Why do we always see the same side of the Moon? - medical - 2023



Since the origins of humanity, the Moon has captivated us. Our satellite has awakened thousands of mystical and scientific reflections to give an explanation of why that "rock" of apparently perfect geometry revolves around us.

And one of the things about the Moon that historically most has fascinated us is that there is the famous "hidden face", that is to say, that there is a whole half of the satellite that is never focused towards us. This obviously implies that we are always seeing the same face of her.

This, which is already mysterious in itself, becomes almost a paradox when we realize that, despite this, the Moon is always rotating on its same axis (as does the Earth). But, if it's always rotating, how can we only see one face?

In today's article, then, we will try to answer this question that was a headache for astronomers. until the phenomenon of synchronous rotation was discovered. And then we will understand perfectly what it consists of.

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What is the Moon?

The Moon, as we well know, it is the only natural satellite of our planet. But what exactly is a satellite? A satellite is, broadly speaking, any celestial body of a rocky nature that orbits around a planet that, being larger than it, traps it by the action of gravity.

The Moon is one of the 146 satellites of the Solar System. Mercury and Venus have neither. Earth, one. Mars, two. Jupiter, 50. Saturn, 53. Uranus, 27. And Neptune, 13. Each of these satellites has very specific characteristics and it is even believed that some of them are where life could most probably exist in the Solar System.

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Returning to the Moon, it is a satellite with a diameter of 3,476 km (the Earth has a diameter of 12,742 km) and a weight 81 times less than that of the Earth. It is located 384,400 km away from the Earth and the gravity on its surface, having such a smaller mass, is one sixth that of the Earth. In other words, on the moon you would weigh a sixth of what you weigh down here.

How was the Moon formed?

To answer this question, we must travel a few 4,520 million years into the past, with a very young Earth that was barely 20 million years old. This, in astronomical terms, is practically a "newborn".

For quite some time, it was believed that the Earth and the Moon formed simultaneously as a result of the compaction of different rocks in two different centers of gravity. One (the Earth) would end up being larger than the other (the Moon), causing the latter to be trapped by the gravity of the former.

This simple explanation seemed reasonable, but as studies in astronomy began to become more complex, it was discovered that this theory did not work, since the forces of inertia observed in the Earth-Moon system collided with what had been said. That is, if the theory were true, the inertia could not be what was seen.

Therefore, a new origin had to be found. And we did it. For now, the most widely accepted hypothesis is that the origin of the Moon is in the collision of a massive meteorite on Earth. This, which happened 20 million years after the formation of the planet, is what would cause the Moon to form.

And we are talking about a huge impact. In fact, it is believed that the collision was against a celestial body the size of Mars (about 6,800 km in diameter), which would be about half that of Earth.

As a result of this colossal explosion, billions of rock particles from both the Earth and the body that impacted were shot into space. These rocks were compacted to form the Moon. Therefore, a part (not all) of our satellite is, literally, fragments of the young Earth.

But the important thing is that once it was formed, as a celestial body "victim" of the action of gravity, it began to move, both around itself and around the celestial body that it orbits.

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What movements does the Moon follow?

Here we are getting closer to answering why we always see the same face. And is that by the force of gravity, the celestial bodies follow different movements. The Moon, like the Earth, follows two main types of movements. Let's see them, because understanding their nature will be essential to later answer the question in the article.

1. Rotational movement

The rotational movement is the one that celestial bodies follow when rotate on their own axis. Just as the Earth does, the Moon constantly revolves around itself, "circling." Simple as this. You simply have to take into account a key aspect, and that is that although the Earth takes one day to complete one turn, it takes the Moon 27 days. Later we will see why this qualification is so important.

2. Translational movement

The translational movement is the one followed by the celestial bodies that orbit around an object more massive than themselves, as they are trapped in their orbit due to the force of gravity, which, by simple physics, makes them follow a generally elliptical motion. The force of gravity pulls inward the celestial body around which they orbit, while inertia pulls them outward. The two forces compensate exactly in the strip where they follow the orbit, since that is where equilibrium is reached.

The important thing is that, just as the Earth revolves around the Sun, the Moon revolves around the Earth. And if it takes the Earth 365 days to complete a return to the Sun, to the Moon, as the Earth-Moon distance is much lower than the Earth-Sun, it takes only 27 days. As we see, it seems that the 27 days are important. And indeed, here is the key to everything.

Synchronous rotation and "hidden face"

We finally get to answer the question in today's article. And, as we have just seen, the rotation time and the translation time are practically the same: 27 days. There are small variations in hours, but they are not noticeable due to distances. In other words, The Moon takes exactly the same time to rotate on its own axis as it does to complete one revolution of the Earth.

And here is the key to everything. When a celestial body has the same period of rotation as of translation, a phenomenon known as synchronous rotation, which explains why we always see the same face of the Moon.

Synchronous rotation is a very strange event in the Universe, because it is a huge coincidence that a satellite takes the same time to make one revolution on its own axis as it does around the planet it orbits. Be that as it may, all the conditions were put together for this to happen with our Moon.

But why does the synchronous rotation make us always see the same face of the Moon? Let's try to explain it. And to understand it, imagine that you are in the field circling a tree. And you're not just spinning around that tree, you're spinning around on yourself or yourself.

Now, three things can happen: that you turn yourself faster than around the tree, that you turn yourself slower than around the tree or that you go at the same speed in both movements.

Let's put ourselves in the first assumption. You can try it with something you have at home. Whatever is. Imagine that your face is the face we see of the moon and your back, the hidden face. If you spin faster than you spin around the tree, what will happen? That in a short time, you will have already turned your back. That is, your hidden face.

Let us now put ourselves in the second assumption. If you turn more slowly, there will come a time when, before completing the turn around the tree, you have already shown your back, because the movement of turning around it is “ahead” of yours.

But beware of the third assumption. And is that if you rotate on your axis at the same speed as around the tree, what happens? Exactly, no matter how much you turn on yourself, you never turn your back on the tree. It seems impossible. But you can prove it. And you will see that even if you really turn on yourself, you will always face.

The same is what happens with the Moon and the Earth. From the perspective of the Moon, she constantly rotates. What happens is that, for the viewer, us, it remains staticbecause it rotates around us at the same speed that it rotates on itself.

If you try the tree thing with a friend, he will become Earth. And he will not have the feeling that you are turning on yourself, because for him, you are always focused on the same side.

In short, that we always see the same face of the moon and that there is a hidden face is due to a huge coincidence: synchronous rotation. If we were at a different distance and the lunar rotational and translational movements were not the same between them, we would not always see the same face from the satellite.

In fact, the Moon separates from the Earth 4 centimeters each year. Therefore, although it is not appreciable, technically every day we are seeing a little more of its hidden side. But, we repeat, this will only be appreciable millions of years from now. For now, we can only see one side of the moon because it takes 27 days to rotate both on itself and around us.