How does the moon rotate - why we always see the same side of the moon?
A remarkable feature about the Moon is that from Earth, we only ever see one side. This is incredible considering that both the Earth and the Moon are spinning while rotating around one another and the sun. So what causes this to happen? Why don’t we ever see the “dark side” of the Moon?
The Earth rotates on its own axis. The Moon also rotates on its own axis. However, the Moon always shows us the same face because Earth's gravity has slowed down the Moon's rotational speed. Millions of years, ago, the Moon rotated at a much quicker speed. Gravity from Earth acted to slow this speed down to that of the Earth. As a result, the Moon takes as much time to rotate once on its axis as it takes to complete one orbit of Earth.
So as the Earth rotates, so does the Moon and we Earth see the same face every night. Both are about 29.5 days. The Moon is “locked in” to this rotation, meaning that it will not change in the future. In astronomy, this is called a synchronous rotation, a phenomena that is also seen with other Moons in the Solar System.
If the Moon didn't spin at all, then eventually we on Earth would see its far side as it orbited around Earth. However, since the rotational period is exactly the same as the orbital period, the same portion of the Moon's surface is always facing Earth.
Given this fact, it would make sense to assume that we can only see 50% of the Moon’s surface from Earth. Oddly enough, due to an interesting phenomenon in the rotational path of the Moon, we can in fact see a little more. Since the Moon's orbit is elliptical, and not circular, the speed of its orbital travel increases and decreases depending on how close it is to our planet. As a result, we can see more of the surface area of the Moon when looking at the ends. The term for this "rocking" motion of the Moon is called libration and it allows for 59 percent of the Moon to be seen in total.