Why do we always see the same side of the Moon?

Contrary to what one might think, the Moon rotates on itself, just like the Earth, but the speed at which it rotates on itself is equal to the speed with which it completes one revolution around the Earth (a little more than 27 days that the lunar cycle lasts), and that is why we always see the same side or face.

This phenomenon is due to synchronous rotation, an astronomical phenomenon that is also known as tidal coupling. The Moon's rotation on itself has been coupled to its rotation around the Earth by the effect of the force of gravity between both objects.

As you can see in this animation, on the right, the Moon rotates on itself at the same speed that it rotates around the central body, which would represent the Earth, and as this effect causes the same face to always point towards the center.

How does tidal coupling occur?

Tidal coupling occurs by effect of the force of gravity when one object orbits another. The term "docking" refers to one face being fixed by pointing towards the surface of the other, and "tide" refers to why this happens.

Tides are rises and falls in sea level that occur depending on the position of the Moon. Tides are an example of what is known as gravitational lumps, deformations of celestial bodies in the direction of the force of gravity. In the rock it is seen with less intensity than in the water, but they also occur.

This same gravitational bulging effect occurs on the Moon, which is "stretched" in the direction of the force of gravity exerted by the Earth. Gravitational bulges cause more mass to accumulate in deformed areas, and as there is more mass, the force of gravity is felt more strongly in these areas.

Because of this greater gravitational force, the gravitational bulges tend to line up in the direction of the gravitational force and slow down the speed of rotation until this rotation becomes synchronous with the speed of translation. This happened billions of years ago, when the Earth was formed, and at that time one of the faces of the Moon became attached.

Docking occurs in any pair of objects orbiting each other. If the difference in mass is small, both will be coupled. But if the difference in mass is substantial, as it is between the Earth and the Moon, the smaller object is coupled but its gravity is not strong enough to couple the larger object.

Synchronous rotation or tidal coupling can be observed on many moons of the Solar Systemall of them in an apparent fixed position and with the same face always pointing to the surface of the planet they orbit.

For example, Phobos, one of the satellites of Mars, completes one revolution around itself and around Mars in approximately 8 hours. It can also be seen in dwarf Pluto and its larger moon Charon, in this case both are coupled to each other.

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