Our side of the moon: Why do we always see the same side of the moon?
Recently I was helping some students in their thesis and one of them categorically asked me why we are only able to see one side of the moon. Trying to answer your question I found that maybe many more people would have the same doubt and that it would be interesting to make a post about it so here it is.
NASA
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The most obvious and perhaps what many believe correct, it would be that we always see the same side of the moon basically because the Moon does not have a rotation movement. But they are wrong, first, because the moon does have rotation movement and second because in case of not having a rotation movement, remember that it revolves around the earth so; even when we assume this hypothesis there should still be a time when we would be able to see the other side of the moon.
The correct answer to why we are only able to see one side of the moon is basically because the Moon performs its rotation and translation movements at the same time. That is to say that while it revolves around the earth (translation), it also rotates on its axis (rotation); besides that both movements take the same time, that is to say, a little more than 27 days and as a consequence of this, it is always the same side that points towards the earth.
Synchronized rotation.
At first the rotations of the moon and the earth were not synchronized in fact it has been speculated that previously the rotation speed of the moon was much higher than it currently has, all because it is believed that the moon has been changing its shape due to the effect of the tidal force to adopt a more spherical shape and therefore to reach synchrony with the earth.
Also, do not forget that usually physical systems tend to get a minimum energy configuration. And in the case of the Earth-Moon system, this minimum energy configuration was obtained with the longest axis of the moon located towards Earth; and that as a result of this there was synchronization of the rotation of the moon with the orbital movement around the earth
Deformation of the moon.
It is believed that approximately 4,500 million years ago a gigantic body with a size similar to Mars impact the Earth, that as a result of this the debris that resulted in the orbit of the Earth were coupled to originate the formation of what we know today as the moon.
It should be noted that it has been determined that many years ago, the Moon was at a very low distance from Earth. For example, the moon is currently about 384,000 kilometers, but previously, it was circling the earth at a distance of between 19,000 and 30,000 kilometers.
Then by means of these facts, it has been possible to determine that for that moment the speed of rotation of the moon and the earth was far superior to the one that they presently demonstrate and that in fact, the moon was able to rotate several times on its axis daily.
Entering more in the matter, it is necessary that we understand that as a result of the "little" distance that separates the Earth from the Moon; the moon was constantly undergoing a great transformation due to the force of the tides. And in addition to the rapid rotation, it was executing, an alteration occurred in the direction of its gravitational bulges outside the line that joins the centers of mass of the two bodies.
Tidal locking.
That Moon that was being deformed by its proximity to the Earth, experienced an extra gravitational force that little by little was causing the bundles that had originated to align with the earth. It should be noted that it was that same force that was responsible for the rotation of the moon becoming progressively slower.
The rotation speed of the Moon normalized and stopped its slowdown when the lumps of the Moon were aligned with the Earth, because when they were in the same position the gravitational force that pressed the packages stopped. And that's how the side we can currently see the moon was set.
Basically, this is the fully explained cause of why we see the side of the moon that we see.
However, it is remarkable that not only the earth exerted a gravitational force that caused the alignment, but the Moon performs the same "aligning" effect on the Earth, but at a slower velocity due to the difference in mass of both.
As a curious fact it is emphasized that while the Moon exerts its gravitational force on the Earth, the speed of rotation of our planet is becoming slower and, therefore, the Earth is taking more time in a return on its own axis.
That is to say, the Moon is prolonging the duration of our days by about 2.3 milliseconds per century.
Sides of the moon.
The two sides of the Moon are very unequal. The side that we can see has some big dark spots that create what we call 'the face of the Moon' or 'the man of the Moon'.
These spots are no more than large plains that were originated by the lava of volcanic eruptions.
While the side that we cannot see, is much more mountainous, almost does not have spots, and its orography consists basically of an immense amount of craters.