally lock Earth, as seen by the fact that the rotation of Earth is slowed down by our satellite over billions of years. Therefore, there are cases where two bodies successfully tidally lock each other, as seen in the system containing Pluto and Charon.

What Are the Effects of Tidal Locking?

Tidal locking means that the rotational period and orbital period of an object are identical. This implies that only one half of the object will be able to see what it’s orbiting while the other half doesn’t. For example, we can only see the near side of the Moon on Earth. That is the side that faces Earth. Only when we send spacecraft out to the Moon can we glimpse the far side of the Moon, which cannot see our planet since our satellite is tidally locked with Earth.

If a planet becomes tidally locked to its star, it is almost assuredly not habitable to life as we know it. One side of it will be blistering as it will face its star 24 hours a day. At this level of exposure, even if a planet is inside the habitable zone, the side facing the star will be hot enough to make water boil away.

On the other hand, the other half of the planet, which faces eternal darkness, will cool down to extremely low surface temperatures that are so freezing that water will freeze no matter how close the planet is to the star.


To conclude, tidal locking is the process where tidal bulges affect the speed of rotation of an object so that the rotational period equals the orbital period of the object. If you want to learn more about this topic, please visit the websites listed in the references below.

References and Credits

  1. Fraser Cain. (2015, November 16). What is Tidal Locking? Retrieved January 18, 2022, from https://www.universetoday.com/123391/what-is-tidal-locking/
  2. (n.d.). What is Tidal Locking? Retrieved January 18, 2022, from https://theplanets.org/what-is-tidal-locking/
  3. Jonathan O’Callaghan. (2013, September 30). What is tidal locking? Retrieved January 18, 2022, from https://www.spaceanswers.com/deep-space/what-is-tidal-locking/

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