Never see the full Moon from Earth

The moon is the only natural satellite of the Earth and is also the only planet in which humans once set foot. However, if viewed from Earth, we can only see one side of the Moon. At any time, one side of the Moon always faces Earth, while the other side we cannot see and is called the dark side of the Moon. So, why can't we see the entire Moon from Earth?

The cause of this phenomenon is because the Moon is on a rotating orbit synchronously with the Earth , the Moon's rotation speed coincides with orbit around the Earth. So no matter where the Moon moves to Earth, only one side of it is facing Earth.

However, when it first formed, the Moon had a very different rotation and orbital speed than it is now. Over time, Earth's gravitational field makes the Moon's rotation speed gradually slow. Until it's stable and coincides with orbit around the Earth. So what makes the Moon stable with the current rotational and orbital speeds? The answer is because the friction effect appears with the phenomenon of tidal deformation.

As we know, the tidal phenomenon on Earth is caused by the gravity of the Moon. Similarly, Earth also exerts a gravitational force on the Moon, but is much larger (about 81,000 times). So when the Moon is trying to move in a straight line, the Earth pulls it back to itself and makes the Moon a satellite orbiting the Earth.

The gravitational pull between these two closely spaced planets causes a phenomenon called tidal hump , the surface of both planets distorted by gravity. Form a hump toward the other planet, it is called tidal hump because this is the cause of the tide phenomenon on Earth.

Initially when the Moon was spinning very fast, its tidal hump ran in front of the Earth-Moon connection because it could not deflect the tumor fast enough to keep the hump on that line. The rotation force causes the hump to always exceed this line. This phenomenon causes torque, reducing the Moon's rotation speed, as a force to tighten the nut. When the Moon's rotation speed is reduced enough to balance its orbital speed, the hump always points towards the Earth, the hump lies on the straight line connecting the Earth-Moon, and the force disappears. This explains why the Moon rotates at the rate of orbit and we always see only one side of the Moon.

On the left is the side facing the Earth, the right side is the dark side of the Moon.

Another interesting fact is that not only can we see 50% of the Moon's surface, but in fact we can see 59% of the Moon's surface . That's because the Moon's orbit around the Earth is not a circle but an ellipse. When the distance between the Earth and the Moon changes, it changes the angular velocity (velocity around the Earth), while the Moon's rotation speed remains the same. As a result, we can see 9% more of the Moon's surface. If the Moon's orbit is a circle, we will only see 50% of its surface.

And if we live about 10 or 20 billion years later, maybe we will always see the Moon, or we will never see the Moon again. Why is that? Just like what the Earth did with the Moon, the gravitational force and tidal force caused by the Moon on Earth also slowed the Earth's rotation. Although this friction effect is not large because the Moon's gravity is too small, we do not see the difference now. However, in a few billion years, there will be only one side of the Earth that is always facing the Moon. This also means that people living on the other side of the Earth (if humans still exist) will never see the Moon again.