system
Given the following post, respond as if you're commenting on Reddit:
human
If the moon also rotates on its axis,why is it said that we can never view its other side?
gpt
The time it takes for the moon to rotate around its own axis is exactly the same as the time it takes for the moon to orbit our planet. As a consequence, we always see the same side of the moon.

Now, you might say "well, that's a bit too neat to be a coincidence" and you would be right. Because the reason that the rotation and orbit of the moon are synced up is a phenomenon called "tidal locking".

The gravity of the Earth causes the moon to be stretched out ever so slightly, along the line that runs from the Earth to the moon. This stretching is due to the tidal effect and the gravity of the moon does the same to the Earth and because the Earth isn't just solid rock, but also has liquid water, we're able to see the consequences of this in the form of the tides.

If the moon would rotate faster than it would orbit, the elongation would rotate ahead of the imaginary line between Earth and moon. And if this happens, the gravity of the Earth would exert a torque on the moon that would pull it back, both slowing down the rotation and speeding up the orbit.

When the rotation and orbit are the same, this torque vanishes and a stable equilibrium is reached. At this point, we say that the body (moon) is tidally locked to the other body (Earth).

Tidal locking happens in other systems as well. Pluto and its satellite Charon are tidally locked to each other, meaning that Charon is only visible from one side of Pluto and from there you always see the same side of Charon. Mercury is tidally locked to the Sun, but because the orbit of Mercury deviates far enough from a circle, this lock is not 1:1. Instead, a Mercury year (the orbital period) is exactly 1.5 times the length of a Mercury day (its rotational period).

The moon is also affecting the rotational period of the Earth. The length of a day is ever so slightly increasing and the Earth would becomes tidally locked to the moon in about 50 billion years, at which point a day on Earth, a lunar month (a full orbit of the moon) and a lunar day (the rotation of the moon around its axis) would all be the same length. However, the expansion of the Sun into a red giant in 4-5 billion years will prevent this from ever actually happening.