Why does the Moon always show us the same visible face?
The gravitational attraction of the Moon on Earth brings up the level of the ocean on both sides of our planet and thus creates two bulges. As the Earth rotates from west to east, these two lumps (of which one always looks towards the Moon and the other in the opposite direction) travel from east to west around the Earth's surface.
When making this displacement, the two packages rub against the bottom of the shallow seas such as Bering or Ireland. Such friction converts rotation energy into heat, and this consumption of terrestrial rotation energy causes the Earth's rotation movement around its axis to gradually decrease. The marks act as a brake on the rotation of the Earth, and as a consequence the terrestrial days are lengthening one second every thousand years.
But it is not only ocean water that rises in response to lunar gravity. The solid crust of the Earth also accuses the effect, although less noticeably. The result is two small rock bulges that revolve around the Earth, one facing the Moon and the other on the opposite side of our planet.
During this displacement, the friction of one rock layer against another also undermines the earth's rotation energy. The packages, of course, do not physically move around the planet, but, as it rotates, they remit in one place and form in another, depending on what portions of the surface pass under the Moon.
The Moon has no seas or tides in the ordinary sense. However, the solid crust of the Moon accuses the gravitational force of the Earth, and we must not forget that it is eighty times larger than that of the Moon. The bulge caused on the lunar surface is much greater than that of the earth's surface. Therefore, if the Moon rotated in a twenty-four hour period, it would be subject to much greater friction than Earth. In addition, since our satellite has a much smaller mass than Earth, its total rotation energy would be already input, for equal periods of rotation, much less.
Thus, the Moon, with a very small initial reserve of energy, quickly undermined by the large bumps caused by the Earth, had to suffer a relatively rapid decrease in its period of rotation. Surely many millions of years ago it must have slowed down to the point that the lunar day matched the lunar month. From then on, the Moon would always show the same face towards Earth.
This, in turn, freezes the bulges in a fixed position. One of them looks towards the Earth from the very center of the lunar face that we see, while the other points in the opposite direction from the very center of the face that we do not see.
Since the two faces do not change position as the Moon revolves around the Earth, the packages do not undergo any new change nor does any friction occur that alters the satellite's period of rotation. The Moon will continue to show us the same face indefinitely; which, as you see, is no coincidence, but an inevitable consequence of gravitation and friction.
The Moon is a relatively simple case. Under certain conditions, friction due to tides can lead to more complicated stability conditions. For about eighty years, for example, it was thought that Mercury (the planet closest to the Sun and the most affected by solar gravity) always offered the same face to the Sun, for the same reason that the Moon always offers the same face to the Land. But it has been proven that, in the case of Mercury, the effects of friction produce a stable period of rotation of 58 days, which is just two thirds of the 88 days that constitute the period of revolution of Mercury around the Sun.
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