As the moon revolves around the Earth, it also rotates on its axis. Why is it that the the same side of the moon is always visible from Earth?
The phenomenon of the same side of the moon always facing the Earth is known as "tidal locking." It occurs because of the combination of the moon's rotation period and its orbital period around the Earth.
To understand this, let's break it down:
1. The rotation period: The moon takes approximately 27.3 days to complete one full rotation around its axis. This means that it spins on its axis once every 27.3 days.
2. The orbital period: The moon orbits around the Earth in roughly 27.3 days as well. So, it takes the same amount of time to complete one orbit around the Earth as it does to complete one full rotation on its axis.
Now, here's how it leads to the same side of the moon always facing Earth:
Initially, when the moon formed, it rotated faster on its axis. However, gravitational forces between the Earth and the moon caused the moon's rotation to gradually slow down over millions of years.
As the moon's rotation slowed down, the gravitational force exerted by the Earth between the bulging parts of the moon on the side facing the Earth caused a "tidal bulge" to form. The gravitational attraction between these bulges and the Earth created a torque that acted to slow down the moon's rotation.
Over time, this gravitational interaction between the Earth and the moon caused the tidal bulges to align with the Earth-Moon line, which means the bulges always face towards or away from the Earth.
Eventually, the moon's rotation slowed down to the point where it has become "tidally locked" with the Earth. This means that it now takes the same amount of time for the moon to rotate on its axis as it does to orbit around the Earth.
As a result, the same side of the moon always faces the Earth, while the other side, known as the "far side" or "dark side," remains mostly invisible from our perspective.
So, in summary, the gravitational forces between the Earth and the moon have caused the moon's rotation to slow down and become tidally locked, resulting in the same side of the moon always facing the Earth.