Approximately 59% of the Moon is visible from Earth because the Moon rotates and revolves in the same period. What most likely causes this phenomenon?
1 Answer
The Moon presents the same face to Earth as it is tidally locked.
Explanation:
When the Moon was formed it was orbiting much closer to the Earth and was also also rotating faster. The Earth's gravity exerted tidal effects on the Moon which slowed its rotation period and made it move further away. Ultimately it became tidally locked and presents the same face to the Earth.
The reason we can see more than 50% of the Moon's surface from the Earth is that the Moon wobbles on its axis. It is called librations.
This tidal locking is not uncommon. Most moons in the solar system are tidally locked to their parent planet.
Interestingly, the planet Mercury is tidally locked to the Sun. As its orbit is quite eccentric the resonance isn't 1:1 like the Moon and Earth, it is 3:2.
As the Moon is quite large, it also exerts tidal effects on the Earth. So, the Earth's period of rotation is slowing and the Moon is moving further away as a result. The Earth would ultimately show the same face to the Moon, except that the Sun will have become a red giant before that could happen.
