The moon has been accompanying our planet on its great space journey for several billion years. And it shows us, earthlings, from century to century, always the same moon landscape. Why do we admire only one side of our companion? Does the moon rotate around its axis or does it float motionless in outer space?
Characteristics of our Space Neighbor
In the solar system there are satellites much larger than the moon. Ganymede is the satellite of Jupiter, for example, twice as heavy as the Moon. But then she is the largest satellite relative to the mother planet. Its mass is more than a percent of the earth, and its diameter is about a quarter of the earth. There are no more such proportions in the solar family of planets.
Let's try to answer the question of whether the moon rotates around its axis, looking more closely at our closest cosmic neighbor. According to the theory accepted today in scientific circles, our planet acquired a natural satellite while still a protoplanet - it was not completely cooled, covered with the ocean of liquid red-hot lava, as a result of a collision with another planet of a smaller size. Therefore, the chemical compositions of the lunar and terrestrial soils are slightly different - the heavy nuclei of the colliding planets merged, which is why the earth's rocks are richer in iron. The moon got the remains of the upper layers of both protoplanets, there is more stone.
Does the moon rotate
To be precise, the question of whether the moon rotates is not entirely correct. After all, like any satellite in our system, it turns around the mother planet and with it circles around the star. But, the axial rotation of the moon is not quite usual.
No matter how you look at the moon, it is always turned towards us by the crater of Tycho and the sea of Tranquility. “Is the moon spinning around its axis?” - From century to century, earthlings have been asking themselves a question. Strictly speaking, if you operate with geometric concepts, the answer depends on the selected coordinate system. Relative to the Earth, the axial rotation of the Moon is indeed absent.
But from the point of view of an observer located on the Sun-Earth line, the axial rotation of the Moon will be clearly noticeable, and one polar revolution up to a split second will be equal in duration to the orbital one.
Interestingly, this phenomenon in the solar system is not unique. So, the satellite of the dwarf planet Pluto Charon always looks at his planet with one side, the same way the satellites of Mars - Deimos and Phobos.
In scientific language, this is called synchronous rotation or tidal capture.
What is the tide?
In order to understand the essence of this phenomenon and confidently answer the question of whether the Moon rotates around its own axis, it is necessary to analyze the essence of tidal phenomena.
Imagine two mountains on the surface of the moon, one of which "looks" directly at the Earth, the other is at the opposite point of the lunar ball. Obviously, if both mountains were not part of one celestial body, but revolved around our planet independently, their rotation could not be synchronous, the one that is closer, according to the laws of Newtonian mechanics, should rotate faster. That is why the masses of the lunar ball, located at points opposite to the Earth, tend to "run away from each other."
How the moon “stopped”
How tidal forces act on one or another celestial body, it is convenient to make out on the example of our own planet. After all, we also revolve around the Moon, or rather the Moon and the Earth, as befits in astrophysics, “circle dance” around the physical center of mass.
As a result of the action of tidal forces both at the nearest and at the farthest point from the satellite, the level of water covering the Earth rises. Moreover, the maximum amplitude of the ebb-tide can reach 15 or more meters.
Another feature of this phenomenon is that these tidal "humps" daily bend around the surface of the planet against its rotation, creating friction at points 1 and 2, and thus slowly stop the globe in its rotation.
The impact of the Earth on the moon is much stronger due to the difference in masses. Although there is no ocean on the moon, tidal forces act no worse on stony rocks. And the result of their work is evident.
So does the moon rotate around its axis? The answer is yes. But this rotation is closely related to the movement around the planet. For millions of years, tidal forces have aligned the axial rotation of the moon with the orbital.
But what about the earth?
Astrophysicists claim that immediately after the big collision that caused the formation of the moon, the angular velocity of rotation of our planet was much greater than now. A day lasted no more than five hours. But as a result of the friction of tidal waves on the ocean floor year after year, millennium after millennium, the rotation slowed down, and this day lasts 24 hours.
On average, every century adds 20-40 seconds to our day. Scientists suggest that in a couple of billion years our planet will look at the Moon in the same way that the Moon looks at it, that is, on one side. The truth of this, most likely, will not happen, since even earlier the Sun, having turned into a red giant, will “swallow” both the Earth and its faithful companion - the Moon.
By the way, tidal forces give earthlings not only an increase and decrease in sea level in the equator. Acting on the masses of metals in the Earth’s core, deforming the hot center of our planet, the Moon helps to maintain it in a liquid state. And thanks to the active liquid core, our planet has its own magnetic field that protects the entire biosphere from the killer solar wind and deadly cosmic rays.