You may have heard references made to the “dark side” of the Moon. This popular term refers to the fact that the same physical half of the Moon, the “near side”, is always facing Earth, which in turn means that there is a far side or so-called “dark side” that is never facing Earth and can only be seen from space.
This phenomenon has nothing to do with illumination or the periodic light and dark we see as the phases of the moon change. Sometimes people refer to a New Moon as a “dark moon” because the moon is fully in shadow as viewed from Earth and we can’t see it, but that’s not the same thing as the dark side of the moon. The side of the moon facing us during a New Moon is the same as any other moon phase, such as a Full Moon when we can see the entire face.
So why can we only see one side of the moon from Earth? We all know that the Earth rotates on its own axis, so theoretically, the Moon should also do the same, allowing us to get a full picture of the planetoid. Why are we limited to seeing only 50 percent? It turns out that the speed at which the Moon rotates has led to this particular phenomenon. Millions of years ago, the Moon spun at a much faster pace than it does now. However, the gravitational influence of the Earth has gradually acted upon the Moon to slow its rotation down, in the same way that the much smaller gravitational influence of the Moon acts upon the Earth to create tides. This influence slowed the rotational period of the Moon to match that of its orbit – about 27.3 days – and it is now “locked in” to this period. (Note that to observers on earth a full moon cycle takes 29.5 days.
If the Moon didn’t spin at all, then eventually it would show its far side to the Earth while moving around our planet in orbit. However, since the rotational period is exactly the same as the orbital period, the same portion of the Moon’s sphere is always facing the Earth.
Another interesting fact is that actually a little bit more than half of the Moon’s surface is observable from Earth. 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. The rotational speed of the Moon is constant however – and this difference between orbital speed and rotational speed means that when the Moon is farthest from the Earth, its orbital speed slows down just enough to allow its rotational speed to overtake it, giving observers a small glimpse of the usually hidden area. 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 (over time).
Finally, one reason that the far side of the Moon is frequently referred to as the “dark side” is because many people mistakenly think that it never sees any light from the Sun. In that sense the term “dark side” is wrong and misleading. In fact, since the Moon is constantly rotating on its own axis, there is no area of the planetoid which is in permanent darkness, and the far side of the Moon is only completely devoid of sunlight during a Full Moon – when the Sun is facing the Moon with the Earth in between.