The Moon always shows us the same face . Regardless of what phase we see it in, what time of day or night, or what part of the sky it is in, it always points the same piece of its surface toward Earth. It seems that he wants to hide something from us, that he is up to mischief behind our backs. But no, what happens is simply that the Moon is gravitationally coupled to the Earth . It suffers from what is known as a tidal coupling .
In the same way that the Moon creates tides on Earth, raising the water level of seas and oceans, the Earth creates tides on the Moon. However, these tides do not affect lunar water , mainly because there is no liquid water on the Moon, but instead it affects its own internal structure . These tides will cause the Moon to create a certain bulge in the direction that joins both bodies and a flattening in the perpendicular direction. Of course, this bulge will be very subtle, deforming the Moon by only a few meters, very little compared to its 3,500 km in diameter. Still, this bulge will be present, and it will circle the Moon as it completes one orbit around Earth.
But in addition to moving around us, the Moon also rotates on itself. Doing so will cause that bulge to be slightly forward to its natural position. In other words , the bulge will move away from that Earth-Moon direction , moving forward if the Moon rotates faster than it takes to orbit the Earth and falling behind if it takes longer. By doing so, the attraction between planet and satellite will be affected and will no longer occur in exactly the same direction as before, but will instead pull harder on the side to which the bulge has moved. By doing so, you will gradually be able to stretch this bulge, thereby counteracting the Moon’s own rotation on its axis, slowing down this rotation. Over millions of years, this effect will cause the Moon’s rotation to lengthen, until it equals its translation.
At that time, from the surface of the Earth we will observe the scene to which we have become accustomed: a Moon that always shows us the same face and we will say that the Moon undergoes a tidal coupling. Of course our satellite is not the only object that undergoes this coupling . Due to the large size of the Moon, the Earth also feels this effect. In addition to the tides that raise the sea level, we also have the other type of tides , the planetary tides, which deform the planet, creating a bulge of just 40 centimeters in the Earth-Moon direction. This effect, although small, is enough to affect the rotation of the Earth.
As the bulge of the Earth advances due to the planet’s rotation, two things happen. On the one hand , the Earth is held back by the attraction of the Moon, lengthening the length of the days . On the other hand, the Moon is accelerated in its translation , increasing the distance between both bodies . This will have the effect that in billions of years the Moon will have receded to a distance of approximately one million kilometers, almost 3 times farther than today, and that the Earth’s rotation will have slowed down to a few 120 days from today to complete a full rotation. That is, at this time, the days will last more than 2800 hours and the Earth and the Moon will be mutually coupled , so that we will always see the same side of the Moon but, in addition, we will always see the Moon at the same point in the sky. .
Tidal coupling is not something that only affects the Moon-Earth system. Many bodies in the solar system and even in distant systems suffer from tidal coupling. The main moons of the giant planets of the solar system suffer from it . I speak of Ganymede , Io, Callisto or Europa in the case of Jupiter, of Titan in the case of Saturn, of Titania and Oberon for Uranus or of Triton with Neptune. Comparable in size to our own, these moons orbit close enough to their planets, though they are not so massive as to return the effect. Pluto and its moon Charon are in the configuration that awaits Earth in the distant future. Both bodies are mutually coupled, rotating in unison and always showing the same face.
Tidal coupling does not necessarily imply synchronous rotation , in which one orbit takes the same time as one rotation. Mercury is a clear example of this. It is coupled to the Sun, but its day and year do not last the same time , but have a ratio of 3:2, so that Mercury rotates 3 times for every 2 orbits around the Sun.
Due to the fact that current exoplanet detection techniques favor the discovery of massive planets that orbit very close to their star, 85% of the exoplanets discovered are attached to their star. However, we think that this should not be the norm and that future techniques that allow the discovery of planets further away from their star should lower this statistic.
REFERENCES:
R. Barnes, 2017, Tidal locking of habitable exoplanets, Celestial Mechanics and Dynamical Astronomy volume 129, https://doi.org/10.1007/s10569-017-9783-7
