The moon has been accompanying our planet on its great cosmic journey for several billion years now. And she shows us, earthlings, from century to century always the same lunar landscape. Why do we admire only one side of our satellite? Does the Moon rotate on its axis, or does it float motionless in outer space?
Characteristics of our space neighbor
There are satellites in the solar system much larger than the moon. Ganymede is a moon of Jupiter, for example, twice as heavy as the Moon. But on the other hand, it is the largest satellite relative to the mother planet. Its mass is more than a percent of the earth's, and its diameter is about a quarter of the earth's. 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 by looking more closely at our nearest space neighbor. According to the theory accepted today in scientific circles, our planet acquired a natural satellite while still a protoplanet - not completely cooled down, covered with an ocean of liquid red-hotlava, as a result of a collision with another planet, smaller in size. Therefore, the chemical compositions of the lunar and terrestrial soils are slightly different - the heavy cores of the colliding planets have merged, which is why the terrestrial 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 parent planet and, together 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 Tycho Crater and the Sea of Tranquility. "Does the moon rotate on its axis?" – from century to century earthlings asked themselves a question. Strictly speaking, if we operate with geometric concepts, the answer depends on the chosen 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 visible, and one polar rotation up to a fraction of a second will be equal in duration to the orbital one.
Interestingly, this phenomenon is not unique in the solar system. So, the satellite of the dwarf planet Pluto Charon always looks at its planet with one side, the satellites of Mars - Deimos and Phobos - behave in the same way.
In scientific language, this is called synchronous rotation or tidal capture.
What is the tide?
In order to understand the essence of this phenomenon andto confidently answer the question of whether the Moon rotates around its own axis, it is necessary to analyze the essence of tidal phenomena.
Let's imagine two mountains on the surface of the Moon, one of which "looks" directly at the Earth, the other is located at the opposite point of the lunar ball. Obviously, if both mountains were not part of the same celestial body, but rotated 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 a particular celestial body, it is convenient to disassemble on the example of our own planet. After all, we also revolve around the Moon, or rather the Moon and the Earth, as it should be in astrophysics, "dance" around the physical center of mass.
As a result of the action of tidal forces, both at the nearest and at the most distant point from the satellite, the level of water covering the Earth rises. Moreover, the maximum amplitude of the tide can reach 15 meters or more.
Another feature of this phenomenon is that these tidal "humps" daily go 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 tomass difference. And although there is no ocean on the Moon, tidal forces act just as well on rocks. And the result of their work is evident.
So does the moon rotate on its axis? The answer is yes. But this rotation is closely related to the movement around the planet. Tidal forces have aligned the Moon's axial rotation with its orbital rotation over millions of years.
What about Earth?
Astrophysicists claim that immediately after the big collision that caused the formation of the Moon, the angular velocity of our planet's rotation was much higher than now. Days 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 the current day lasts for 24 hours.
On average, each 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 as the Moon looks at it, that is, on one side. True, 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 satellite, the Moon.
By the way, tidal forces give earthlings not only rise and fall in the level of the world's oceans near the equator. By affecting the masses of metals in the earth's core, deforming the hot center of our planet, the Moon helps to keep it in a liquid state. And thanks to an active liquid core, our planet has its own magnetic field, protecting the entire biosphere from the deadly solar wind and deadly cosmic rays.
