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

reason why we always see the same side of the moon

We are all aware that the Moon always shows us the same face, that is, from Earth we cannot see the hidden face of the Moon. Unfortunately, this fact has led many to believe that the moon does not rotate. Many people don't know why do we always see the same side of the moon.

For this reason, we are going to dedicate this article to explaining to you step by step why we always see the same side of the moon.

The moon turns on itself

moon rotation

First, you have to understand how the moon moves around the earth (translation), then you can understand why the moon rotates. This lunar translation period is 27,3 days, which means that if there is a full moon tonight, the moon will be in exactly the same phase as today for 27,3 days. When the moon revolves around the earth, it also revolves.

The key point here is the amount of time you spend doing both moves. In fact, the moon takes the same 27,3 days to complete one revolution around the Earth as it takes to complete one revolution around its axis. It is this synchronous movement, the coincidence of this movement, that we always see the same face of our natural satellite.

The fundamental aspect behind this brilliant coincidence is the action of gravity. The Moon's gravity slightly deforms the Earth and makes tidal activation much easier. In the same way, Earth's gravity "pulls" on the moon, creating a brake-like bump on the moon. This brake slows down the rotation of the moon at its current rotation speed.

When that happened, About 4.500 billion years ago, the Moon was "blocked" by so-called tidal forces, and it has shown us the same face ever since. There is also a tendency to think that the side we do not see is colder than the one we do see because it does not receive solar radiation. However, this is also wrong. Both sides of the Moon, or rather the entire lunar surface, receive the same amount of radiation during its motion around Earth.

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

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

Most of the planets in the solar system have moons. For example, Mars has two moons, Jupiter 79 and Neptune 14. Some are icy, some are rocky, some are geologically active, but others have little or no activity. But what about the moon? What does that have to do with it?

There is a simple answer to this question: the moon is like an excellent dance partner, constantly looking at its partner: it always looks at the Earth with the same face. The face is "unique" because the moon takes exactly the same amount of time to rotate around its axis as it does to orbit the Earth.

That equates to a little over 27 days, so we always see the same lunar hemisphere. This is a phenomenon called gravitational coupling. Or the same thing: its rotation and translation movements are synchronized, so we always see the same face.

We can practice it ourselves without traveling to space and away from it: just take a stick and two papers of two colors and rotate it around you while it rotates by itself. So if you can see yellow paper at first, you will only see yellow paper the rest of the time. This is what happened to the fifth largest moon in the solar system.

What about the dark side of the moon?

full moon

But there's more, what about that face we can't see? Since 1959, people can see pictures thanks to Soviet space probes. Today we got high-resolution images from all directions in the distance, and we can see that it's more cratered: that's because it's more exposed to outer space.

Thus, the visible side is made up of 40% ocean, and a large expanse of land comes from the volcanic flow. However, on the hidden side only 8%. It is a mystery today, and it is precisely in trying to understand why the crusts of the two sides are so different.

According to a 2019 Chinese survey by the Change 4 probe, this coupling could have an effect: “When the Earth and Moon formed, they were actually glowing lights. The satellites got smaller and cooled down quickly, but our planet continued to emit heat. At that point, the orbits must have docked, and the heat prevented the formation of a thicker crust on the visible side," he explained.

Movements of the moon

Since between the moon and the Earth there is a force of gravitational attraction, there are also natural movements of this satellite. Just like our planet, it has two unique movements known as rotation about its own axis and translation in an orbit around the Earth. These movements are what characterize the moon and are related to the tides and the phases of the moon.

During the different movements that he has, he spends a certain time to finish them. For example, a complete translation lap takes an average of 27,32 days. Curiously, this means that the moon always shows us the same face and it seems that it is totally fixed. This is due to numerous geometric reasons and to another type of movement called lunar libration that we will see later.

When the Earth revolves around the Sun, the moon is doing it too but on the Earth, in an East direction. The distance of the moon from the Earth throughout its movements varies greatly. The distance between the planet and the satellite is 384 km. This distance varies completely depending on the moment in which it is in its orbit. Since the orbit is quite confused and at some moments far away, the Sun influences a lot with its gravitational force.

The nodes of the moon are not fixed and move 18,6 light years away. This makes the lunar elliptical non-fixed and the perigee of the moon occurs for every 8,85-year turn. This perigee is when the moon is in its full phase and is closest to its orbit. On the other hand, the apogee is when it is farthest from orbit.

I hope that with this information you can learn more about why we always see the same side of the moon.


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  1.   Caesar Salad said

    As always, the information you present to us is excellent, so I invite you to continue enriching our knowledge...