Why do we see only one side of the Moon?

Why do we see only one side of the Moon?

Let’s talk about the Moon. Our friendly floating next-door neighbor, the only celestial object on which man has stepped foot other than Earth, of course! Now for a while imagine yourself as a young astronomer who observes the Moon for many successive nights and find beautiful craters and huge lava plains. But one astonishing feature you would surely come across is that the modest Moon always looks the same. Isn’t it quite peculiar for an object that is both rotating and revolving to seem similar to us on every night, but actually the fact is that it isn’t. There is a part of the moon that we never see from Earth (since it always keeps the same side towards us). It's just the "far side of the moon" also known as "dark side of the moon".

Is this dark side of moon is really dark?

First, the dark side isn’t really any darker than the near side. Like Earth, it gets plenty of sunlight. 
The first images of the far side of the moon were taken in 1959 by the Soviet Luna 3 Spacecraft. Since then, missions like the Lunar Reconnaissance Orbiter have been able to tell us much more about the side we never see – and the far side doesn’t look anything like the near side. Scientists believe the moon was molten, or hot liquid when it first formed, and then it cooled. But the dark side cooled first, making it older with more craters. Though they don’t know why, the near side also has more radiation than the far side, perhaps contributing to why the near side didn’t cool as fast as far side.

If dark side isn't really dark then why we only see one side of moon?

The logic is quite simple - "Tidal Locking".  Tidal locking is the name given to the situation when an object's orbital period matches its rotational period. Our moon takes 28 days to go around the Earth and 28 days to rotate once around it's axis. This results in the same face of the Moon always facing the Earth. The concept of tidal locking is based on the concept of gravity. As gravity weakens with distance, the side of the Earth that faces the Moon is closer to the Moon than the rest of the Earth and so is attracted strongly to the Moon. In contrast, the far side of the moon feels a weaker than average attraction towards the Moon.  Gravity from Earth pulls on the closest tidal bulge, trying to keep it aligned. This creates tidal friction that slows the moon’s rotation. Over time, the rotation was slowed enough that the moon’s orbit and rotation matched, and the same face became tidally locked, forever pointed toward Earth.

Why Tidal locking  happen?

Tidal locking occurs because the planet deforms the satellite into an oval, with long axis pointing towards the planet. If the satellite is rotating the long axis will move away from being pointing towards the planet, and the gravity of the planet will tend to pull it back, slowing the rotation until one face is permanently facing the planet. Tidal locking isn't a result of the formation processes, but a consequence of satellites not being perfectly rigid. Let's consider this scenario, when a planet orbits a star, it is being pulled by the gravity of that star. The different sides of the planet are pulled to different degrees, with the side closest to the star receiving a small but noticeably larger pull. This bends the planet out of shape, from a ball into an eclipse. No water is necessary for this to happen. Even solid rock stretched out - the surfaces of both the Earth and the Moon stretch toward each other. This stretching doesn't happen immediately, though. It takes time for the planet to stretch its solid mass towards the sun and to settle back, and while it is stretching and settling, it is moving. At first, it is moving in two different ways. It is rotating on its axis, the way the Earth does to produce night and day. It is also orbiting the star, as the Earth does to produce a year. Those two movements rarely sync up. For example, sometimes the rotation speeds past the orbit. In that case, instead of the bulges in the ellipse "pointing" directly at and away from the star, they turn past it. 

The problem is, the near bulge is closer to the star than the rest of the planet, and it feels a gravitational pull dragging it backwards - so it's once again aligned with the center of the star. It doesn't necessarily get pulled all the way back, but it gets shifted a little bit. That shift happens every time the planet rotates. If the rotation is too slow and the orbit is fast, the bulge lags behind as the planet orbits forward, and the gravitational pull of the star drags it forward. No matter what, the planet gets a tug until its rotation is exactly the same period of time as its orbit. When that happens, it's tidally locked. It shows one face to the sun at all times.

Lunar libration

Much like a race car drifts when it turns on the curved portions of an oval racetrack, the moon does have a tendency to want to spin faster. Earth’s gravitational pull holds it in place. The moon’s shape is key to keeping it in sync with the Earth. Long ago, scientists believe, the moon had its own spin. Over time, frictional forces, including gravity, helped mold the moon into the shape it is now — spherical, but not a perfect sphere. If the moon were a perfect sphere, then the gravity felt on the far side and the near side (or Earth’s side), would cancel each other out. But because it isn’t a perfect sphere, as it turns, a smaller portion of the moon moves in toward Earth and a larger portion moves away. This uneven distribution in gravity causes a torque, or a rotational force, making the moon spring back into place. The spring-like motion is referred to as lunar libration.

Early life of Moon
Now, imagine the Moon very early on in its life. There is no reason to suppose that the Moon was tidally locked from the beginning of its existence. In fact, it probably wasn’t. It was either rotating faster than its current revolution, or slower. It really doesn’t matter which, as we shall soon see. Now, the bulge caused by the Earth had to be near the point where the Earth is directly overhead. If the Moon was rotating faster than it was revolving, it would be ahead. The Earth’s attraction would give rise to a torque (think: a twisting force) until it slowed down. The moment the bulge passed underneath the Earth, there would be no more unbalanced force, so it would stay locked in that position. This would also have an effect of forcing the bulge to always stay underneath the Earth, because if it moved away, even a little bit, it would be pulled back. It has no choice but to stay put exactly like that. If it was moving any slower, the twist would simply be in the other direction, but the outcome would be the same.

The impact of gravity

 
The Earth would be a very different place if the moon did not exist or we can say the same about gravity. Gravity exists as a gradient. Not only did the Earth gravity slow down the Moon’s rotation, but the Moon gravity is slowing down the rotation rate of the Earth. Since the moon’s formation, the Earth has been slowing its rotation due to the friction of the tides caused by the moon, and in reaction to this exchange of energy, the moon has been moving farther away from the Earth. Turns out, Even now, the Moon is continuing to slow down our day by 15 microseconds every year. In fact, at the time of the moon’s formation the Earth rotated much faster than it does today; a single day lasted for about a quarter of our day (6 hours, more or less). But the Moon, being small in relation to Earth, will take more than twice the age of the solar system to slow Earth’s spin rate to the Moon’s orbital rate. So, there’s one less thing to worry about, right?

An exclusive case for our Moon
As you can see, there is no reason why this has to be an exclusive case for our Moon. In fact, most satellites (barring some small ones that revolve around Jupiter and Saturn) are tidally locked. For Pluto and its moon Charon, the situation is even more extreme. Both of them are so close in mass that they have both become tidally locked to each other. That means that they both show only one face to the other. This is less like a satellite revolving around a planet (okay, dwarf planet) and more like two balls attached to a rod that is twisting on its own. Well, we have no right to be all smug about it. It might have slowed to a crawl and gotten kicked out of the planetary group of our solar system, but we face some of the same problems.