The Moon’s “dark side” is actually the “far side” and gets cycles of day and night. Tidal locking, caused by gravitational pull and friction, explains why the same side always faces Earth. The Moon’s rotation has slowed to match the tidal bulge, creating an equilibrium. The Earth’s rotation also slows, but to be perfectly synchronized with the Moon it would need to rotate only once per lunar cycle.
The same side of the Moon always faces the Earth. The “dark side” isn’t actually dark – it gets cycles of day and night just like most places on Earth – the “far side” is a more correct term. The reason one side is never visible from Earth is because it rotates once on its axis in exactly the same time it takes to rotate around the Earth. If its rate of rotation were slightly different from its rate of revolution, those on Earth would eventually be exposed to the entire surface of the Moon. These two ranges have been the same throughout recorded history, however, and probably have been for millions of years or more.
This otherwise bizarre phenomenon can be explained in terms of a subtle effect generated by gravitation and friction called tidal locking. Through their mutual gravitational pull, the Earth and the Moon create tidal bulges on each other, with one bulge facing in the direction of each other’s body and one facing outward. These bulges generate heat through the friction of the rock rubbing against itself, and also turn into a major orbital force for the Moon, meaning it is continually moving away from Earth. Over time, they subtract energy from the rotational momentum of both bodies, producing a braking effect.
Since the mass of the Earth dominates the Earth-Moon system, the Moon experiences the greatest braking effect. Over time, its rotation progressively slowed until the rate of rotation reached the rate at which the tidal bulge moves around the body. Today, the lunar tidal bulges are in a constant position with respect to the rotation of the Moon, meaning that some sort of equilibrium has been reached.
The rate of Earth’s rotation also slows over time due to tidal forces, but the braking effect is much smaller: to be perfectly synchronized with the Moon, the Earth would need to rotate only once per lunar cycle, or about every 29.5 days . So the Moon would always be in the same place in the sky and only visible from one side of the Earth, but that’s not the case. In some planetary systems, such as that of the dwarf planet Pluto and its satellite Charon, both bodies are locked to each other.
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